Modeling the CH Stretch/Torsion/Rotation Couplings in Methyl Peroxy (CH₃OO)

The manifestations of CH stretch/torsion/rotation coupling in the region of the CH stretch fundamentals are explored in the CH₃OO radical. Following our earlier study of the fundamental in the totally symmetric CH stretch (the ν₂ fundamental), this work focuses on the other two CH stretch fundamentals, ν₁ and ν₉, which would be degenerate in the absence of a barrier in the potential along the methyl torsion coordinate. The simplest model, which assumes a decoupling of the CH stretch vibrations from the torsion, fails to reproduce several important features of the spectrum. Specifically, the absence of a strong peak around the origin of the ν₁ fundamental and broadening of the strong peak near the origin in the observed spectrum of the ν₉ fundamental are not captured by this model. The origins of these features are explored through two more sophisticated treatments of the torsion/CH stretch couplings. In the first, a four-dimensional potential based on the three CH stretches and the torsion is developed and shown to reproduce both of these features. On the basis of the results of these calculations, the calculated parameters are adjusted to simulate the recorded spectrum. To further explore the torsion/CH stretch couplings in CH₃OO, a 9-state model Hamiltonian is developed and discussed. The implications of various types of couplings on the observed energy level patterns are also discussed.