Novel Patterns of Torsion-Inversion-Rotation Energy Levels in the ν11 Asymmetric CH-Stretch Spectrum of Methylamine

The high-resolution infrared spectrum of methylamine (CH3NH2) has been recorded using slit-jet direct absorption spectroscopy in the ν11 CH-stretch region (2965–3005 cm−1) with a resolution of 0.0025 cm−1. The 621 lines assigned by ground state combination differences represent 27 substates with |K′| ≤ 2 for the A, B, E1, and E2 symmetries. The spectrum of CH3NH2 is complicated by torsion and inversion tunneling connecting six equivalent minima. The upper states K′ = 0, ± 1 for E1 and E2 are substantially perturbed by “dark” states. The result in the spectrum is multiplets of 2 or 3 states with mixed bright/dark character. The analysis of the spectrum reveals two qualitative differences in the energy level pattern relative to the vibrational ground state and relative to available data on the lower frequency vibrations (NH2 wag and CN stretch). First at J′ = 0, there is a different ordering of the levels connected by torsion-inversion tunneling. Second, the low-J splittings indicative of torsion-rotation coupling are greatly reduced in the ν11 excited state relative to the vibrational ground state for both the E1 and E2 species, suggesting the partial suppression of torsional tunneling in the ν11 CH-stretch excited state