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), H(3)C-CH(SH)-COOH, undergoes the C-OH bond cleavage on excitation to the S(2) 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+/-25 K. 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+/-2.0 kcal/mol, which implies 25.6 kcal/mol of energy in relative translation of photoproducts corresponding to the f(t) value of approximately 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 S(1), T(1), and T(2) states. But, we could locate the transition state structure for OH formation in the S(1) state alone. Thus, although thiolactic acid is excited to the S(2) state at 193 nm, it undergoes internal conversion to S(1) 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.

Photolysis of o‐Nitrobenzaldehyde in the Gas Phase: A New OH. Formation Channel

“…Rotational population distribution of the OH fragments generated from photodissociation of o-NBA at five different photolysis wavelengths. 2 P 3/2 state has also been observed in the photodissociation of several other molecular systems, such as benzoic acid, [20] thiolactic acid, [21] and so on. [22][23][24][25][26] The measured nonstatistical spinorbit ratio can be explained partly on the basis of the energy difference between these two states.…”

Formation of Hydroxyl Radical from the Photolysis of Salicylic Acid

“…On the basis of the thermochemical data, the bond energy of C–OH in phenol (110.8 kcal/mol) is higher than that of O–H (90.4 kcal/mol), so the OH radical of SA photolysis cannot be produced from the direct dissociation of the phenolic OH group. In addition, like other carboxylic acids, ,, we can exclude the possibility of OH as a secondary product from HOCO. According to the thermochemical data, the C–C bond and the HO–CO bond cleavage were estimated to be endothermic by 105.2 and 36.0 kcal/mol, respectively.…”

“…The hydroxyl radical is the most important chemical cleaning agent in the atmosphere. Pushpa et al 6 and Kumar et al 7 studied the photodissociation of saturated and unsaturated carboxylic acids at 193 and 248 nm. They observed an appreciable amount of energy being channeled into the relative translation of OH and its cofragment.…”

“…The photochemical and photodissociation dynamics of polyatomic molecules is of considerable interest toward the fundamental understanding of the dynamics of the elementary reactions. − In contrast to small molecules, photodissociation of polyatomic molecules may involve more complex dynamics, from which one can obtain the dynamical information on the dissociation pathways and the nature of the dissociative state potential energy surface by detecting the energy disposal in the photofragmentation process. The photochemistry and photodissociation of carboxylic acids have been investigated extensively − because it generates the important OH radical. The hydroxyl radical is the most important chemical cleaning agent in the atmosphere.…”

Wavelength-Dependent Photodissociation of Benzoic Acid Monomer in α C−O Fission

“…[2][3][4][5][6] However, the photodissociation of BAM and the other carboxylic acids in UV plays more critical roles in atmospheric, combustion, and interstellar chemistry. [7][8][9][10][11][12][13][14][15] Aromatic decarboxylation processes also play a part in different areas of organic synthesis and in biochemical reactions. Ruelle 16,17 reported the first theoretical study of the decarboxylation of benzoic acid and salicylic acid.…”