Vibrational contributions to vicinal proton–proton coupling constants3 J HH

Abstract

Vibrational contributions to the couplings of six mono- and five 1,1-di-substituted ethanes, three mono-substituted cyclohexanes, three norbornane-type molecules, and 11 three-membered 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 Qk , required to estimate the vibrational contributions, have been obtained from six Jk values computed for molecular geometries positively and negatively displaced from the equilibrium geometry along the normal coordinate Qk 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.