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.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry