Unimolecular reactions in the gas and liquid phases: A possible resolution to the puzzles of the trans-stilbene isomerization

Abstract

Previous theoretical and experimental investigations of the trans-stilbene isomerization reaction in the excited S1 state indicated that the gas phase thermal rate at room temperature is much smaller than the thermal rate in the liquid phase. This was based on the observations that: (a) A combination of measured energy-dependent rates and RRKM calculations led to an isolated molecule thermal rate at T=300 K of 2×109 s−1; (b) An experiment of Balk and Fleming [J. Phys. Chem. 90, 3975 (1986)] in which stilbene vapor at 300 K excited at the S0 to S1 zero point to zero point electronic transition energy (000), gave a lifetime in the excited state of ∼780 ps. The liquid state lifetime in ethane is ∼30 ps. In this paper we present theoretical computations of the rate in the gas and liquid phases, based on a new potential model of Vachev et al. [J. Phys. Chem. 99, 5247 (1995)]. We find that: (a) RRKM rates are in agreement with measured energy-dependent rates; (b) The thermal rate derived from the new RRKM rates is the same as the thermal rate in liquid ethane; (c) The laser excitation experiment of Balk and Fleming leads to laser cooling of the excited state suggesting that their measured lifetime is longer than the lifetime in the liquid. The surrounding liquid heats up the molecule on a time scale which is faster than the isomerization lifetime. Experiments are suggested to verify this interpretation.

Photoisomerization of trans-Stilbene in Moderately Compressed Gases: Pressure-Dependent Effective Barriers

“…RRKM fit to experimental k(E) measured in the supersonic jet 21 using the Vachev, Gershinsky, Pollak potential model67 and barrier heights E0 of 900, 930, and 1000 cm -1 , from top to bottom.…”

Experimental evidence of laser cooling of room temperature trans-stilbene upon excitation to the S1 state

“…Gershinsky and Pollak have recently suggested 16 that this lowering of the rate in the gas phase is due to cooling of the excited state vibrational energy upon excitation at the 00 frequency. This cooling arises from the lowered vibrational frequencies of the S 1 state caused by the loosening of the carbon-carbon double bond.…”

“…Using the harmonic model of stilbene in the S 1 state, as given in Ref. 16, the average energy was translated into the initial temperature of the distribution. The resulting dependence of the initial temperature on the laser frequency is plotted in Fig.…”

Communication: Optical cooling of trans-stilbene

“…Fast collisions with a buffer gas or liquid reheat the molecule and the reaction rate increases. [25][26][27][28] (iv) Intramolecular vibrational redistribution (IVR) may be slow (restricted) in isolated stilbene, but becomes faster (unrestricted) in solution due to solute-solvent interactions. 15,[29][30][31][32] (v) Solute-solvent collisions may directly increase the isomerization rate; and (vi) dynamic polarization in the excited state.…”