FROM BARRIER CROSSING TO BARRIERLESS RELAXATION DYNAMICS - PHOTOISOMERIZATION OF TRANS-STILBENE IN COMPRESSED N-ALKANOLS

We report the first observation of a reaction which, in a single solve nt, starts as a barrier crossing process at low pressure and turns int o a relaxation on a barrierless potential at high pressure. The pressu re and temperature dependences of trans-stilbene photoisomerization in n-alkanols were investigated by picosecond transient absorption spect roscopy. As in n-alkane solvents, at constant temperature, the rate co efficients k for rotation about the central double bond in each solven t exhibit a fractional power dependence on solvent viscosity eta, k ap proximate to eta(-alpha), with 0 < alpha equal to or less than 1. alph a varies little with solvent, but increases with temperature. This obs ervation is discussed in terms of a solvent shift effect which causes with increasing solvent density a lowering of the barrier height. In n -propanol at high pressure the barrier height approaches zero. Even un der these conditions, however, the observed decay dynamics remain mono exponential.