Proton NMR relaxation of the dipolar quasiinvariants of nematic PAAd6 within the high-temperature Redfield relaxation theory

H. H. SEGNORILE; L. BARBERIS; C. E. GONZÁLEZ; R. C. ZAMAR

Facultad de Matemática, Astronomía, y Física - Universidad Nacional de Córdoba

Workshop; Magnetic Resonance in a Cordubensis Perspectiv; 2006

Facultad de Matemática, Astronomía, y Física - Universidad Nacional de Córdoba

We use the high temperature Redfield spin-lattice relaxation theory to study the relaxation properties of the NMR  dipolar quasiinvariants of the 8-proton system corresponding to the thermotropic nematic methyl deuterated para-azoxyanisole (PAAd6).According with previous experiments, this system can be considered as composed of weakly coupled pairs of strongly interacting spins, the ortho protons of the aromatic rings. Thus, it possesses four quasiinvariants of the motion: The Zeeman, dipolar intrapair and interpair, and singlet orders.We calculate the  relaxation constants in terms of spectral densities of the lattice motions. The calculated relaxation constants show that the Zeeman and singlet quasiinvariants are uncoupled from the dipolar ones, and that the relaxation time of  the singlet order is much longer than the Zeeman and dipolar relaxation times. We also obtain  a small value for the spin-lattice cross relaxation rate  between the dipolar orders, which is not large enough to deviate the time dependence of the dipolar inverse temperatures from a single exponential trend, within the range of a few relaxation times.We find that the nonsecular term associated with  the ODF dominates relaxation of the intrapair and interpair energies in PAAd6, while the local motions do not play a significant role, in qualitative agreement  with the reported experimental behavior. However,  the predicted dipolar relaxation times are significantly larger than the experimental ones, the difference being even more pronounced for the interpair quasiinvariant.  This striking feature reinforces previous indications that the usual high-temperature Redfield relaxation theory underestimates the role of the collective fluctuations.