Vibronic structure of nonadiabatic and fluxional states: Two-photon absorption spectroscopy of jet isolated 3s ¹E′ sym-triazine

The nature of nonadiabatic effects in the Rydberg and ground ionic states of jet-cooled sym-triazine is investigated by means of ultraviolet two-photon absorption spectroscopy. A highly resolved band system is observed in the region from 55 000 to 60 000 cm^-1, which can be associated with excitation from an e′ lone pair to the 3s Rydberg orbital. A complete assignment of the low-energy bands of this system, as determined by isotope effects and comparison to model calculations, reveals a clear example of the dynamical Jahn-Teller effect for the case where only one mode, the ν₆ ring distortion, is significantly active. On the basis of simple limiting models we derive approximate vibronic coupling parameters and present a quantitative description of vibronic motion in terms of adiabatic molecular coordinates. We conclude that sym-triazine, in its ground state one of the most rigid of medium-sized polyatomics, becomes strikingly fluxional in its Rydberg and ground ionic states, as the dynamic Jahn-Teller effect introduces a ring-distortion vibronic pseudorotation which can be characterized adiabatically in terms of a rotor frequency no greater than 80 cm^-1. This is a limit, however, which the data shows is valid only well below a vibronic energy of 1100 cm ^-1, at which point the conical intersection is reached and all levels become intrinsically nonadiabatic.