Rotational tunneling dynamics of methyl groups measured by spectral hole burning

Rotational tunneling of methyl groups has been studied extensively in the past decades [1]. The specific features of the dynamics of rotational tunneling are a consequence of the basic laws of quantum mechanics. The reason is that a 2π3-rotation of a methyl group is equivalent to a two-fold permutation of identical particels. As a consequence, the symmetry of the total wave function of the methyl group has to be invariant with respect to a rotation. This symmetry correlation has a dramatic influence on the relaxation dynamics between different rotational states. To observe a rotational tunneling relaxation an interaction is necessary, which breaks the strict symmetry correlation. For the CH3-methyl group this interaction is the nuclear spin-spin coupling and for the CD3-methyl group it is the nuclear quadrupole interaction of the deuteron with the electric field gradient. The magnitude of this interaction is so small that the respective relaxation times can be as long as months at low temperatures. Because of the influence of the nucleus involved in the rotational tunneling dynamics the relaxation of the rotational states is called nuclear spin conversion. Theoretical models have been published for the protonated [2, 3] as well as for the deuterated methyl group [4]. They basically predict three different kinds of relaxation processes (Direct, Raman and Orbach process) depending on the rotor parameters which are the V3-potential, the tunneling splitting and the phonon coupling. These, in turn depend on isotopic substitution.