The computed harmonic fundamental frequencies are, in line with previous findings,79 found to be overestimated compared to experiment. Even with anharmonic corrections, the frequencies are in many cases overestimated compared to the experimental data, but lead to a significantly better agreement with experiment. The differences in the calculated vibrational frequencies for the different rotamers are in general very small. Indeed, very similar vibration frequencies are found for the two rotamers of CH3NO2 at both the harmonic and anharmonic level of calculation, the largest difference being 9 cm–1. The calculated vibrational frequencies are also in very good agreement with the experimental assignments of the modes.80–83