State-to-state rate constants for vibrational energy transfer from the 6l level of SI benzene induced by collisions with N2 have been measured as a function of temperature in the range 41-1 3 Kin a supersonic free jet expansion. It is found that there is a change in the propensity for transfer into the destination channels as the temperature is decreased. The efficiency of transfer to the 162 level increases by a factor of -2 compared with transfer to the spectrally unresolved pair of levels 1 1 I/l6l with the decrease in temperature over this range. Comparison with the room temperature results of Parmenter and Tang (Chem. Phys. 1978,27,127) shows that the branching ratio of 162 to 1 1 I/16I increases from <0.18 at room temperature to -1 .O at 13 K. The increase in propensity for 162 occurs despite the fact that transfer to this level involves a total change in vibrational quanta of 3, compared with 2 for 11 and 16l. The trend in the state-to-state propensities for collision-induced vibrational energy transfer from 300 to 13 K suggests that they are approaching those seen in van der Waals predissociation as the temperature approaches 0 K.
Triplet-state lifetimes for 2,4-dinitrostilbene and 2,2',4,4'-tetranitrostilbene of 50 and 25 ns respectively have been determined in acetonitrile solutions using laser flash photolysis. Five other highly nitrated stilbenes, possessing the common structural feature of three nitro groups in the 2, 4, and 6 positions of one benzene ring, have also been studied under the same conditions. For these molecules, rapid photoproduct formation precludes the determination of triplet-state lifetimes and the observed transient species has a lifetime of 34 ns .
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