Measurement and assignment of long-range C-H dipolar couplings in liquid crystals by two-dimensional NMR spectroscopy

Journal Article · Thu Aug 29 04:00:00 EDT 1996 · Journal of Physical Chemistry

DOI:https://doi.org/10.1021/jp960972m· OSTI ID:380768

Hong, M; Pines, A ^[1]; Caldarelli, S ^[2]

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1. Lawrence Berkeley National Lab., CA (United States)
2. BCH-Dorigny, Lausanne (Switzerland)

We describe multidimensional NMR techniques to measure and assign {sup 13}C-{sup 1}H dipolar couplings in nematic liquid crystals with high resolution. In particular, dipolar couplings between aromatic and aliphatic sites are extracted, providing valuable information on the structural correlations between these two components of thermotropic liquid crystal molecules. The NMR techniques are demonstrated on 4-pentyl-4`-biphenylcarbonitrile (5CB), a well-characterized room-temperature nematic liquid crystal. Proton-detected local-field NMR spectroscopy is employed to obtain highly resolved C-H dipolar couplings that are separated according to the chemical shifts of the carbon sites. Each {sup 13}C cross section in the 2D spectra exhibits several doublet splittings, with the largest one resulting from the directly bonded C-H coupling. The smaller splittings originate from the long-range C-H dipolar couplings and can be assigned qualitatively by a chemical shift heteronuclear correlation (HETCOR) experiment. The HETCOR experiment incorporates a mixing period for proton spin diffusion to occur, so that maximal polarization transfer can be achieved between the unbonded {sup 13}C and {sup 1}H nuclei. To assign the long-range C-H couplings quantitatively. we combined these two techniques into a novel reduced-3D experiment, in which the {sup 1}H chemical shift-displaced C-H dipolar couplings are correlated with the {sup 13}C chemical shifts. 34 refs., 6 figs., 2 tabs.

• DOE Contract Number: AC03-76SF00098
• OSTI ID: 380768
• Journal Information: Journal of Physical Chemistry, Journal Name: Journal of Physical Chemistry Journal Issue: 35 Vol. 100; ISSN JPCHAX; ISSN 0022-3654
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CHEMICAL SHIFT
COUPLING
LIQUID CRYSTALS
MEASURING METHODS
NMR SPECTRA
ORGANIC COMPOUNDS
STRUCTURAL CHEMICAL ANALYSIS