“…For ease of discussion, this approach is termed here the high-barrier model. The Hamiltonian H xy,ν 12 21 has been used extensively to characterize the intervibrational coupling in CH 3 SiH 3 (1), CH 3 SiD 3 (13), CH 3 CH 3 (19), and CH 3 CD 3 (20). However, this approach has the drawback that the term involving sin 3α does not emerge naturally in this order from the formal treatment of the expansion of the vibration-torsion-rotation Hamiltonian.…”