Vibrational contributions to the vicinal proton-proton coupling constan ts 3-J(HH).

A. L. ESTEBAN, E. DÍEZ, M. P. GALACHE, J. S. FABIAN, J. CASANUEVA, R. H. CONTRERAS

MOLECULAR PHYSICS

TAYLOR & FRANCIS LTD

Año: 2010 vol. 108 p. 583 - 595

0026-8976

Vibrational contributions to the 3JHH 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 Je and the respective vibrational contributions J300 vib at 300K are multiplied by a factor of 0.8485, the corresponding predicted couplings Jpre are in good agreement with the experimental couplings Jexp with a standard deviation of 0.18 Hz. The same results when Jpre values are obtained by multiplying the Je values by 0.9016. However, the vibrational contributions must be taken into account, together with the Je values, in order to achieve a procedure for a reliable and accurate prediction of 3JHH couplings since, globally, J300 vib contributions amount to about 7% of the Je values and the correlation coefficient between J300 vib and Je is only 0.68 with a 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 ðþQkÞ and negatively  ðQkÞ displaced from the equilibrium geometry along the normal coordinate Qk and using for  the values 0.01, 0.05 and 0.10. The computational precision of the results obtained when using one, two and three  values is analysed.