Bibliothek

Notizen: Abstract Seven workers at a work site where methylenedianiline (MDA) was used as a curing agent for an epoxy resin were studied during 4 workdays and one weekend. Internal exposure was monitored by analysis of 4,4′-MDA in hydrolysed urine and blood. After acidic hydrolysis, MDA was extracted, derivatised by use of pentafluoropropionic anhydride, and determined by gas chromatography and mass spectrometry, with negative ion chemical ionization. There was wide variation of MDA concentrations in hydrolysed blood between the workers (range 10–60 nmol/l). Also, the urinary excretion rate varied considerably both between and within the individual workers (0–90 μmol/h). The urinary excretion displayed some variation in relation to exposed periods. The urinary levels were linearly related to the concentrations in blood (r s= 0.93,P = 0.02). The present results show the value of excretion of MDA in hydrolysed urine and concentrations in blood as means for the biological monitoring of MDA exposure. In this case, the exposure was probably mainly dermal, in spite of extensive protection measures.

Notizen: Abstract Interrenal-like tissue in male African catfish is localized in paired organs, situated retroperitoneally anterior of the kidney. Histological and enzyme-histochemical (3β-hydroxysteroid dehydrogenase) reactions were carried out on sections of these organs in order to localize steroid-producing cells. In vitro incubations were carried out to determine the steroidogenic capacity of the interrenal-like tissue. Twenty-one steroids could be identified and quantified in the incubation medium, by means of gas chromatography followed by mass spectrometry. Cortisol, 5α-and 5β-androstanedione and androstenedione together comprised about 80% of the total steroid content. In a previous study, twenty testicular steroids were detected in plasma after castration. In the present investigation we have shown that most of these steroids can be produced by the interrenal-like tissue, which thus can be considered as an extra-testicular source of gonadal steroids.

Notizen: Abstract. Interrenal-like tissue in male African catfish is localized in paired organs, situated retroperitoneally anterior of the kidney. Histological and enzyme-histochemical (3β-hydroxysteroid dehydrogenase) reactions were carried out on sections of these organs in order to localize steroid-producing cells. In vitro incubations were carried out to determine the steroidogenic capacity of the interrenal-like tissue. Twenty-one steroids could be identified and quantified in the incubation medium, by means of gas chromatography followed by mass spectrometry. Cortisol, 5α- and 5β-androstanedione and androstenedione together comprised about 80% of the total steroid content. In a previous study, twenty testicular steroids were detected in plasma after castration. In the present investigation we have shown that most of these steroids can be produced by the interrenal-like tissue, which thus can be considered as an extra-testicular source of gonadal steroids.

Notizen: Summary Statistical and factor analysis have been used to test the ability of several molecular descriptors to predict the retention indexes (I) of polycyclic aromatic hydrocarbons determined on SPB-5 and SPB-20 stationary phases. Regression analysis with both columns showed that linear regression with connectivity index and bending energy of the molecules as independent variables gave the best correlation coefficients. ΔI 20–5, considered to be a measure of the polar forces in retention, gave a good correlation with the van der Waals radii of the molecules, or strain, and bending energy.

Notizen: Summary This report was conceived as a source for the relations between five pneumatic parameters of carrier gas-inlet and outlet pressure, inlet and outlet velocity, and average velocity. Any pair of these parameters can be independent while the remaining three can be expressed as functions of the independent pair. As the total number of the independent pairs is ten, thirty different functions describing relations between arbitrary independent pairs and dependent parameters can be identified. All thirty were derived below together with the complete set of bounds for variations of independent parameters. To derive some relations, a third order rational equation had to be solved. Some properties of that solution are discussed. A simple case of vacuum operations with zero outlet pressure has also been considered.

Notizen: Summary If the dependence of HETP on temperature is specified under isothermal conditions, it is possible to predict the HETP for programmed temperature elution and subsequently peak width at half-height. This requires knowledge of isothermal retention time at retention temperature, which is computed by means of a model including the variation with temperature of dead time estimated from 3 homologs with carbon number: n, (n+j), (n+jk), where n, j and k are any integers. Predicted and measured peak widths corresponded within 4–9%.

Notizen: Summary This paper presents the results obtained using fused-silica fibers coated with graphitized carbon black (Carbograph-Alltech) for solid-phase microextraction (SPME). The extraction and calibration curves relative to organic micro-pollutants present in gaseous and aqueous samples are reported. Examples of applications of this extraction procedure to GC and GC-MS analysis of organic micro-pollutants in actual samples are also reported.

Notizen: Summary The synthesis of 3-bromo-4-(4-methylbenzyloxy)azobenzene (BMBOA) and its use as a stationary phase in packed columns are described. Chromatographic characteristics such as column efficiency, polarity (McReynolds constants) and operating temperature range were studied. Retention factors, specific retention volumes and activity coefficients for alcohols, esters, ketones, aromatic hydrocarbons andn-alkanes were determined. Phenols and 2,3, and 4-chlorobenzyl chlorides were separated on the new phase. Based on experimental data some assumptions were made on the possible types of intermolecular forces between solute and solvent.

Notizen: Summary An extension of Rohrschneider's concept to the field of the linear temperature programmed gas liquid chromatography, is proposed, which is based on Antoine's equation for the temperature dependence of the reduction index of Takács et al. and Tekler. The new method presented in this paper, applied to retention data of two stationary phases of low polarity, PS-255 and OV-105, gives a new possibility of precalculating retention data.

Notizen: Summary A capillary gas-liquid chromatography method for the analysis of free short chain fatty acids in intestinal samples was developed. Analytical results obtained with intestinal samples are influenced not only by high molecular mass matrix constituents but also by matrix components with a molecular weight smaller than 5000 daltons. To achieve a faithful transfer of SCFA into the chromatographic system it was important to avoid accumulation of involatile deposits on the surface of the inlet liner. Cleaning of the inlet linear lead to a significant weakening of matrix effects. The method described, using an Innowax column, is more effective, particularly with respect to precision and sensitivity, and is clearly superior to packed columns. The capillary is the column of choice for the separation of short fatty acids in samples from the small intestine and stomach.

Notizen: Summary The correlation equations between Kovats retention indices ofn-alkenes andn-alkynes, column temperature and number of carbon atoms in molecules of these compounds on OV-101, OV-225 and PEG 20M capillary columns have been calculated. The two variable equationI=A+B·n+C/T give a good fit and are the simplest for prctical use.

Notizen: Abstract Clarkia breweri (Onagraceae) is the only species known in its genus to produce strong floral fragrance and to be pollinated by moths. We used gas chromatography-mass spectrometry (GC-MS) to identify 12 abundant compounds in the floral headspace from two inbred lines ofC. breweri. These volatiles are derived from two biochemical pathways, one producing acyclic monoterpenes and their oxides, the other leading from phenylalanine to benzoate and its derivatives. Linalool and linalool oxide (pyran form) were the most abundant monoterpenoids, while linalool oxide (furan form) was present at lower concentrations. Of the aromatic compounds detected, benzyl acetate was most abundant, whereas benzyl benzoate, eugenol, methyl salicylate, and vanillin were present as minor constituents in all floral samples. The two inbredC. breweri lines differed for the presence of the additional benzenoid compounds isoeugenol, methyleugenol, methylisoeugenol, and veratraldehyde. We also analyzed floral headspace fromC. concinna, the likely progenitor ofC. breweri, whose flowers are odorless to the human nose. Ten volatiles (mostly terpenoids) were detected at low concentrations, but only when headspace was collected from 20 or more flowers at a time. Trans-β-ocimene was the most abundant floral compound identified from this species. Our data are consistent with the hypothesized recent evolution of floral scent production and moth pollination inC. breweri.

Notizen: Summary A gas chromatographic method is presented which distinguishes phosphine from hydrogen sulfide and other possible headspace gases of anaerobic microbial cultures. In anaerobic cultures spiked with phosphine, this gas is recovered in the liquid and gaseous phase down to 10 pg per ml of gas or liquid. No biogenically produced phosphine was found. Phosphine in amounts as small as 30 ng per 1 can be stored for several days in glass bottles covered with rubber septa, filled with nitrogen, in the presence or absence of water or of an anaerobic bacterial culture. Due to the selectivity of the detector and the retention characteristics of the porous layer open tubular polymer column alkanes, alkenes and organosulfur compounds routinely found in anaerobic bacterial headspaces do not interfere with the analytical quantification of phosphine.

Notizen: Summary Thermal and analytical properties of two liquid crystals containing internal [2-nitro, 3-methyl 4-(4-heptylbenzoyloxy) 4′-methylazobenzene] and external NO2 [2,3-dimethyl 4-(4-heptylbenzoyloxy) 4′-nitroazobenzene] have been investigated by gas chromatography. Transitions temperatures are determined by inverse gas chromatography using standard columns. Good agreement with values determined by differential scanning calorimetry are found. Analytical capabilities of the two liquid crystals are shown in glass capillary columns. Compared to examples in the literature, the crystals used separated different kinds of solutes. In addition, high molecular mass solutes were eluted at relatively low temperatures. Typical separations are observed for hydrocarbon positional isomers, aromatic hydrocarbons, geometric isomers, and volatile aroma compounds. Comparison between the performance of internal and external, NO2-liquid crystals is made.

Notizen: Summary The analysis of spirits with a single stationary phase is still an unsolved problem; the two amyl alcohols, ethyl acetate and acetal are not separated on a polar column, methanol and acetaldehyde coelute from an apolar column. Trials with coupled columns of different polarities showed that optimum results were obtained with a 40 m capillary column, comprising 6 m of Carbowax 20 M, 12 m of 1∶1 Carbowax 20M:OV1 mixture and 30 m of PS 264. Best results were achieved when a 1 m retention gap was used, injecting a small amount of sample and using a 7 cm syringe needle.

Notizen: Summary By application of a chromatographic column filled with Supelclean ENVI-Carb/Celite and elution with three solvents of different polarity three PCB fractions were obtained. Fraction A contained poly-ortho PCBs, Fraction B mono-ortho PCBs, and Fraction C non-ortho PCBs. The Supelclean ENVI-Carb/Celite column was used in combination with a sample preparation procedure for pre-cleaning of acid-stable chlorinated hydrocarbons such as DDT and its metabolites, HCH isomers, and regulation-relevant PCB congeners. The method was optimized using standard solutions of 55 PCB congeners, 8 chlorinated pesticides and contaminated cod liver oil samples. The influence of traces of remaining matrix on the elution profile of the organochlorine compounds on Supelclean ENVI-Carb/Celite was observed. Quantitation was carried out by GC-ECD with fused silica capillary columns of different polarity.

Notizen: Summary We report a method for the analysis of triterpenes in aqueous alcoholic extracts ofM. ilicifolia orM. aquifolium leaves, using high temperature capillary gas chromatography in polar columns (OV-17-OH). These species are popularly known in Brazil as “espinheira santa” and have proven antiulcer properties. The method consists of the pre-concentration of the triterpenes from the aqueous alcoholic extract, followed by GC analysis, which permits the determination of the presence of friedelan-3-ol and friedelin. Direct comparison with several triterpenol standards that are usually found in falsified “espinheira santa” samples but are absent in the authentic drug, showed that GC analysis clearly allows detection of the presence of friedelan-3-ol and friedelin and, therefore, permits distinguishing between authentic and false “espinheira santa” aqueous alcoholic extracts.

Notizen: Summary A rapid and efficient multiresidue extraction procedure using ethyl acetate and sodium sulfate has been applied to the analysis of diazinon, methamidophos, chlorpyrifos, malathion, parathion, parathion-methyl, dimethoate and monocrotophos residues in many different kinds of vegetables. No cleanup step was required Concentrated extracts were analysed by gas chromatography with flame photometric detection in phosphorus mode. Recovery studies were performed in six kinds of matrices at two fortification levels. Recoveries were in the range 80–115%. The limit of quantification of the analytical method has been estimated as 0.01 ppm for diazinon, methamidophos and malathion, 0.03 ppm for chlorpyrifos, parathion, parathion-methyl and dimethoate and 0.1 ppm for monocrotophos. Experiments showed that potentially it should be possible to develop a rapid and universally applicable method for organophosphate pesticide residues in different matrices.

Notizen: Summary A continuous-flow method that combines on-line preconcentration and isolation with gas chromatography for the direct determination of vitamins D2 and D3 in oily solutions is reported. A silica gel column permits preconcentration and isolation of analytes from the vitamin D matrix, although some triglyceride (ca. 25%) is also retained. To overcome problems associated with the low volatility of triglycerides, their retained fraction is further transesterified with potassium methylate to fatty acid methyl esters after elution. The proposed method was applied to the determination of vitamin D in pharmaceutical preparations.

Notizen: Summary A rapid and direct gas chromatographic (GC) method for determining free cholesterol in milk fat using a capillary column and programmed-temperature vaporizerinjector was assayed. It was compared with other procedures involving saponification of fat, solvent extraction of unsaponifiable matter—with and without fractionation by thin-layer chromatography—and transformation of sterols into silyl derivatives prior to GC analysis. This paper proposes an alternative to other published procedures. Repeatability of the method was assessed and the coefficient of variation determined as 2.1%. The alternative saponification method exhibited comparable accuracy (coefficient of variation=1.8%). Recovery ranged from 99.1% to 105.6%.

Notizen: Summary Silica was modified with γ-aminopropyltriethoxylsilane, and then the amino group was converted into an N-benzoylthiourea compound. Treatment with anhydrous CuCl2 solution, gave a copper(II) complex on the surface. In order to estimate the ability of the complex to interact specifically with electron donors, a number of retention parameters were determined for three groups of compounds showing electron-donor properties: ketones, ethers and nitroalkanes. On the basis of the results, the effect of adsorbate structure on the strength of specific interactions was established. The packings studied may be useful for the analysis of the mixtures of ketones, ethers, and nitroalkanes.

Notizen: Summary A comparison between different element selective detectors for the determination of organophosphorus and organochlorine pesticide residues, from fruit and vegetables, was performed by capillary GC with electron capture detector (ECD), nitrogen phosphorus detector (NPD), flame photometric detector (FPD) in the sulphur and phosphorus modes, and mass spectrometry detector (MSD) in selected ion monitoring (SIM) mode. Pesticides were extracted from the different foodstuffs by Matrix Solid Phase Dispersion (MSPD). Recoveries of 41–108 % with relative standard deviation of 2–14 % in the concentration range 0.5–10 μg L−1 were obtained in oranges, lemons, grapefruit, pears, plums, lettuces and tomatoes. The results demonstrated that the extracts of all the samples can be analyzed by the detectors used since no interfering co-extracted compounds were detected.

Notizen: Summary Experimental Design methodology allows the modelling and optimization of the chromatographic separation of similar pesticides (triazine family) by GC and HPLC. The GC separation of simazin and atrazin is well modelled by a first degree equation, involving injected volume, carrier gas pressure and rising oven temperature. The LC is modelled by a second degree equation, depending on injected volume, eluent flow and composition. These calculated models allow easy optimization of the separations, using isoresponse curves.

Notizen: Summary A comparison of some adsorbents, commonly used for trapping trace volatile organic pollutants followed by thermal desorption, is presented. The evaluation has been performed on 12 compounds representative of the main classes of organic substances. Breakthrough volumes and desorption recoveries have been measured. Every adsorbent gave good results for a range of compounds, but none were able to cover all the compounds examined. Thus, a multilayer trap utilising the best features of each adsorbent was introduced with very good results.

Notizen: Summary The polysiloxane backbone of the gas chromatography phase Chirasil-Val has been modified by replacing one methyl group per dialkylsiloxy unit by pentyl and hexyl groups, respectively. The new phases offer the advantage of reduced polarities and softening points, while the separation factors (α) of enantiomeric pairs of N,O-trifluoroacetyl amino acid propyl esters show small but intriguing deviations from those determined on Chirasil-Val.

Notizen: Summary An on-line, solid-phase extraction gas chromatography atomic-emission detection (SPE-GC-AED) system has been set up using an on-column interface to transfer 100 μl of desorbing solvent to the GC part of the system. Analytical characteristics such as recovery, precision and linearity of calibration plots were comparable with those of the off-line combination of SPE-GC-AED using organophosphorus pesticides as test compounds. The fully on-line set-up causes a marked improvement in detection because of the quantitative transfer of the analytes from the SPE module to the GC: detection limits are as low as 5–20 ng l−1 for the analysis of 10 ml raw and spiked surface water samples using the phosphorus channel. Detection levels can be further enhanced by processing up to 100 ml samples. The integrated analytical system is robust. The potential of the on-line set up has been demonstrated for the analysis of surface water and waste water.

Notizen: Summary In order to enable the coupling of reversed-phase liquid chromatography (RPLC) with capillary gas chromatography (GC), the performance of an anion-exchange micromembrane device has been studied to remove the ion-pair reagent methanesulphonic acid from an acetonitrile/water LC eluent. The regenerant in the membrane was tetrabutylammonium hydroxide dissolved in acetonitrile/water, which effects an anion-exchange of methanesulphonate ions for regenerant hydroxide ions. The efficiency of the exchange process was found to be 99.9%. This enabled the direct introduction of the LC eluent, free of ions and with the proper acetonitrile/water ratio, into the GC. The applicability of the on-line LC-micromembrane-GC system has been illustrated for the potential drug eltoprazine, which is quantitatively recovered with a coefficient of variation for standard solutions of 3% at the 150 μg/ml analyte level.

Notizen: Summary An automated sample preparation module, (the automated sample preparation with extraction columns, ASPEC), was interfaced with a capillary gas chromatograph (GC) by means of an on-column interface. The system was optimised for the determination of the antidepressant trazodone in plasma. The clean-up of untreated plasma was carried out with disposable C18 cartridges, using several washing steps. The analyte was desorbed with methanol, the extract was diluted on-line with buffer and preconcentrated on a PLRP-S trapping column. The trapping column was dried by purging with nitrogen gas. Desorption (phase switching) of the trapping column was carried out with ethyl acetate which was introduced into a retention gap using partially concurrent solvent evaporation conditions. Trazodone was determined by GC with flame ionisation detection (FID). With a 1 ml sample, this resulted in a detection limit for trazodone in plasma of 3 ng/ml. The system showed good linearity and repeatability in the range 0.01–1 μg/ml, thus covering the range of pharmacokinetic/dynamic-to-therapeutic concentrations of trazodone in plasma. Preliminary results for benzodiazepines are promising. They indicate that the use of a selective detector such as the nitrogen/phosphorus detector, is to be recommended.

Notizen: Summary A number of oxygenated compounds such as C1−C4 alcohols, methyl tert-butyl ether (MTBE), and tertamyl methyl ether (TAME) are increasingly being used in gasoline as octane boosters and to reduce vehicular emissions. Monitoring of individual oxygenates in gasoline is important for quality control and regulatory purpose. Although, several analytical methods based on gas chromatography have been reported, a need was still felt to develop a simple and cost effective method of analysis. In this paper, a packed column gas chromatographic method is described for the analysis of C1−C4 alcohols, MTBE and TAME over a wide concentration range (2–20% V/V) in gasoline. The method involves determination of alcohols and ethers separately on two different columns. The alcohols are determined on a 20 ft column packed with poly(ethylene glycol) 400 as a stationary phase while the ethers are analysed on a 20 ft OPN-Poracil C column. The analysis of alcohols on poly(ethylene glycol) 400 column also provides data on aromatic components of gasoline.

Notizen: Summary A detailed statistical analysis, comparing the validity of the best literature recommended equations for prediction of retention indices of alkylbenzenes on squalane and methylsilicone stationary phases with the unified retention index concept, is presented. A comparison with literature sources shows that the unified retention index provides the smallest residual error. Hence, it can be applied for prediction purposes.

Notizen: Summary Kinetic parameters for the ring opening of cyclohexane over modified ZSM-5 zeolites can be measured simultaneously under non-steady state conditions using reversed-flow gas chromatography. At relatively low temperatures, the main product detected and measured was propene. The mathematical relations used for the calculation of adsorption-desorption rate constants, surface reaction rate constants and adsorption equilibrium constants for the reactant cyclohexane are different from previously studied catalytic reactions, owing to the different experimental arrangement for feeding the catalytic bed. The diffusion bands obtained experimentally for reactant or product are described by the sum of two or three exponential functions of time, respectively. From the exponential coefficients of time and the pre-exponential factors, all determined by simple PC programs, the above kinetic parameters are calculated.

Notizen: Summary The dependence of specific retention by a binary stationary phase in GC can be expressed as the sum of the products of the specific retention of the pure components times their respective volume fractions. In this study, however, one component has a site, which is not only mainly responsible for the selectivity, but also participates in strong self-association. This requires introduction of a concentration-dependent factor (μx) in the corresponding term of the equation correlating Vg mix x with x. In the GC resolution of N-trifluoroacetyl-amino acid isopropyl esters on a binary phase, N-lauroyl-L-amino t-butyl amide-squalane, data for the values of μx were obtained. Adapting a previously developed LC retention model to the above GC data, an equation was derived for the dependence of μx on the weight fraction (x) of the selector, namely μx=√1/x. This relationship permits calculation of retention volumes on the binary phase for a given x, as well as corresponding resolution coefficients of enantiomeric amino acid derivatives with generally excellent agreement with experiment. The chirality of the system is not relevant to application of the equation.

Notizen: Summary A new method of analysis using headspace gas chromatography with microcapillary columns is proposed. Small diameter (50 μm I.D.) fused-silica capillary columns with non-extractable SE-54 and PS-255 polysiloxane stationary phases were used for the analysis of low boiling organic compounds. The small diameter columns possess the usual very high efficiency so that the method can be employed for the headspace analysis of complex mixtures. The use of short microcolumns reduces the analysis times in comparison to conventional capillary columns. Good performances were obtained in the analysis of volatile compounds in some lemon essential oil, perfumes, and water samples.

Notizen: Summary The separation of some aromatic bicyclic and tricyclic spiro ketones on fused silica capillary columns coated with polydimethylsiloxane, cyanopropylmethylsiloxane and poly(ethylene glycol) stationary phases was investigated. Retention indices were determined at two temperatures in order to enable understanding of the compounds' chromatographic behaviour. The respective standard deviation values were 0.8, 0.5 and 0.3 index units. The influence of the polarity of the stationary phases on the chromatographic retention of these cyclic spiro ketones is discussed.

Notizen: Temperature programmable injectors with liner diameters ranging from 1 to 3.5 mm are evaluated and compared for solvent split injection of large volumes in capillary gas chromatography. The liner dimensions determine whether a large sample volume can be introduced rapidly or has to be introduced in a speed controlled manner. The effect of the injection technique used on the recovery of n-alkanes is evaluated. Furthermore the influence of the liner diameter on the occurrence of thermal degradation during splitless transfer to the analytical column is studied. Guidelines are given for the selection of the PTV liner internal diameter best suited for specific applications.

Notizen: Materials used in the manufacture of silicone breast prostheses were analyzed by supercritical fluid chromatography (SFC) and high temperature gas chromatography (GC) coupled with direct deposition FT-IR detection. The chromatographic resolution and deposition quality were found to be unsatisfactory for SFC/FT-IR; however, high temperature GC/FT-IR produced excellent results. The chromatograms and spectra obtained allowed for the identification of poly-dimethyl silicones and mono-, di-, and tri-diphenylsubstituted poly-dimethyl silicones. An automated method of baseline correction was developed that allowed functional group chromatograms due to molecules containing Si-phenyl bonds to be constructed without significant interference from Si-methyl bonds despite the fact that the Si-C stretching mode of Si-phenyl bands overlapped significantly with the strong bands due to Si-CH3 groups.

Notizen: The use of effective core potentials in the calculation of the geometrical parameters of the ferrocene molecule and its heavier analogs is reported. It is shown that a critical factor in these calculations is the efective core-core (ECC) potential and, in the absence of ECCs for first-row atoms that are involved in short bonds, calculations of the geometrical parameters are not reliable. Good agreement with experimental geometries may be obtained by using the Los Alamos ECPs for atoms of the second and higher rows of the periodic table at the MP2 level. DFT calculations have been performed and found to give numerical results comparable to MP2 in the same basis. © 1995 John Wiley & Sons, Inc.

Notizen: In this article, we report an efficient computational procedure for electron scattering matrix elements in the previously developed cubic-grid Gaussian basis sets. The Green function matrix elements derived for the cubic-grid basis set are simpler and easier to calculate than are those available in the literature for conventional Gaussian basis sets. Special features of the cubic-grid basis sets may also be exploited for a very efficient computation of Coulomb and exchange integrals. Inelastic scattering amplitudes for vibrational excitations may be efficiently calcualted in the harmonic approximation by numerical differention of the T-matrix elements. © 1995 John Wiley & Sons, Inc.

Notizen: Previously developed cubic-grid basis sets of various sizes were used for the calculation of cross sections for elastic and inelastic electron scattering by the He and Ne atoms and the H2O molecule by the T-matrix expansion method. The aim was to test the invariance of calculated cross sections with respect to the translation of the target molecule and to examine the effect of basis-set size on the results. We also present a simple procedure for accounting for long-range interactions from the part of space that lies outside the volume that contains the cubic-grid basis set. © 1995 John Wiley & Sons, Inc.

Notizen: Atomic charge and momentum densities of 91 atoms (He—U) are classified in terms of their L1, L2, and entropic measures of distance from the densities of the preceding atoms. The relationship between these distances and the first ionization energies is also considered. © 1995 John Wiley & Sons, Inc.

Notizen: The subject of this article is the self-consistent-field (SCF) treatment of low-lying molecular vibrations in molecules subject to solvent effects and light atom migration. The analyses use a Cartesian Gaussian basis and Gaussian functional expansions of potential energy operators. The objective of the work was to establish approximate and practical methods of analysis of vibrational degrees of freedom in molecules that build on and compare well with the highly accurate treatments of vibrations in small molecular systems of the past decade. An application to a system in which hydrogen bonding contributes the major anharmonic effect illustrates the method. © 1995 John Wiley & Sons, Inc.

Notizen: This article presents methods for computing matrix elements with Cartesian Gaussian wave functions of potential energy operators that depend on functions of the form (r-r0)n exp[-a(r - r0)] as well as matrix elements of the class of polynomial many-body potentials developed by Murrell et al. The matrix elements arise in the analyses of anharmonic vibrations in molecules. © 1995 John Wiley & Sons, Inc.

Notizen: The extension of the PPP Hamiltonian for alternant cyclic polyenes to noninteger values of the pseudomomentum by imposing modified boundary conditions is discussed in detail. It is shown that a computer program for periodic boundary conditions can be easily adapted to the new boundary conditions. Full CI computations are carried out for some low-lying states of the PPP model of altemant cyclic polyenes (CH)N, (N even) at half-filling. The energy values obtained by using periodic (Bloch) and antiperiodic (Möbius) orbitals are used to perform energy extrapolations for N → ∞. © 1995 John Wiley & Sons, Inc.

Notizen: General formalism for evaluation of multiparticle integrals involving J̌2 and J̌z operators over explicitly correlated Cartesian Gaussian functions is presented. The integrals are expressed in terms of the general overlap integrals. An explicitly correlated Cartesian Gaussian function is a product of spherical orbital Gaussian functions, powers of the Cartesian coordinates of the particle, and exponential Gaussian factors, which depend on interparticular distances. This development is relevant to both adiabatic and nonadiabatic calculations of energy and properties of multiparticle systems. © 1995 John Wiley & Sons, Inc.

Notizen: The molecular g-tensors for the molecules NO2, CO2-, C3H5, H2CO+, and NF3+ are calculated at the unrestricted Hartree-Fock (UHF) level using the Rayleigh-Schrödinger perturbation approach. All one-electron terms have been evaluated, including the relativistic mass correction, the one-electron spin-Zeeman gauge correction, and a second-order term involving the spin-orbit coupling and the orbital-Zeeman interaction. The relative importance of the first-order terms is found to be quite substantial. For first-row molecules, their total contribution is often of the same order of magnitude as of the second-order contribution, in some instances even larger. A study on the basis-set dependence of these g-tensors has been conducted. At this level, the most important basis-set criterion is shown to be a satisfactory description of the valence regions. Finally, the gauge dependence of these g-tensor calculations has been systematically studied. This dependence seems to be substantially reduced for basis sets which include polarization functions. © 1995 John Wiley & Sons, Inc.

Notizen: A scaling procedure is used for the force constants generated by the SINDO1 method in internal coordinates to achieve better agreement with experimental frequencies of molecules. The procedure is subsequently used to calculate frequency shifts for adsorbed molecules. The results for CO2 and CO adsorption at NaCl cluster surfaces are in good agreement with experimental data. © 1995 John Wiley & Sons, Inc.

Notizen: Ab initio self-consistent-field calculations are reported for electronic states of beryllium clusters comprised of 93, 105, 111, and 123 atoms. The respective clusters correspond to coordination shells 12-15 of a central Be atom with internuclear separations derived from the lattice constants of the bulk metal. Ab initio effective core potentials have been employed to replace the 1 s electrons, thereby reducing the complexity of the calculations. In addition, use of the full D3h point group symmetry of the clusters results in a substantial reduction of the numbers of two-electron integrals that must be computed and processed. Binding energies, orbital energies, electric field gradient, nuclear-electrostatic potential, diamagnetic shielding constant, second moments, and Mulliken populations are calculated for selected electronic states. Calculated binding energies when compared among the different clusters as well as to smaller and larger fragments from earlier studies provide evidence for the onset of convergence to the Hartree-Fock limit of the bulk. Lowest-state ionization potentials are consistently above and agree to within 14% of the experimental workfunction. The net charge on the central beryllium atom decreases toward zero. The variability of observed bulklike behavior for the different properties indicates that the transition between cluster and bulklike behavior is not sharp and depends on the quantity of interest. © 1995 John Wiley & Sons, Inc.

Notizen: The discrete variable representation method (DVR) is applied to the calculation of the J = 0 vibrational energy levels of the ground electronic state of nitrogen dioxide, a molecule which shows a large amplitude bending vibration. The Hamiltonian is expressed in Johnson hyperspherical coordinates and developed on a DVR basis set for each coordinate. A successive diagonalization-truncation method is applied which gives accurate values for the energy levels up to ≃ 7000 cm-1. © 1995 John Wiley & Sons, Inc.

Notizen: A procedure for finding transition states ( TS) that does not require the evaluation of the second derivatives during the search is proposed. The procedure is based on connecting a series of points representing products Pi and reactants Ri and taking conservative steps along the difference vector from Pi toward Ri and from Ri toward Pi until the two points coalesce. Although the points Po and Ro represent the product and reactant specifically, other Pi and Ri are determined by minimization in hyperplanes perpendicular to Pi -1 and Ri-1. We apply this technique to six well-known potential functions and compare these results with those obtained from other well-known procedures. © 1995 John Wiley & Sons, Inc.

Notizen: Potential energy curves for the ground and low-lying excited states of the AH2+ (A = Mg - Ar) dications have been calculated using high-level ab initio methods with large atomic orbital basis sets. Quasi-bound potential energy curves with local minima and deprotonation barriers have been found for most of the dications studied. The energies, tunneling lifetimes, and widths of the quasi-bound states have been calculated by numerical solution of the radial Schrodinger equation using the Numeov method. All these dications except ArH2+ have low-lying states which support quasi-bound vibrational states. The ArH2+ dication has a 2∏i potential energy curve with a minimum so shallow that it does not support any quasi-bound vibrational states. Results of our calculations are compared with previous ab initio calculations and available experimental data. © 1995 John Wiley & Sons, Inc.

Notizen: A bond-energy formula deduced by means of the Hellmann-Feynman theorem is applied to selected simple hydrocarbons. The required potentials at the nuclei are calculated with the help of large basis-set expansions including polarization functions. The carbon-carbon bond energy of ethane is evaluated at ∼ 70 kcal mol-1. The CC bond energies of ethane, ethylene, acetylene, benzene, and cyclopropane are approximately in a ratio of 1: 2.0: 3.0: 1.65 1.0. Limitations and possible improvements in future applications of this energy formula are discussed. © 1995 John Wiley & Sons, Inc.

Notizen: Ab initio molecular orbital (MO) calculations with the 3-21G and 6-31G basis sets are carried out on a series of complexes of NH3 with Li+, C≡N-, LiCN, and its isomer LiNC. The BSSE-corrected interaction energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies are evaluated for 15 species. Complexes with trifurcated (C3v) structures are calculated to be saddle points on the potential energy surfaces and have one imaginary frequency each. Calculated energies, geometrical parameters, internal force constants, and harmonic vibrational frequencies of the various species considered are discussed in terms of the nature of association of LiCN with ammonia. The vibrational frequencies of the relevant complexed species are compared with the experimental frequencies reported earlier for solutions of lithium cyanide in liquid ammonia. © 1995 John Wiley & Sons, Inc.

Notizen: When two electronic potentials present an avoided crossing, the adiabatic approximation breaks down in the energy region near to the crossing. In particular, the correspondence between exact energy levels of the two-state system and the adiabatic levels of the lower and upper adiabatic potentials becomes ambiguous. This implies that the term “nonadiabatic effect,” used for the difference between exact and adiabatic energy eigenvalues, loses its meaning in the crossing regime unless an unambiguous way of assigning an adiabatic to an exact level is defined. This is important in order to investigate where nonadiabatic schemes, such as the generator coordinate approximation, fit in between the adiabatic approximation and quasi-exact approaches. © 1995 John Wiley & Sons, Inc.

Notizen: The application of the Σ-separation method to the calculation of multicenter two-electron molecular integrals with Slater-type basis functions is reported. The approach is based on the approximation of a scalar component of the two-center atomic density by a two-center expansion over Slater-type functions. A least-squares fit was used to determine the coefficients of the expansion. The angular multipliers of the atomic density were treated exactly. It is shown that this approach can serve as a sufficiently accurate and fast algorithm for the calculation of multicenter two-electron molecular integrals with Slater-type basis functions. © 1995 John Wiley & Sons, Inc.

Notizen: The intersystem crossing (ISC) between the lowest triplet and singlet states occurring in the reaction of atomic oxygen with ethylene was studied. The importance of spin-orbit coupling (SOC) in oxirane biradicals (ĊR′R″ - CRR* - Ȯ) is stressed through calculations where the spin-orbit matrix elements over the full Breit-Pauli SOC operator has been obtained in the singlet-triplet crossing region. The calculations are performed with a multiconfigurational linear response approach, in which the spin-orbit couplings are obtained from triplet response functions using differently correlated singlet-reference-state wave functions. Computational results confirm earlier semiempirical predictions of the spin-orbit coupling as an important mechanism behind the ring opening of oxiranes and addition of oxygen O(3P) atoms to alkenes. © 1995 John Wiley & Sons, Inc.

Notizen: We consider a crystal as partitioned into a localized molecular cluster (containing a defect or not) and an embedding region. Within the Hartree-Fock formalism, an expression is derived for an effective potential due to the embedding region of crystal. This potential is part of the cluster Fock operator and requires input from a perfect crystal calculation. Special features of the derivative are rigorous inclusion of cluster-embedding overlap and orthogonality between single-electron states of the embedding region and the function-space manifold of the cluster; physically correct normalization of the Fock eigenstates; and a nontrivial total-energy algorithm. Computational requirements are qualitatively compared with those for an isolated cluster. The method allows for intracluster (and intraembedding) correlation and can be adapted straightforwardly to local density functional approaches. Fundamental aspects of the embedding problem are addressed in a general formulation that is, nevertheless, oriented toward explicit calculations. © 1995 John Wiley & Sons, Inc.

Notizen: An efficient procedure for third-order electron propagator calculations of ionization energies and electron affinities is reported. Diagonal self-energy expressions that are suitable for large molecules are empolyed. The outer-valence Green's function method also is implemented. An integral transformation program for direct and semidirect algorithms is modified to store only nonzero integrals according to Abelian point group symmetry. Contributions to self-energy matrix elements that depend on electron repulsion integrals with four virtual orbital indices are computed in a direct way. Intermediate batches of integrals are created by sort procedures while avoiding storage of transformed integrals in the main memory. This method permits calculation of electron binding energies for C72- with a 231 atomic orbital basis and for Zn(C5H5)2 with a 220 atomic orbital basis on an IBM RISC/6000 Model 550. During these calculations, the CPU is engaged approximately 90% of the time. © 1995 John Wiley & Sons, Inc.

Notizen: The intermolecular potentials for the X 2σ and A2Π states of Li… Ar were studied by a variety of multiconfiguration, single-configuration, and perturbation methods (CASPT2). The A 2Π excited state was calculated to have a well depth of 811 cm-1 at an internuclear separation of 2.59 Å, in excellent agreement with the 810 cm-1 derived from experimental data. A smaller well of 77 cm-1 was found for the X 2σ ground state at an intermolecular separation of 4.8 Å. These results are in better agreement with experimental results than were the previously reported pseudopotential calculations. The comparison of CI calculation with the CAPST2 results shows that the latter is able to give good results for interacting metal-rare gas systems. © 1995 John Wiley & Sons, Inc.

Notizen: This article is a brief review of the work formalism of electronic structure, its recent developments, and the results of its application to spherically symmetric and nonspherical density atoms. The formalism, which is founded in Schrödinger theory, is derived by physical arguments based on Coulomb's law. The fundamental quantity in the formalism is the pair-correlation density that constitutes the nonlocal quantum-mechanical source charge distribution giving rise to both a local potential representing electron correlations as well as the electron interaction energy. The potential is the work done to move an electron in the force field of the pair-correlation density and the energy the interaction energy between the electronic and pair-correlation densities. (For systems for which the curl of the force field may not vanish, the potential is obtained from the irrotational component of the field, the solenoidal component being neglected). The differential equation governing the system is a Sturm-Liouville equation, and as such, the exact wave function can in principle be obtained as an infinite linear combination of Slater determinants of the self-consistently determined spin-orbitals of the occupied and virtual states. The correctness of the interpretation for the local potential representing electron interaction is evidenced as follows: In the Pauli-correlated and central field approximations, ground-state energies of atoms (2 He -86Rn) lie within 50 ppm of those of Hartree-Fock theory, differing by less than 10 ppm for atoms with Z 〉 35. The densities thus generated clearly exhibit atomic shell structure and also satisfy the Kato-Steiner electron-nucleus cusp condition to 2 ppm. Another attribute of the formalism is that the asymptotic structure of the potential (when both Pauli and Coulomb correlations are considered) is that of the Pauli-correlated approximation. This is rigorously the case as shown for the He atom for which the potential vanishes in the classically forbidden region, the potential there being the exchange potential. As such, it is meaningful to compare the highest occupied eigenvalue of the differential equation in the Pauli-correlated approximation to experiment. A comparison for atoms and atomic ions of this eigenvalue to experimental ionization potentials and electron affinities show them to be consistently superior to the corresponding eigenvalue of Hartree-Fock theory. Transition energies determined from eigenvalue differences are also superior to those obtained from total energy calculations via Hartree-Fock theory when compared to experiment. Further, by considering the carbon atom in one of its degenerate ground states for which the curl of the field due to the Fermi hole does not vanish, it is shown that the solenoidal component of the field is negligible and two orders of magnitude smaller than is the irrotational component. Thus, the approximation of obtaining a path-independent potential for nonspherical density systems from the irrotational component of the field is accurate. Finally, Coulomb correlation effects can be incorporated within the work formalism in practice via the configuration interaction approximation. The self-consistent orbitals thus obtained explicitly incorporate the effects of both Pauli and Coulomb correlations in their structure because the source charge from which they are generated is a pair-correlation density. Furthermore, these orbitals possess the correct asymptotic structure since they are also generated by a potential that is local. The work formalism also provides a physical interpretation for the local potential representing electron correlations of Kohn-Sham density functional theory. Further, the exchange potential of the work formalism satisfies analytically two requisite conditions of the Kohn-Sham theory exchange potential. These are the scaling requirement and the sum rule relating the exchange energy to its functional derivative. The work formalism also leads to a deeper understanding of electron correlations in various approximations within Kohn-Sham theory. For example, it can be rigorously shown that the pair-correlation density in the local density approximation contains a term proportional to the gradient of the density. Thus, in contrast to the Kohn-Sham theory interpretation that electron correlations in this approximation are those of the uniform electron gas assumed valid locally, we learn that the nonuniformity of the electronic density is, in fact, explicitly accounted for by the approximation. This then explains the accuracy of the approximation. © 1995 John Wiley & Sons, Inc.

Notizen: The aziridine analogues of the epoxide metabolites of carcinogenic polycyclic aromatic hydrocarbons have greater mutagenic potency than the epoxides. Like their well-studied analogues, the aziridines undergo a pH-dependent decomposition that leads to a reactive carbocation intermediate. In aqueous solution the nucleophile with which the carbocation generally reacts is water. The kinetics of this pH-dependent reaction have been experimentally characterized by others. Although the effect of DNA on this reaction has not been studied, we hypothesize that, like their epoxide analogues, the aziridine derivatives of polycyclic aromatic hydrocarbons undergo a DNA-catalyzed reaction leading through a carbocation to either a DNA-adduct or a hydrolysis product. Using Poisson-Boltzmann calculations in conjunction with Metropolis Monte Carlo simulations and energy-minimized conformations, we predict the DNA-dependence of the acidcatalyzed decomposition of the K-region aziridine, phenanthrene[9,10]imine. © 1995 John Wiley & Sons, Inc.

Notizen: A QSAR analysis of a series of Valproic Acid (vpa) derivatives is given, which shows the importance of hydrophobic and electronic effects as determinants of the anticonvulsivant activity. The statistical analysis allows one to infer that the electron acceptor capability of the carboxylic carbon atom may guide electrostatic interactions of the molecules with the receptor site, in those cases where the lipophilic requirements are satisfied. Both the anticonvulsivant activity and the calculated lipophilic parameters (log P values) are taken from the literature, whereas the electronic descriptors result from Intermediate Neglect of Differential Overlap calculations at the Configuration Interaction level, (INDO/S-CI parametrization), for the most stable conformers of each derivative. The protein environment is modeled as a dielectric continuum in a Self-Consistent Reaction Field approach. The conformational analysis is based on AMI calculations. © 1995 John Wiley & Sons, Inc.

Notizen: A new form of second-order multireference perturbation theory coupled with finite-field perturbation theory is applied to evaluate some one-electron molecular properties. Several possible definitions of the zeroth-order Hamiltonian are considered and results tested against bench-mark full CI calculations. © 1995 John Wiley & Sons, Inc.

Notizen: The correlation present in the nondegenerate ground state of an interacting Fermi system is discussed in terms of reduced density matrices and their cumulant expansion. By generalizing a result obtained for the interacting uniform electron gas (correlation induced exchange-hole narrowing), possible measures of the correlation strength in terms of natural occupation numbers (the eigenvalues of the true one-particle density matrix) are introduced. These quantities-the v-order nonidempotency and the information entropy of the natural occupation numbers-result from the correlated many-body wave function and characterize the ground-state correlation in addition to the usual correlation energy. The uniform electron gas serves as a first illustrative example. © 1995 John Wiley & Sons, Inc.

Notizen: In the present work, we reexamined the gradient expansion of the exchange energy of an electron gas with a slowly varying charge density. We stay within the exchange-only approximation of Sharp, Horton, Talman, and Shadwick but go to second order in the deviation from the homogeneous limit. The coefficient of the lowest-order gradient correction is obtained analytically both for a bare and a screened Coulomb interaction - the former yielding the value previously obtained by Kleinman numerically and by Engel and Vosko analytically. A screened Coulomb interaction gives Sham's coefficient in the limit of infinite screening length. The cause of the difference between the coefficients of Kleinman and Sham is clearly exhibited. The coefficients of the two next highest-order gradient corrections, one of which originates in second-order response theory, is shown to diverge as the screening length becomes large. The bare Coulomb interaction gives finite coefficients of which the one originating from linear response is obtained analytically and differs from the presumably correct result obtained by Engel and Vosko. This discrepancy demonstrates the extreme sensitivity of the analytical expressions to different regularization procedures. We suggest that coefficients should rather be chosen according to the performance of the resulting gradient approximations in weakly perturbed electron gases. © 1995 John Wiley & Sons, Inc.

Notizen: The focal point of the present work is the single-particle kinetic energy density tensor in D dimensions. This quantity enters both differential and various integral forms of the virial theorems, which are again set up in D dimensions. Major new results lie in (i) demonstrating that, by one-dimensional quadrature, it is possible to construct the Pauli potential directly from the kinetic energy tensor, without the need for functional differentiation and (ii) generating the gradient expansion for the kinetic energy tensor, in D dimensions. © 1995 John Wiley & Sons, Inc.

Notizen: The geometries of the two most important tautomeric forms of adenine and the corresponding methyladenines are investigated by means of quantum chemical calculations at different level of sophistication, ranging from semiempirical methods to correlation corrected ab initio methods at second-order Møller-Plesset perturbation theory level (MP2). The relative stability of the N(7)H ↔ N(9)H tautomeric forms of adenine are investigated with highly correlation corrected methods, MP3 and MP4. The relative stability is also corrected for solvent interactions and compared with experimental information. N(9)H-adenine is predicted to be the most stable tautomer in both vacuum and in solution. The relative stability is predicted to be between 24.5 and 35.0 kJ/mol in vacuum depending on computation method. In water solution N(7)H-adenine is stabilized more by the solvent, and the corresponding relative energies were found to be between 4.9 and 10.2 kJ/mol. We also found that correlation effects are essential to describe the ground state geometry with a high accuracy. The geometries predicted by semiempirical methods and ab initio calculations without correlation correction show large deviations in some parts of the molecule compared to the MP2 results as well as compared with experimental geometries. © 1995 John Wiley & Sons, Inc.

Notizen: Ab Initio crystal orbital calculations on three-dimensional crystals/crystallohydrates of a number of diprotonated mononucleotides have been performed using the CRYSTAL92 routine package. The present results help to gain a deeper insight into the physical mechanisms of nucleic acid semiconductivity, as well as into the essence of intermolecular interactions and solvent effects in solid-state samples of nucleic acids. © 1995 John Wiley & Sons, Inc.

Notizen: Experimentally measured rates for the oxidation of p-substituted benzyl amines by bovine monoamine oxidase type B (MAO-B) derived from the literature were examined with respect to the effects of molecular (semiempirically (AM1) derived) electronic, steric, and lipophilicity parameters. These properties included vertical and adiabatic ionization potential, LUMO energy, the LUMO-HOMO difference, molecular hardness, absolute electronegativity, calculated log P values, molecular volume, surface area, and ovality. Substrate oxidation rates (log kcat/Km) were found to correlate with molecular ovality and vertical ionization potential while the rate of enzymatic (flavin) reduction associated with substrate oxidation (log kred) was described by a two-parameter model containing an ovality and an absolute electronegativity term. These results are consistent with an initial one-electron substrate oxidation mechanism. In previous work, use of classical Hansch analysis suggested that electronic terms were not important in the enzymatic reactions. This discrepancy may be related to nontransferability inherent in fragment approaches which assume that the substituent of interest behaves similarly in all molecular scaffolds. Analysis of substrate binding (log Kd) to the enzyme was described by a two-parameter model containing a calculated log P term as well as LUMO energy. The significant correlation found with LUMO energy is consistent with studies suggesting that this property is important for drug-receptor interactions. © 1995 John Wiley & Sons, Inc.

Notizen: The integrated molecular transform (FTm) is a unitary structure index that has been successfully used for the correlation of 2- and 3-dimensional structure representations with their physicochemical and pharmacological properties. In the present instance the reported pKa values in a series of 30 compounds consisting of five subseries were correlated with their FTm indices. The omission of four outliers gave a moderate correlation across the entire series; within the individual subseries the correlations were considerably improved even with inclusion of the outliers. In general, the logarithmic transform of the pKa did not improve the correlations. This method gives a relatively simple means of estimating pKa in several structure classes. © 1995 John Wiley & Sons, Inc.

Notizen: We investigated Raman and infrared spectra of the Watson-Crick type of the guanine cytosine base pair and of the individual guanine and cytosine nucleic acid bases by ab initio Hartree-Fock theory using the 6-31G* basis set. IR and Raman intensities and Raman depolarization ratios were predicted using the double-harmonic approximation. The effects of a polar solvent were modeled by the self-consistent reaction field (SCRF) approximation. Variations in geometries, harmonic force constants, and vibrational spectra of the studied nucleobases due to the specific hydrogen-bonding interactions are discussed. © 1995 John Wiley & Sons, Inc.

Notizen: We present here a perturbative analysis of the coupled cluster response method for molecular static properties with Euler and extended coupled cluster functionals under cubic truncation. Comparative analysis is meant to cater to both pedagogical and practical interests. Comprehensive tables for energy-derivative expressions and equations at the stationary point are presented. © 1995 John Wiley & Sons, Inc.

Notizen: The ground-state potential energy curve of the F2 molecule as well as spectroscopic constants were calculated by means of the second-order quasi-degenerate many-body perturbation theory within a full (eight) valence orbital space using a DZP basis set. The problem encountered with a large number of valence electrons is avoided by a proper redefinition of the Fermi vacuum. A comparison with other related multireference techniques is also provided. © 1995 John Wiley & Sons, Inc.

Notizen: Correlation holes of electrons with the same (Fermi hole) and different (Coulomb hole) spins in the ground (X1Σ+), first (A1Σ+) and second (B1II) excited states of LiH were constructed from full configuration interaction (CI) wave functions. It was found that the shapes of both the Fermi and Coulomb holes in these states are dependent on the location of the reference electron. When the reference electron is chosen to be close to the Li nucleus, the Fermi correlation results in a large negative hole for all three states. However, the A1Σ+ excited state is further characterized by displaying a second hole around the H nucleus, and in the B1II state, the hole is elongated along the molecular axis. Coulomb correlation shows up strongly in the A1Σ+ state and, in addition, there is clearly correlation of electrons at the two nuclei. These features of the correlation holes were compared with those from a two-Slater-determinant model wave function. The Hartree, Fermi, and Coulomb screening potentials in these states were also studied in the light of possible modeling of the correlation functionals for the excited states. © 1995 John Wiley & Sons, Inc.

Notizen: The link between a uniform scaling of the electron density and a scaling of the electron-electron interaction is reviewed. The effective potential along the coupling constant path which connects a noninteracting and a fully interacting system with the same electron density is considered. The effective potential for an arbitrary coupling constant is here expressed in terms of the exchange-correlation potential at a coupling constant of unity. The effective potential is then investigated for ionization processes. Use of the fact that the ionization energy is determined by the exponential decay of the electron density allows us to derive new formulas for the ionization energy. Based on the Taylor expansion of the effective potential along the coupling constant path, a density functional perturbation theory is introduced which leads to a formally exact Kohn-Sham KS formalism. To first order, this formalism gives identities for the exchange potential in terms of KS orbitals and orbital eigenvalues. Moreover, higher-order terms give identities for the correlation potential as well as for the exchange potential. These identities are pointwise as well as integrated. Hence, various new requirements for the exchange and correlation functionals are derived. New insight into the optimized effective potential method is gained by discussing it in the light of the results obtained here. © 1995 John Wiley & Sons, Inc.

Notizen: Available approximations to Exc[{nσ}], the exchange-correlation energy functional of spin-density-functional theory, do not include self-interaction corrections (SIC). This leads to Kohn-Sham (KS) potentials, Vxcσ, that fail to satisfy some important analytic properties known to be exhibited by the exact potential. To resolve these difficulties, we consider a KS theory for orbital-dependent exchange-correlation energy functionals that explicitly includes SIC. Recent work by Krieger, Li, and lafrate (KLI), which considers the analytic properties of the spin-polarized optimized effective potentials (OEP), Vxcσ0, i.e., the KS potentials corresponding to Exc = Exc[{φiσ,}], is reviewed as well as the properties of VxcσKLI, an easily calculated approximation to the exact result which, unlike Vxcσ0, can be employed for systems of arbitrary symmetry. In addition, we compare the results of the exact and approximate OEP calculation of the properties of the ground state of atoms and singly charged negative ions in the exchange-only case in which Ex = ExHF [{φiσ}] where HF = Hartree-Fock. We conclude that VxσKLI maintains most of the important analytic properties of Vxσ0, and provides an excellent numerical approximation to the exact result. We also give detailed consideration to the calculation of the ionization potential, I, and the electron affinity, A, in the exchange-only approximation for atoms with Z ≤ 20. We find that the KLI results for both I and A are always within 0.1 milli-au of the exact KS results, whereas both the local spin density (LSD) approximation and the Becke exchange only energy functional lead to deviations which on average are two orders of magnitude larger and significantly exceed the criterion for quantum chemistry accuracy. Finally, using the KLI method for orbital dependent Exc, we compare the KS results for I and A for Z ≤ 20 with the experimental values by employing various approximations for Exc[{φiσ] including: (1) HF exchange with LSD correlation with SIC, (2) LSD approximation for exchange and correlation with SIC, (3) the conventional spin density LSD approximation, and (4) the Becke exchange-energy functional with LSD correlation. In addition, we examine how closely the ionization theorem for ∊m, the energy eigenvalue for the highest occupied orbital, is satisfied in these approximations. © 1995 John Wiley & Sons, Inc.

Notizen: First-principles local density functional approximation (LDA) calculations using the linear-response method are more efficient than are traditional supercell or dielectric matrix techniques for determining phonon spectra and dielectric constants. We have implemented this approach using a linearized-augmented-plane-wave (LAPW) basis, which facilitates such calculations for systems containing transition-metal and other atoms with localized orbitals. The accuracy of the method is illustrated with applications to some semiconductors and ferroelectrics. Theoretical work on the perovskite ferroelectrics has focused on the possible roles of disorder and soft-phonon behavior. A complete mapping in the Brillouin zone (BZ) of the ferroelectric instability of KNbO3 has been carried out, revealing a pronounced two-dimensional character. In real space, this instability corresponds to chains oriented along 〈100〉 directions, of displaced Nb atoms. Such instabilities are discussed in relation to the static chain structures in the eight-site order-disorder model introduced by Comes et al. © 1995 John Wiley & Sons, Inc.

Notizen: Electrostatic and dispersion contributions of solute-solvent interactions have been considered in the self-consistent reaction field scheme and implemented in the LCGTO-DF (linear combination of Gaussian-type orbitals-density functional) method. Results for the tautomeric equilibrium of formamide-formamidic acid, for the cis-trans energy difference in dichlorodiammineplatinum(II), and for H2O—HF hydrogen-bond systems are in agreement with the available experimental and previous high-level ab initio data. The role of the dispersion energy is discussed for the different studied systems. © 1995 John Wiley & Sons, Inc.

Notizen: Local (LSD) and nonlocal (NLSD) spin density calculations using different exchangecorrelation functionals have been performed to determine equilibrium geometries, harmonic vibrational frequencies (ωe), ionization potentials (IP), electron affinities (EA), dipole moments (μ), and singlet-triplet energy gaps (Δ EST) of SiH2, GeH2, and SnH2. Geometrical structures as well as vibrational frequencies are in agreement with the available experimental data and compare favorably with the most sophisticated postHartree-Fock computations performed until now. Both computed IPS (9.15 and 9.25 eV for SiH2 and GeH2, respectively) and EA of SiH2 (1.17 eV) compare favorably with experimental data (9.17, 9.21, and 1.2 eV). Accurate values are obtained also for singlet-triplet energy gaps. We report for the first time the electron affinities of all neutral systems and the spectroscopic constants of the cations and anions. © 1995 John Wiley & Sons, Inc.

Notizen: The coupled clusters singles and doubles (CCSD) method for calculations of open-shell systems with the single restricted Hartree-Fock (ROHF) reference determinant is extended by the noniterative triples to give CCSD(T). Our approach profits from the fact that (a) single- and double-excitation amplitudes are spin-adapted, which directly leads to a computationally less demanding algorithm than are nonadapted procedures and produces the spin-adapted CCSD wave function and (b) triple excitations calculated from converged spin-adapted (SA) CCSD amplitudes are also obtained more effectively. Altogether, computer demands of our SA CCSD(T) approach, applicable to high-spin open-shell cases which are well represented by a single-determinant reference is comparable to that for closed-shell systems. Our approach is not based on semicanonical orbitals, applied by Bartlett's group. However, we compare some other possible choices of ROHF orbitals to this “standard.” Numerical results for a series of atoms and molecules demonstrate little sensitivity to this selection. © 1995 John Wiley & Sons, Inc.

Notizen: Ab initio accurate all-electron relativistic molecular orbital Dirac-Fock self-consistent field calculations are reported for the linear symmetric XeF2 molecule at various internuclear distances with our recently developed relativistic universal Gaussian basis set. The nonrelativistic limit Hartree-Fock calculations were also performed for XeF2 at various internuclear distances. The relativistic correction to the electronic energy of XeF2 was calculated as ∼ -215 hartrees (-5850 eV) by using the Dirac-Fock method. The dominant magnetic part of the Breit interaction correction to the nonrelativistic interelectron Coulomb repulsion was included in our calculations by both the Dirac-Fock-Breit self-consistent field and perturbation methods. The calculated Breit correction is ∼6.5 hartrees (177 eV) for XeF2. The relativistic Dirac-Fock as well as the nonrelativistic HF wave functions predict XeF2 to be unbound, due to neglect of electron correlation effects. These effects were incorporated for XeF2 by using various ab initio post Hartree-Fock methods. The calculated dissociation energy obtained using the MP2(full) method with our extensive basis set of 313 primitive Gaussians that included d and f polarization functions on Xe and F is 2.77 eV, whereas the experimental dissociation energy is 2.78 eV. The calculated correlation energy is ∼ -2 hartrees (-54 eV) at the predicted internuclear distance of 1.986 Å, which is in excellent agreement with the experimental Xe - F distance of 1.979 Å in XeF2. In summary, electron correlation effects must be included in accurate ab initio calculations since it has been shown here that their inclusion is crucial for obtaining theoretical dissociation energy (De) close to experimental value for XeF2. Furthermore, relativistic effects have been shown to make an extremely significant contribution to the total energy and orbital binding energies of XeF2. © 1995 John Wiley & Sons, Inc.

Notizen: Based on the EHMO approach, the band structures for the Y—Ba—Cu—O superconductors doped by La were calculated. The influence of the partial substitutions of La for Y and Ba in YBa2CU3Oy on its electronic structures was investigated. The results demonstrate that the La doping at the Ba site has a great effect on the electronic structures of the Y—Ba—Cu—O superconductors, whereas the change in the band structures caused by the La doping at the Y site is very small. The increase in the oxygen content caused by the La doping results in an increase in the densities of states at Ef, N(Ef), for La1+x Ba2-xCu3Oy, but the increase in N(Ef) cannot compensate the decrease caused by the La doping at the Ba site. In addition, the 2D Cu—O planes are much more sensitive to the change in N(Ef) than are the 1D Cu—O ribbons, which implies an important role of the 2D Cu—0 planes in the Y—Ba—Cu—O superconducting system, regardless of whether La substitutes for Y or for Ba. © 1995 John Wiley & Sons, Inc.

Notizen: The proof of Parr and Chattaraj for the Principle of Maximum Chemical Hardness is very general and can be applied to many more observables. The case of the physical, or mechanical, hardness of a solid is taken as an example. It is shown that this also should be a maximum in an equilibrium system. Assuming the validity of the argument leads to new information about the compressibility of solids. A simple expression is also given for the Gruneisen constant. These maximum principles are necessary consequences of the equilibrium conditions of quantum mechanics and thermodynamics. © 1995 John Wiley & Sons, Inc.

Notizen: The field theoretical background of relativistic density functional theory is emphasized and its consequences for relativistic Kohn-Sham equations are shown. The local density approximation for the exchange energy functional is reviewed and the importance of relativistic corrections for an accurate representation of the exchange functional is demonstrated. © 1995 John Wiley & Sons, Inc.

Notizen: The dynamics of Jahn-Teller systems has recently been discussed in terms of generalized electronic charge and current densities in nuclear-coordinate space. The introduction of the electronic phase as a function of both electronic and nuclear coordinates, in addition to the electronic density, was a crucial component of this formulation. Here, a densitybased treatment of Born couplings is derived from first-principles quantum mechanics beyond the Born-Oppenheimer approximation. Because of the degenerate electronic configuration of a Jahn-Teller molecule, there are an infinite number of ways in which the charge distribution can be oriented for the same energy, leading to a vanishing bond hardness for the molecule in the symmetric nuclear configuration. Further, the moving nuclear framework serves as the perturbation necessary to define the orientation of the charge density, leading to unhindered rotation of the charge cloud. This leads to the dynamical Jahn-Teller problem, namely, the coupling of electronic and nuclear motions through the Born coupling terms. Applications to superconductivity theory are discussed. © 1995 John Wiley & Sons, Inc.

Notizen: It is possible to reformulate the reaction field (RF) model of continuum solvent effects, by considering an approximate expression describing the energy changes from one ground state to another, in the frame of density functional theory (DFT). The energy functional for an arbitrary electronic system coupled to a spin-independent electrostatic external perturbation is used to derive the well-known Born expression giving the electrostatic component of the solvation energy of an atomic ion. The approximate RF-DFT model is illustrated for a series of representative singly positive and negatively charged atomic ions. A Kohn-Sham (KS)-like formalism is then proposed to compute solvation energies within a self-consistent field scheme. The extension of the RF-DFT model to molecular systems is also outlined. © 1995 John Wiley & Sons, Inc.

Notizen: Recent reactivity concepts formulated within charge analysis (CSA) are outlined. The charge stability criteria of equilibrium states in open and closed systems are conveniently characterized in terms of the condensed reactant hardness quantities of reactants; their implications for catatytic systems are examined. A use of characteristics associated with selected collective charge displacement modes, including the populational normal modes and minimum-energy coordinates, as diagnostic tools in the theory of chemical reactivity is proposed. The importance of the mapping relations between modes defined in the electron population and nuclear position spaces, respectively, as the unifying concept linking the conjugate charge and geometry displacements, is commented upon. Recent results for model catalytic clusters are used to illustrate some of the concepts introduced. Finally, the relevant contributions to the quadratic interaction energy between reactants are reexamined and expressed in terms of relevant charge sensitivities. © 1995 John Wiley & Sons, Inc.

Notizen: The Boltzmann-Shannon (BS) information entropy Sρ = ∫ ρ(r)log ρ(r)dr measures the spread or extent of the one-electron density ρ(r), which is the basic variable of the density function theory of the many electron systems. This quantity cannot be analytically computed, not even for simple quantum mechanical systems such as, e.g., the harmonic oscillator (HO) and the hydrogen atom (HA) in arbitrary excited states. Here, we first review (i) the present knowledge and open problems in the analytical determination of the BS entropies for the HO and HA systems in both position and momentum spaces and (ii) the known rigorous lower and upper bounds to the position and momentum BS entropies of many-electron systems in terms of the radial expectation values in the corresponding space. Then, we find general inequalities which relate the BS entropies and various density functionals. Particular cases of these results are rigorous relationships of the BS entropies and some relevant density functionals (e.g., the Thomas-Fermi kinetic energy, the Dirac-Slater exchange energy, the average electron density) for finite many-electron systems. © 1995 John Wiley & Sons, Inc.

Notizen: The local density approximation (LDA) to exchange and correlation effects has well-known limitations. The nonlocal weighted density approximation (WDA) corrects some of those defects. This is illustrated here by applications to free atoms and small atomic clusters. The WDA also induces a nonlocal kinetic energy functional that is tested for atoms. © 1995 John Wiley & Sons, Inc.

Notizen: We present a density functional scheme for calculating the frequency-dependent linear response of superconductors. The central result is a set of integral equations determining the linear response of the normal and anomalous densities to external perturbations. Analytic solutions of these integral equations are obtained for homogeneous systems with separable effective interactions. For inhomogeneous superconductors, the formalism leads to a scheme for calculating the critical temperature without explicitly solving the gap equation. © 1995 John Wiley & Sons, Inc.

Notizen: The ensemble N-representability problem for the k-th order reduced density matrix (k-RDM) as well as the problem of reconstruction of the N-particle system density matrices (N-DM) from a given k-RDM are studied. The spatial parts of the k-RDM expansion in terms of spin tensorial operators Θλk are represented using particular values (at specially chosen ) of the Radon transform of the N-DM spatial parts (or their sums) DNλ(x′ | x″) (here, is a d-plane in the n-space ∝n of x = (x′, x″)), with n = 6N, d = 3 (N - k), x′ ≡ (r′1,⃛, r′N), x′ ≡ (r1″,⃛, r″N ()). In this way, the problem is reduced to investigation of the properties of the functions . For a normalizable N - DM, it is proved that are bounded functions. The properties of implied by the N-DM permutational symmetry, Hermiticity, and positive definiteness are found. A formal procedure of reconstruction of all N-DM corresponding to a given k-RDM is proposed. © 1995 John Wiley & Sons, Inc.

Notizen: The π-allylic organonickel compounds are considered to be key structures in the catalytic reactions of butadiene. Bis(η3-allyl)nickel, bis(η3-methallyl)nickel, and the syn-crotyl-cyclooctadiene-nickel cation have been calculated with DZ and TZ basis sets in all-electron and pseudopotential (ECP) Hartree-Fock (HF) methods and compared with experimental structures. In the second part, we report a systematic investigation of these compounds by density functional theory (DFT). The DFT-optimized structures are generally in better agreement with experimental data than are the HF results. © 1995 John Wiley & Sons, Inc.