Methyl group rotational tunneling in vibrational spectra of crystals at low temperatures

The effect of methyl group nuclear spin conversion on the Raman spectra is reported for two crystals: 2,6-dibromomesitylene and 4-methylpyridine (4MP). In both crystals the tunneling splitting is large and population changes of the tunneling sublevels upon cooling to 4.2 K are easily observable as changes of intensity and lineshape in the Raman spectra of the CH3 torsional modes and of modes coupled to these. The spin conversion time at 4.2 K is 350±30 min for DBM and 170±20 min for 4MP. The intensity ratio of the lowest frequency Raman transitions of the two spin isomers is determined.