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Abstract

Vibration-torsion-rotation interactions in CH₃CF₃ and CH₃SiF₃ have been investigated using Fourier transform spectroscopy at a resolution of 0.00125 cm⁻¹. Although CH₃CF₃ has a high barrier at ~ 1100 cm⁻¹ and CH₃SiF₃ has an intermediate barrier at ~ 415 cm⁻¹, internal rotation plays an important role in determining the form of the spectrum in both molecules. The internal degree of freedom in both molecules provides excellent opportunities to study the Hamiltonian from small amplitude oscillation to large amplitude internal rotation. The general theory of the vibration-torsion-rotation interactions for AB₃XY₃ type molecule has been reviewed. Sequential contact transformations have been applied to the vibration-torsion rotation Hamiltonian. The formalism for the effective Hamiltonian has been developed in terms of the fundamental molecular parameters. Earlier works have used the principal axis method and the internal axis method at the same time. A new approach has been introduced to simplify the effective Hamiltonian in the principal axis method so that it has the advantages of both methods. For CH₃CF₃, four bands have been investigated, namely the vibrational fundamentals (v₁₂ = 1