Near threshold photochemistry of propanal: barrier height, transition state structure and product state distributions for the HCO channel

Abstract

The photodissociation dynamics of propanal have been investigated at photolysis wavelengths between 300 and 327 nm. The threshold for production of HCO fragments was found to be 326.26 nm, which corresponds to 30645 cm-1 (366.6 kJ mol-1) above the zero-point of the S0 state. From known thermochemical data, this threshold lies 25.0 ± 3.6 kJ mol-1 above the bond dissociation energy. The nascent HCO rotational and translational energy distributions were determined following dissociation at threshold. The rotational population was measured as a function of N, Ka, Kc, and S. The distribution of rotational states followed a Gaussian function with an average rotational energy of 2.5 ± 0.5 kJ mol-1. The population of the near-degenerate spin-rotation states was equal, while the population in the asymmetry doublets favored the upper energy component by about 3:1. Careful measurement of the Doppler profiles of individual Ka = 0 lines in the LIF spectrum revealed that the translational energy also shows a Gaussian-like distribution with an average energy of 6.5 ± 1.0 kJ mol-1. The ethyl fragment must also have an average translational energy of 6.5 ± 1.0 kJ mol-1 and therefore an average internal energy of 9.5 kJ mol-1 is inferred. The observed energy partitioning in the fragments is consistent with a model in which the HCO rotational and translational excitation is determined mostly by the transition state geometry, a barrier on the triplet surface, and the fixed energy in the exit channel. A modified impulsive model was satisfactory in reproducing the energy deposited into the various degrees of freedom. The model implied impact parameters at infinite separation corresponding to an in-plane HCO angle of 40 and an out-of-plane angle of 60. The strongly pyramidal nature of the transition state produces more angular momentum about the b axis than the c axis, which causes the preference for the upper energy component of the asymmetry doublets.