A comparison of proton‐detected ¹³C local field experiments with deuterium NMR at natural abundance for studying liquid crystals

Abstract

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.

Keywords:

• proton‐detected ¹³C local field experiment
• deuterium NMR at natural abundance

Acknowledgment

This work is supported in part by a grant from PRIN 2005 (Italian MUIR).