Proton Magnetic Resonance in Nematic Solvents: Orientation and Structure of Anthraquinone Derivatives and a Linewidth Analysis of the Benzene Spectrum.
The PMR spectra of 1,5 and 1,8 Dichloroanthraquinone (ClAQ) were recorded in nematic solvents (EBBA and MBBA). Computer simulated theoretical spectra were obtained from the static spin Hamiltonian. Chemical shift and dipolar couplings were adapted in an iterative process. Molecular structure and orientation were derived from the dipolar couplings. The results can be summarized as follows: (1) The PMR interproton distance ratios are in agreement with X-ray data within experimental error limits. (2) The planes of the molecules orient preferably parallel to the nematic director. (3) The principal values of the in plane order parameters for 1,5 ClAQ are 0.274 and 0.014; for 1,8 ClAQ they are 0.131 and 0.110. The great difference between the order parameters in 1,5 ClAQ cannot be explained on the basis of geometrical anisotropy of the molecule. From comparison with other available data, this peculiarity is attributed to the steric interactions between Cl and O atoms. In the PMR spectrum of partially oriented Benzene, variations of linewidth from 0.7 to 2.6 HZ have been observed. This selective line broadening is analyzed on the basis of intramolecular and intermolecular dipolar relaxation mechanisms. The relaxation matrix was calculated in the framework of the Redfield theory. It was assumed that motion of the Benzene molecule around its C(,6) axis is fast in comparison to other molecular motions. Two computer subroutines (INTEG and RELAX) were linked to the existing program (LAOCOONOR) for numerical calculations of relaxation elements. The calculations show that intermolecular dipolar interactions of the spins provide a dominating relaxation mechanism. For a satisfactory fit with the experimental spectrum it was, however, necessary to include the zero frequency terms of the intramolecular interactions. They are of some importance because of nematic order fluctuations. On the assumption that the mean square magnitude of the local field fluctuations is 3 Gauss('2), we find a correlation time of 10('-10) sec. for the random local field.
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- Physics: Molecular