Torsion–Inversion Tunneling Patterns in the CH-Stretch Vibrationally Excited States of the <i>G</i>₁₂ Family of Molecules Including Methylamine

Two torsion<i>-</i>inversion tunneling models (models I and II) are reported for the CH-stretch vibrationally excited states in the <i>G</i>₁₂ family of molecules. The torsion and inversion tunneling parameters, <i>h</i>_2<i>v</i> and <i>h</i>_3<i>v</i>, respectively, are combined with low-order coupling terms involving the CH-stretch vibrations. Model I is a group theoretical treatment starting from the symmetric rotor methyl CH-stretch vibrations; model II is an internal coordinate model including the local–local CH-stretch coupling. Each model yields predicted torsion–inversion tunneling patterns of the four symmetry species, A, B, E₁, and E₂, in the CH-stretch excited states. Although the predicted tunneling patterns for the symmetric CH-stretch excited state are the same as for the ground state, inverted tunneling patterns are predicted for the asymmetric CH-stretches. The qualitative tunneling patterns predicted are independent of the model type and of the particular coupling terms considered. In model I, the magnitudes of the tunneling splittings in the two asymmetric CH-stretch excited states are equal to half of that in the ground state, but in model II, they differ when the tunneling rate is fast. The model predictions are compared across the series of molecules methanol, methylamine, 2-methylmalonaldehyde, and 5-methyltropolone and to the available experimental data