A TRIMER VIBRONIC COUPLING MODEL FOR TRIPTYCENE - THE JAHN-TELLER AND BARNETT EFFECTS

The first singlet electronic excited state of triptycene, as measured by resonant two-photon ionization in a supersonically cooled beam, has been found to be a textbook example of the E' x e' Jahn-Teller effect. Here it is shown that this E' x e' vibronic coupling can be profitably viewed as a subset of a (A1' + E') x (a2' + e') vibronic coupling scheme which results from a simple trimer model. The enlarged coupling scheme has a simple physical interpretation where the wagging coordinates of the benzene subunits are strongly coupled to their excimer formation. The previously obtained parameters, in which there is a large reduction between the ground and excited electronic state frequencies of the lowest frequency e' mode as well as an unusually large second-order vibronic coupling constant, are shown to arise naturally from a trimer viewpoint. Features of the spectra have been found which are attributed to the involvement of an a2' vibration which couples through nonzero momentum rather than coordinate matrix elements. A coupling of this type has been termed the ''molecular Barnett effect'' and has been predicted to appear in Jahn-Teller systems when certain conditions are fulfilled. This effect has not been previously observed in molecular spectroscopy. It indicates that crude adiabatic basis functions are inadequate to describe the electronic states of the present system.