Search

Very accurate bond distances can be derived from dipolar couplings, Dij, 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 Dij is tested on benzene for which a very accurate structural study is available. [ABSTRACT FROM AUTHOR]

The non-polar rigid molecules 1,4-difluorobenzene and p-benzoquinone dissolved in ZLI1132 and EBBA nematic solvents show a bizarre ‘double face’ rod-like or disk-like orientational behaviour, depending on the used solvent. In the present Letter it is shown that the observed effect, unpredictable on the basis of solely short-range orientating mechanisms, can be qualitatively accounted when also electrostatic long-range interactions are taken into account (in particular, the coupling between the solute quadrupole moment and the solvent electric field gradient). The obtained result confirms the non-negligible role played by electrostatic interactions, even in the case of non-polar molecules. [Copyright &y& Elsevier]

A comparison is made between the information which can be derived on structure, conformation and orientational order of the molecules in a nematic liquid crystal 4-pentyl-4'-cyanobiphenyl (5CB) from the NMR spectra of the deuterium atoms at natural abundance (NAD NMR), and the two-dimensional proton-detected 13C local field experiment (PDLF). The nine residual quadrupolar splittings, Δν k , obtained experimentally have been compared with quadrupolar tensors and a geometry and conformational potentials calculated by the DFT method B3LYP/6-311G**. The PDLF experiment yielded 42 scaled 13C-1H residual dipolar couplings, kD CiHj . The scaling factor, k, is determined experimentally by comparing unscaled and scaled residual dipolar couplings in a sample of fluorobenzene dissolved in a nematic liquid crystalline solvent. The corrected residual dipolar couplings, D CiHj , are used to investigate the structure and rotational potentials about each bond in the molecule. [ABSTRACT FROM AUTHOR]