Dissociation dynamics of thiolactic acid at 193 nm: detection of the nascent OH product by laser-induced fluorescence

Abstract

Electronically excited thiolactic acid (2-mercaptopropionic acid), H₃C-CH(SH)-COOH, undergoes the C-OH bond cleavage on excitation to the S₂ state at 193 nm, generating the primary product OH (v,J), which is detected by laser-induced fluorescence technique in a collisionless condition of flow system. The partitioning of the available energy between vibrational, rotational, and translational degrees of freedom of nascent photofragments is obtained from relative intensities of ro-vibronic lines in laser-induced fluorescence spectrum of OH, and their Doppler profiles. The rotational population of OH (v' = 0) is characterized by rotational temperature of 408 ± 25K. OH is produced in a vibrationally cold state, i.e., mostly in v'= 0. The average translational energy of OH (v' = 0,J') is found to be 21.5 ± 2.0kcal/mol, which implies 25.6 kcal/mol of energy in relative translation of photoproducts corresponding to the f_t value of ~0.6. The observed high translational energy is due to the presence of a barrier in the exit channel, implying that the C-OH bond scission takes place on an electronically excited potential energy surface. The observed partitioning of the available energy between various degrees of the photofragments is theoretically modeled, and the hybrid model, with 26.0 kcal/mol of barrier in the exit channel, is found to explain the measured data quite well. The experimental results are also supported with ab initio molecular orbital calculations for both the ground and the excited electronic states. Time-dependent density functional theory is used to understand the nature of various electronic transitions connecting the lower excited states. Potential energy curves as a function of the C-OH bond length of thiolactic acid suggest distinct exit barriers in the S₁, T₁, and T₂ states. But, we could locate the transition state structure for OH formation in the S₁ state alone. Thus, although thiolactic acid is excited to the S₂ state at 193 nm, it undergoes internal conversion to S₁ where it dissociates to yield OH. In addition to the OH channel from excited electronic states, we studied theoretically all probable dissociation channels occurring on the ground electronic state of thiolactic acid.