HIGH-PRESSURE STUDIES OF THE ACRIDINE FLUORENE PHOTOREACTION - VIBRATION ASSISTED TUNNELING

We report a multifaceted investigation of the hydrogen transfer photor eaction in acridine-doped fluorene crystals at higher temperature. The purpose is to elucidate the role of vibrationally assisted tunneling in this reaction system. Raman experiments were conducted at various p ressures and 77 K to document the change of vibrational frequency for the promoting models). Upon compression, a line with a large pressure coefficient emerges from under the strong phonon mode at 96.5 cm(-1) T hrough polarization studies under pressure, we have identified it as a molecular butterfly mode of B-1 symmetry. We have measured the reacti on rate at 150 K in order to examine the effect of a suggested promoti ng mode at similar to 440 cm(-1). The reaction rate again increases ex ponentially with pressure, but with a significantly higher pressure co efficient than that at 1.4 and 77 K. Mode patterns based on a recently published [J. Phys. Chem. 98, 12 223 (1994)] normal coordinate analys is of fluorene are used to help establish the promoting modes for this reaction. This consideration suggests that the 95 and 238 cm(-1) mode s are likely promoting modes in addition to the 125 cm(-1) libration. A computation of the Franck-Condon factor for the H-transfer process i ndicates that a small population of a high overtone of a promoting mod e may make a disproportionally large contribution to the reaction rate . This calculation fails to account for the greater pressure coefficie nt of the reaction rate at higher temperature. Instead, such an increa se may come partly from a greater compressibility at higher temperatur e. (C) 1995 American Institute of Physics.