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 solvent, starts as a barrier crossing process at low pressure and turns into a relaxation on a barrierless potential at high pressure. The pressure and temperature dependences of trans-stilbene photoisomerization in n-alkanols were investigated by picosecond transient absorption spectroscopy. As in n-alkane solvents, at constant temperature, the rate coefficients k for rotation about the central double bond in each solvent exhibit a fractional power dependence on solvent viscosity eta, k approximate to eta(-alpha), with 0 < alpha equal to or less than 1. alpha varies little with solvent, but increases with temperature. This observation 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 under these conditions, however, the observed decay dynamics remain monoexponential.