DYNAMIC POLARIZATION AND RELAXATION OF METHYL GROUP SPINS IN RARE-GAS MATRIX ISOLATED MOLECULES

Solid state NMR techniques have been used to investigate methyl groups in selected molecules isolated in rare gas matrices. These molecules CH$\sb3$-R, where -R is a linear substituent, were chosen for the absence of internal steric hindering to the rotation of the methyl group. Moreover, this rotation is almost completely unrestricted by the rare gas lattice. Since the spin and rotational states available to the methyl group are symmetry coupled according to the Pauli Exclusion principle, characteristics of the rotational system in the free rotation limit could be determined from spin relaxation experiments. Among the consequences of coupled spin and rotational relaxation is the dynamic polarization of dipolar spin order following a sudden change in the matrix temperature. The rate constants for this process, referred to as the Haupt effect, were determined in several matrices as a function of temperature. Well-resolved dipolar resonance lines were obtained by virtue of magnetic isolation and permitted anisotropies in the polarization process to be observed for the first time. This resolution also contributed to the discovery of additional polarizations, namely those associated with heteronuclear dipolar and spin-rotation interactions. The lineshape associated with heteronuclear dipolar magnetization was explained by including the orientational dependence of the polarization amplitude. Measurement of the temperature dependent width of the spin-rotation resonance line led to a solid state determination of the z component of the spin-rotation coupling tensor for Kr-isolated CH$\sb3$CN. In experiments carried out at fixed temperatures, transverse relaxation of methyl protons was investigated by comparisons of experimental and calculated responses to several Rf pulse sequences. The quantum beating observed in the FIDs and spin echoes is very different depending on the spacing of the pulses. These experiments were extended to include higher order coherences, observed in the form of coherence transfer echoes. The decay of these coherences was attributed to static intermethyl dipolar coupling in the matrix. Finally, Zeeman relaxation was measured as a function of temperature to determine potential energy barriers to methyl group rotation in the matrix. The results indicate that a distribution of barrier heights is involved, associated with the variation of trapping sites in the matrix