From barrier crossing to barrierless relaxation dynamics. Photoisomerization of trans-stilbene in compressed n-alkanols
✍ Scribed by J. Schroeder; D. Schwarzer; J. Troe; P. Vöhringer
- Publisher
- Elsevier Science
- Year
- 1994
- Tongue
- English
- Weight
- 714 KB
- Volume
- 218
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
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 banierless 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 q, kz:r]-OL, with O<cu< 1. CY 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.