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  • 1
    Digitale Medien
    Digitale Medien
    Erratum: Accurate ab initio quartic force fields for the ions HCO+ and HOC+ [J. Chem. Phys. 99, 286 (1993)] (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 9326-9326 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.466241
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  • 2
    Digitale Medien
    Digitale Medien
    Accurate ab initio quartic force fields for the ions HCO+ and HOC+ (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 286-292 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The quartic force fields of HCO+ and HOC+ have been computed using augmented coupled cluster methods and basis sets of spdf and spdfg quality. Calculations on HCN, CO, and N2 have been performed to assist in calibrating the computed results. Going from an spdf to an spdfg basis shortens triple bonds by about 0.004 A(ring), and increases the corresponding harmonic frequency by 10–20 cm−1, leaving bond distances about 0.003 A(ring) too long and triple bond stretching frequencies about 5 cm−1 too low. Accurate estimates for the bond distances, fundamental frequencies, and thermochemical quantities are given. HOC+ lies 37.8±0.5 kcal/mol (0 K) above HCO+; the classical barrier height for proton exchange is 76.7±1.0 kcal/mol.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.465806
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  • 3
    Digitale Medien
    Digitale Medien
    First principles computation of thermochemical properties beyond the harmonic approximation. II. Application to the amino radical NH2 (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 3530-3536 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The anharmonic potential surface of NH2 has been computed ab initio using the quadratic configuration interaction method and large basis sets. The all-electron augmented coupled cluster surface with the [5s4p2d1f,3s2p] basis set reproduces the experimental fundamentals to within a few cm−1. For this quantitative agreement with experiment, both f functions and core correlation seem to be prerequisite. From our best computed force field, a set of spectroscopic constants has been derived for all important isotopomers of NH2. Using a hybrid analytic/direct summation method recently developed by the present authors, the thermodynamic functions gef(T), hcf(T), S0, and Cp are computed including exact accounts for anharmonicity and rovibrational coupling, and very good analytical approximations to centrifugal distortion and quantum rotation effects. Thermodynamic tables in JANAF style from 100–3000 K, as well as a full set of rovibrational spectroscopic constants, are presented for NH2 and all important isotopomers. The effect of the A 2A1 state is found to become significant around 2000 K.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462987
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  • 4
    Digitale Medien
    Digitale Medien
    First principles computation of thermochemical properties beyond the harmonic approximation. I. Method and application to the water molecule and its isotopomers (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 96 (1992), S. 7633-7645 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The anharmonic potential energy surface of water has been computed ab initio using an augmented coupled cluster method and various basis sets. Whereas the Pople 6–311 G family is manifestly unsatisfactory, Huzinaga–Dunning basis sets perform quite well. The [5s4p2d1f,3s2p] surface reproduces harmonic frequencies and anharmonicity constants to better than about 2 and 1 cm−1, respectively. For quantitative agreement with experiment, both f functions on oxygen and inclusion of core correlation seem to be prerequisite. Comparison with various experimentally derived force fields reveals that the ab initio force field is of comparable quality. From the best computed force field, a set of spectroscopic constants has been derived for all important isotopomers of water. Using a hybrid analytic/direct summation method recently developed by the present authors, the thermodynamic functions gef(T), hcf(T), S0, and Cp are computed including exact account of anharmonicity and rovibrational coupling, and very good analytical approximations to centrifugal distortion and quantum rotation effects. The computed functions substantially revise previous literature results at high temperatures. Differences between thermodynamic functions from various computed force fields are an order of magnitude smaller than these errors. Thermodynamic tables in JANAF style from 100 to 3000 K, as well as a full set of rovibrational spectroscopic constants, are presented. It is concluded that obtaining force fields of near-spectroscopic accuracy, and thermodynamic tables of very high accuracy, is presently feasible for small polyatomic molecules.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.462364
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  • 5
    Digitale Medien
    Digitale Medien
    On the heat of formation of C5 and higher carbon clusters (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 9420-9421 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The heat of formation of C5 has been computed ab initio using a variation on G1 theory. The computed ΔH0f = 251.7 ± 4 kcal/mol disagrees quite appreciably with the third-law measurements of Drowart et al.; however, the cause of this is shown to be in the use of very approximate free energy functions by these authors. Using better free energy functions leads to a revised experimental value of 253.3±5.8 kcal/mol, in excellent agreement with our ab initio value. A revision of previous estimates for C6 to C10 by Raghavachari and Binkley is presented.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461173
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  • 6
    Digitale Medien
    Digitale Medien
    On the effect of centrifugal stretching on the rotational partition function of an asymmetric top (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 8374-8389 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Correction terms (up to third order in temperature) for the effect of centrifugal distortion on the rotational partition function of linear molecules, spherical, symmetric, and asymmetric tops are evaluated by means of the classical partition function. It is shown that for the linear, spherical, and symmetric cases, the expressions thus obtained differ from the exact quantum expressions only in the absence of a very small constant correction term. It is then proposed that the partition function for a nonrigid asymmetric top, for which no exact expression has as yet been derived, be evaluated as the product of Watson's asymptotic expansion for the rigid rotor and the centrifugal correction factor derived in the present work. Numerical comparison with direct numerical summation shows that, even for troublesome cases, this approximation holds very well even at 2000 K. Similar performance is observed for the heat content function, except for pathological cases at high temperatures. In the rigid-rotor case, Watson's asymptotic series holds very well. As an example application, pilot calculations have been performed on the thermodynamic functions of water and formaldehyde. The present method yields functions in excellent agreement with those obtained by direct rovibrational summation, whereas computer-time requirements are reduced by 3 or 4 orders of magnitude.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.461265
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  • 7
    Digitale Medien
    Digitale Medien
    Ab initio study of the structure, infrared spectra, and heat of formation of C4 (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 3753-3761 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The lowest-lying 1Ag and 3Σ−g states of C4 have been studied using basis sets including f functions, quadratic configuration interaction, and full fourth-order Møller–Plesset perturbation theory with multiple spin projection. Basis set effects on the linear-cyclic separation have been found to be fairly significant. Use of the 6-311G* and [5s3p1d] basis sets leads to qualitatively different conclusions; this problem is remedied by expansion of the polarization space. At the highest level of theory considered, the cyclic structure lies about 1 kcal/mol below the linear structure. A G1-type estimate of its heat of formation at 0 K of 249.6 kcal/mol is proposed. This value is in disagreement with the generally quoted experimental values, but in good agreement with third-law values by Drowart et al. [J. Chem. Phys. 31, 1131 (1959)] corrected for improved partition functions. Geometries and harmonic frequencies are reported. It is tentatively suggested that a matrix ir band at 1284 cm−1 may belong to cyclic C4; to aid experimentalists in confirming or rejecting this assignment, theoretical isotope shifts are reported.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.459747
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  • 8
    Digitale Medien
    Digitale Medien
    On the geometrical structure of the C+3 cation—an ab initio study (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5037-5045 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The potential energy surface of the C+3 cation has been investigated using coupled cluster techniques and large basis sets. The results are particularly sensitive towards the level of electron correlation. Spin contamination even produces a "false stationary point'' at the UHF/6-31G* level. C+3 has a cyclic 2B2 ground state with predicted geometry r=1.3242 A(ring), θ=73.06° (MP2/6-311G*, empirically corrected bond distance). At the highest level of theory considered, the linear structure (2Σ+u state) lies about 2 kcal/mol above the ground state: this might imply quasilinearity. There is also a low barrier towards degenerate isomerization: at high temperatures, C+3 will be extremely floppy. Harmonic frequencies (UHF/6-31G*) as well as double-harmonic IR and Raman intensities are given for various structures of C+3. Interesting analogies of C+3 with B3 and B2N are pointed out. The heat of formation at 298.15 K, vertical and adiabatic ionization potentials of C3 are predicted as 194.9±2 kcal/mol, 11.92±0.1 eV, and 11.84±0.1 eV, respectively.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.458641
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  • 9
    Digitale Medien
    Digitale Medien
    Ab initio study of the infrared spectra of linear Cn clusters (n=6–9) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 8850-8861 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The harmonic frequencies, infrared intensities, and isotopic substitution bands of C6 have been evaluated at the MP2/6-31G* level. They confirm previous assignments of the 1952 and 1197 cm−1 matrix IR bands to C6. Isotopic substitution spectra at the MP2/6-31G* and HF/4-21G levels, as well as those with a newly developed ad hoc modified neglect of diatomic overlap (MNDO) method are shown to be in good agreement with experiment for C4, C5, and C6. On the basis of RHF/4-21G, RHF/6-31G*, MP2/4-21G, MP2/6-31G*, and ad hoc MNDO data, including theoretical frequencies, IR intensities, and isotopic substitution spectra, it is shown with virtual certainty that the 2128 and 1893 cm−1 matrix IR bands belong to C7. A recent assignment of the very intense 1997 cm−1 matrix IR feature to C8 is contradicted on the same grounds, and an alternative assignment to C9 put forward.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.459224
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  • 10
    Digitale Medien
    Digitale Medien
    The dissociation energy of N3 (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 4485-4487 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The dissociation energy of N3 has been computed ab initio using coupled cluster techniques and large basis sets. Our best theoretical estimates (accuracy ±2 kcal/mol) are ∑De=234.0, ∑D0=228.4, ΔH0 f =109.3, and ΔH0 f,298=108.4 kcal/mol. These values are in excellent agreement with an ICR determination of ΔH0 f=112±5 kcal/mol. Very large basis set effects are observed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.458686
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