The nonexponential character of the Zeeman relaxation of reorienting methyl groups in solids

Abstract

Articles you may be interested inMethyl group rotation, 1H spin-lattice relaxation in an organic solid, and the analysis of nonexponential relaxation J. Chem. Phys. 136, 054508 (2012); 10.1063/1.3677183 Solid state proton spin relaxation and tbutyl and methyl group reorientation in 1bromo2,4,6tritbutylbenzene J. Chem. Phys. 95, 4778 (1991); 10.1063/1.461720 Methyl reorientation in methylphenanthrenes. I. Solid state proton spin-lattice relaxation in the 3methyl, 9methyl, and 3,9dimethyl systems J. Chem. Phys. 87, 20 (1987); 10.1063/1.453617 Spin-lattice relaxation of reorienting or tunneling deuterated methyl groups J. Chem. Phys. 73, 172 (1980); 10.1063/1.439903 Proton spinlattice relaxation studies of reorienting methyl groups in solidsIf the rapid reorientations of methyl groups in solids are the dominant relaxation mechanisms, it is theoretically found that the Zeeman system is dynamically coupled to the so-called rotational polarization system, resulting in a nonexponential Zeeman relaxation, Though this result has been confirmed experimentally, in many samples within measuring error an exponential Zeeman relaxation has been observed, In order to decide whether or not this is contradictory to the theory the degree of nonexponentiality has been further investigated in this paper. It is shown that the nonexponential character depends stronly on the following parameters: (i) the way in which the spin system is prepared for the observation of the Zeeman relaxation, (ii) the number of protons not belonging to the methyl groups, (iii) the relaxation due to the intermethyl dipolar interactions, and (iv) the temperature. It is found that under specific conditions the observed relaxation can become almost exponential. The corresponding time constant, however, can differ considerably from that predicted by the usual spin temperature theory. Therefore, the interpretation of the experiments with this theory can lead to incorrect values of the different parameters characterizing the relaxation process. The various results have been illustrated with experiments on polycrystalline samples of 2,6-dimethylphenol, methylcyanide, 2methylpiperazine, and 1,4-dimethoxy-2,6-dimethylbenzene.

Investigation of Methyl Reorientations in 1,4-Dimethoxy- 2,6-dimethylbenzene by NMR and Inelastic Neutron Scattering

“…(ii) It has been shown extensively in [5] that for (O0TO < 1 the observed non-exponential relaxation can be used to determine the contribution of the inter-methyl relaxation to the total relaxation. This method has been applied.…”

“…(i) In case of non-exponential relaxation the initial rate value of the curve In (Mo -Mz)/Mo versus t (Mz is the magnetization and Mo its thermal equilibrium value) has to be taken in order to obtain the value of the Zeeman autorelaxation rate Az [5]. This has as a consequence that the accuracy of the values becomes rather poor: 10 to 15% of its value (a more accurate procedure would be obtained by rotating the sample externally because then the non-exponentiality has disappeared [6], but to perform this as a function of the temperature was not possible at the time the experiments were carried out).…”

“…Until recently this phenomenon had not been recognized, and the measurements were interpreted by the traditional relaxation theory, where complete spin diffusion was assumed. It has been shown [5] that this may lead to incorrect values of the correlation time. For an extensive treatment we refer to the references [6][7][8][9][10].…”

“…pulse, e.g. a 90° pulse (for other preparation methods the observed relaxation may become different [5,6]). In this case Az is equal to the initial rate value of the observed non-exponential Zeeman relaxation.…”