The vibrational Jahn–Teller effect in E⊗e systems

Highlights: • The vibrational Jahn–Teller effect is documented for three E⊗e molecular systems. • The spontaneous vibrational Jahn–Teller distortion is very small. • Vibrational Jahn–Teller splittings are substantial (1–60 cm⁻¹). • Vibrational conical intersections in CH₃OH are accessible at low energies. - Abstract: The Jahn–Teller theorem is applied in the vibrational context where degenerate high-frequency vibrational states (E) are considered as adiabatic functions of low-frequency vibrational coordinates (e). For CH₃CN and Cr(C₆H₆)(CO)₃, the global minimum of the non-degenerate electronic potential energy surface occurs at the C_3v geometry, but in CH₃OH, the equilibrium geometry is far from the C_3v reference geometry. In the former cases, the computed spontaneous Jahn–Teller distortion is exceptionally small. In methanol, the vibrational Jahn–Teller interaction results in the splitting of the degenerate E-type CH stretch into what have been traditionally assigned as the distinct ν₂ and ν₉ vibrational bands. The ab initio vibrational frequencies are fit precisely by a two-state high-order Jahn–Teller Hamiltonian (Viel and Eisfeld, 2004). The presence of vibrational conical intersections, including 7 for CH₃OH, has implications for spectroscopy, for geometric phase, and for ultrafast localized non-adiabatic energy transfer.