For all rotamers of each isotopomer, the frequency corresponding to the rotation of the methyl group is found to be quite small at the harmonic level and negative at the anharmonic level, which is consistent with what can be expected for a quasi-free rotating methyl group.76,77 The observed spectra should therefore come from the average over all the rotamers. For this study, only the extremum rotamers (staggered and eclipsed) have been considered (cf. Fig. 1), and the system has been treated as having only 14 normal modes (instead of 3N – 6 = 15) by not considering the derivatives with respect to the methyl rotation mode in the anharmonic calculations.