Calculated Versus “Experimental” Force Fields: The Influence in the Structure Determination of Benzene by NMR Spectroscopy in Liquid Crystal Solvents

Abstract

Very accurate bond distances can be derived from dipolar couplings, D _ij , obtained by analysis of the NMR spectra of samples dissolved in anisotropic solvents (LXNMR). To do this, however, the couplings must be corrected for the averaging produced by vibrational motion. The vibrational corrections require a knowledge of the force field, which may be obtained by fitting experimental vibrational frequencies or, much more easily, from quantum-mechanical computations. In the present study, the reliability of the methods used to calculate the force field and the effects on the accuracy of the vibrational corrections to the D _ij is tested on benzene for which a very accurate structural study is available.

Keywords:

• ¹³C satellite spectra
• liquid crystals
• NMR
• vibrational corrections

Acknowledgments

This work has been supported by MIUR PRIN ex 40%. The authors are grateful to Prof. J.W. Emsley for stimulating discussions.

Notes

†Theoretical number of lines for benzene.

‡Approximated number of calculated lines.

†Δ(a/b) = {[(r _a /r _b ) _measured  − (r _a /r _b ) _hexagon ]/(r _a /r _b ) _hexagon }*10⁴ as in [2].

‡From: *, “Experimental”; **, B3LYP Force Field