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High resolution infrared study of vibration-torsion-rotation interactions in CH3CF3 and CH3SiF3 Wang, Shixin


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 <— 0), (v₁₁ = 1 <— 0), (v₅ = 1 <— 0), as well as the torsional overtone (v₆ = 2 <— 0). For each of the three vibrational fundamentals, both the spectrum originating in the ground torsional state (t>6 = 0) and the first torsional hot band originating in the torsional state (v₆ = 1) were studied. A total of about 12,000 frequencies were included in the global analysis. Effective parameters which characterize the effective vibration-torsion-rotation Hamiltonian in the three excited vibrational states were determined. For each of the (v₁₂= 1) and (v₁₁ = 1) vibrational states, the magnitude of the centrifugal distortion constant e that characterizes the (Δl = 0, Δk; = ±3) matrix elements has been determined. The Fermi-like interaction parameters which couple the torsional states in the ground vibrational state with those in the (v₅ = 1) vibrational state were obtained. The theory of vibrational contact transformations was tested by comparing the results of different models. For CH₃SiF₃, the second and third lowest lying vibrational fundamentals, namely (v₁₁ = 1 <— 0) and (v₅ = 1 <— 0), have been studied. A total of about 3,000 frequencies were identified. The (v₁₁ = 1 <— 0, V₆ = 0 <— 0) band was strongly affected by the resonance of the (v₁₁ = 0, v₆ = 3) state with the (v₁₁ = l, v₆ = 0) state through the Coriolis-like interactions. The Coriolis-like parameters were determined along with the parameters which describe the effective vibration-torsion-rotation Hamiltonian for the (v₁₁ = 1) vibrational state. The parallel band (v₅ = 1 <— 0, v₆ = 0 <— 0) was resonantly perturbed by the Fermi-like interactions between the (v₅ = 0, v₆ = 4) state and the (v₅ = 1, v₆ = 0) state. The Fermi-like interaction parameters were obtained together with the effective parameters for the (v₅ = 1) vibrational state.

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