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  • 1
    Electronic Resource
    Electronic Resource
    A basis set study of the NMR chemical shift in PH3 (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 2814-2820 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Chemical shift calculations varying the heavy atom valence shell basis functions have been carried out for PH3 as a prototype molecule containing second row atoms. A triple valence split with two sets of d polarization functions, (66211/6211/11)=[5s,4p,2d], appears to be adequate for second-row atoms in the GIAO-SCF approach. Chemical shift calculations on SiH4, H2S, and HCl as well as PH3 employing this basis yield results in good agreement with experiment and with calculations of others involving larger bases. Bond length and bond angle shift derivatives in these hydrides are negative, continuing the general trend observed before for molecules containing first-row atoms.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451039
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Nuclear magnetic resonance chemical shifts using optimized geometries (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 6238-6245 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Isotropic chemical shifts and shift anisotropies for carbon, nitrogen, oxygen, and fluorine in first-row-atom molecules have been calculated in the perturbed Hartree–Fock gauge including atomic orbital scheme for both experimental and optimized molecular structures using a 6-311G* basis for heavy atoms and a scaled 4-31G basis for hydrogen. Structure optimization leads to the expected shortening of bond lengths, which is accompanied by an increase in the isotropic chemical shifts. The increased shifts show much improved agreement with gas phase experimental values for nitrogen and oxygen, while the results for carbon are only mildly affected and remain good; shift anisotropies for all species tend to decrease in magnitude and also generally improve. Fluorine is anomalous, its increasing shifts upon structure optimization moving further away from experiment at this level of basis set. The trend in the optimized isotropic shifts is explained in terms of the general tendency for atoms in the right-hand portion of the first row of the periodic table to have negative shift derivatives with bond extension.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457390
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    Paper (German National Licenses)
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  • 3
    Electronic Resource
    Electronic Resource
    Heisenberg spin exchange effects in powder spectra of weakly perturbed triplet states (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 5495-5500 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Heisenberg spin exchange effects on the powder spectra of weakly perturbed triplet states have been studied by simulation techniques. From very small exchange frequencies on, the derivative powder spectrum is dominated by a sharp, essentially constant width, central line whose amplitude exhibits very close to a square-root dependence on the exchange frequency. The derivative powder edge lines are shown to behave according to the single crystal limiting exchange width equation and it is shown that the frequency dependence of the central dominating line provides yet another measure of exchange parameters in physical systems. Application of these methods to the (φ3AsCH+3) (TCNQ)−2 ion radical salt yields results in good agreement with those obtained by other methods.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449670
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  • 4
    Electronic Resource
    Electronic Resource
    Chemical shifts and bond modification effects for some small first-row-atom molecules (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 852-861 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Chemical shifts and shift changes with bond modification have been calculated for some small first-row-atom molecules employing a (6311/311/1)=[4s, 3p, d] heavy atom, (31)=[2s] hydrogen basis in the GIAO approach. Shifts and shift anisotropies using this intermediate size basis agree as well with experiment as other, more extensive basis sets; heavy atom shifts are determined reasonably well on an absolute basis and hydrogen shifts are acceptable on a relative scale. Nearly all shift first derivatives are negative with respect to bond lengthening as are the second derivative terms. These results suggest that in most cases a negative temperature coefficient for the chemical shifts of the heavy atoms will be observed, an effect which in fact does obtain in most cases known experimentally.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450529
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    Paper (German National Licenses)
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  • 5
    Electronic Resource
    Electronic Resource
    Modified MINDO/3 13C chemical shift calculations for simple hydrocarbons (1988)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 9 (1988), S. 416-423 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Calculations of 13C chemical shifts in some simple hydrocarbons have been carried out using the GIAO approach in the MINDO/3 semiempirical formalism. In order to achieve reasonable agreement with experiment it is necessary to modify (increase) the vacant orbital energies in the MINDO/3 calculation in order to reduce the magnitude of the paramagnetic contribution, and to also modify this dominant term by generally reducing it as a function of the number of hydrogen and carbon atoms bonded to the resonant nucleus in question. For a set of 34 resonant nuclei of the simpler hydrocarbons, agreement with experiment of the order of 7.8 ppm is attained; however, pathological cases such as cyclopropane and some simple allenes continue to cause problems, increasing the standard deviation of the full set to 12.5 ppm. Our results indicate that the MINDO/3 approach is as viable for 13C chemical shift calculations as other semiempirical approaches, all of which seem currently to be limited to a standard deviation of the order of 10 ppm.
    Additional Material: 2 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540090417
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    Paper (German National Licenses)
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  • 6
    Electronic Resource
    Electronic Resource
    Locally dense basis sets for chemical shift calculations (1989)
    New York, NY [u.a.] : Wiley-Blackwell
    Journal of Computational Chemistry 10 (1989), S. 648-659 
    Details
    ISSN: 0192-8651
    Keywords: Computational Chemistry and Molecular Modeling ; Biochemistry
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology , Computer Science
    Notes: Calculations of chemical shifts have been carried out using “locally dense” basis sets for the resonant atom of interest, and smaller, attenuated sets on other atoms in the molecule. For carbon, calculations involving a 6-311G(d) triply split valence set with polarization on the resonant atom and 3-21G atomic bases on other heavy atoms result in good agreement with experiment, and are virtually identical to those found employing the larger basis on all atoms. For species such as nitrogen, oxygen, and fluorine where standard balanced basis sets do not agree well with experiment, use of attenuated sets fail as well. The use of locally dense basis sets permits calculations previously impractical, and the successful application to carbon suggests that the chemical shift is most dependent on the local basis set, and less so on whether or not a balanced or unbalanced calculation is being carried out.
    Additional Material: 5 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/jcc.540100507
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  • 7
    Electronic Resource
    Electronic Resource
    Intramolecular van der waals interactions and chemical shifts: A model for β- and γ-effects (1985)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 23 (1985), S. 625-638 
    Details
    ISSN: 0749-1581
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: A correlation has been discovered between the local steric van der Waals energy as calculated by the MM2 force field and the chemical shift of a resonant nucleus within classes of saturated aliphatic phosphines, amines, hydrocarbons and monofunctional alcohols. Root-mean-square errors found for 37 phosphorus, 35 nitrogen and 164 carbon nuclei are 10.9, 5.9 and 2.8 ppm, respectively. It is found that repulsive van der Waals interactions are associated with deshielding effects in the observed chemical shifts, and attractive interactions with smaller shielding effects. A simple physical picture emerges which describes both β- and γ-effects under one common model. It is found that β-effects are associated with repulsive interactions while, contrary to earlier approaches γ-effects are due basically to attractive interactions.
    Additional Material: 7 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260230807
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    Paper (German National Licenses)
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  • 8
    Electronic Resource
    Electronic Resource
    Intramolecular van der Waals interactions and 13C chemical shifts: Substituent effects in some cyclic and bicyclic systems (1986)
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 24 (1986), S. 93-100 
    Details
    ISSN: 0749-1581
    Keywords: Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: In recent work, a reasonably good linear correlation was found to exist between the local van der Waals steric energy as calculated by the MM2 force field and the chemical shift of the resonant nucleus within classes of saturated aliphatic phosphines, amines, hydrocarbons and monofunctional alcohols. This study extends this work to some additional cyclic systems: 91 trans- and 31 cis-decalin carbons, 57 bicyclo[2.2.1]heptane and 27 bicyclo[2.2.2]octane nuclei have been investigated and yield root-mean-square errors of 3.3-3.7 ppm and correlation parameters in good agreement with those found previously for other carbon species. In the more strained bicyclo systems it is found that additional class parameters must be defined. The theory is applied to a variety of α,β,γ,… ubstituents effects in cyclohexane and decalin systems, and correlated these resonant shifts under one simple, physical model.
    Additional Material: 4 Ill.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1002/mrc.1260240202
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