Vibrational contributions to vicinal protonproton coupling constants^{3} J _{HH}
Abstract
Vibrational contributions to the ? couplings of six mono and five 1,1disubstituted ethanes, three monosubstituted cyclohexanes, three norbornanetype molecules, and 11 threemembered rings have been calculated at the DFT/B3LYP level for the Fermi contact term using a moderate sized basis set. When, for a data set of 70 couplings, the sums of the values for the equilibrium configurations J _{e} and the respective vibrational contributions ? at 300 K are multiplied by a factor of 0.8485, the corresponding predicted couplings ? are in good agreement with the experimental couplings ? with a standard deviation σ of 0.18 Hz. The same σ results when ? values are obtained by multiplying the J _{e} values by 0.9016. However, the vibrational contributions must be taken into account, together with the J _{e} values, in order to achieve a procedure for a reliable and accurate prediction of ? couplings since, globally, ? contributions amount to about 7% of the J _{e} values and the correlation coefficient between ? and J _{e} is only 0.68 with a σ deviation of 0.20. The first and diagonal second derivatives of J with respect to each normal coordinate Q_{k} , required to estimate the vibrational contributions, have been obtained from six J_{k} values computed for molecular geometries positively ? and negatively ? displaced from the equilibrium geometry along the normal coordinate Q_{k} and using for δ the values 0.01, 0.05 and 0.10. The computational precision of the results obtained when using one, two and three δ values is analysed.
 Publication:

Molecular Physics
 Pub Date:
 March 2010
 DOI:
 10.1080/00268971003630687
 Bibcode:
 2010MolPh.108..583E
 Keywords:

 vicinal coupling constants;
 vibrational contributions;
 numerical precision;
 DFT/B3LYP