Vibrational contributions to vicinal proton–proton coupling constants³J_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 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.

Keywords:

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

Acknowledgements

ALE and MPG gratefully acknowledge support from the Dirección General de Investigación of Spain (grant CTQ2006-15141-C02-01). RHC gratefully acknowledges support from UBACYT (X047) and CONICET (PIP 5119/05).