L. Calucci et al., DYNAMICS OF LIQUID-CRYSTALS BY MEANS OF H-2-NMR - A COMPARISON BETWEEN 4,4'-BIS(HEXYLOXY)AZOXYBENZENE AND THE DERIVATIVE PD(II) COMPLEX AZPAC, Zeitschrift fur Naturforschung. A, A journal of physical sciences, 53(6-7), 1998, pp. 427-435
In the present work we report a molecular dynamics investigation of th
e two mesogens 4,4'bis-(hexyloxy)azoxybenzene (HL) and its Pd(II) acet
ylacetonate derivative Azpac in their nematic phases. Deuterium Zeeman
and quadrupolar spin-lattice relaxation times have been measured at 4
6.04 MHz on two isotopomers of Azpac (Azpac-d(4) and Azpac-d(26), deut
eriated on the aromatic core and on the alkoxy chains, respectively) a
nd on HL-d(4), an isotopomer of HL partially deuteriated on the aromat
ic core, by means of the Wimperis pulse sequence. The spectral densiti
es obtained from the measured relaxation times are discussed in terms
of internal and overall molecular motions. A small step rotational dif
fusion model for the overall molecular motions, superimposed on a free
rotational model for internal motions, allowed diffusion coefficients
for molecular spinning and tumbling and for phenyl ring rotations to
be derived for HL; the same models were used in describing the dynamic
s of Azpac.