Contrasting patterns of coupling between the CH stretches and the large-amplitude motions in the molecules, CH3NH2, CH3OH2+ and CH3CH2

- First principles 2D maps of the variation of the CH stretch force constants.
- Similarity of torsion-inversion-vibration couplings in CH3NH2 and CH3CH2.
- Torsion-inversion-vibration coupling in CH3OH2+ is weaker and different in form.