Dynamics of OH formation in photodissociation of pyruvic acid at 193 nm

Abstract

The dynamics of the formation of OH radical upon 193 nm excitation of pyruvic acid has been investigated by the laser-photolysis laser-induced-fluorescence technique. OH radicals were generated in the ground electronic state, with no vibrational excitation. The estimated rotational temperature is 720±90 K, and the translational energy is 18.7±6.5 kcal mol−1. Ab initio calculations on excited electronic states were performed at the configuration interaction with single electronic excitation level with 6-31+G(d,p) basis function. All low-lying electronic excited states (S1–S3 and T1–T6) were characterized and the transitions were identified. A transition state for the C–OH dissociation channel has been obtained from the T1 state with a late exit barrier. A mechanism for the formation of OH radicals involving internal conversion and intersystem crossing from the initially populated S3 state to T1 state and the dissociation from the T1 potential energy surface with the calculated barrier is proposed, which reproduces the observed partitioning of available energy in the fragments according to the hybrid model.

Photochemistry of aqueous pyruvic acid

“…UV irradiation of aqueous PA is known to result in phosphorescence from a 3 (n,π*) state (53). This triplet is presumed to be involved in the aqueous phase photochemistry of PA. Ab initio calculations (36) have suggested that the S 1 state of PA is 1 (n,π*) and that this is higher in energy than both the T 1 and T 2 states, which are found to be 3 (n,π*) and 3 (π,π*), respectively. Assuming these calculations to be at least qualitatively correct, it seems likely that the UV photochemistry of PA with light of λ > 300 nm proceeds according to the sequence S 0 → S 1 → T 2 → T 1 .…”

“…PA undergoes decarboxylation in the gas and aqueous phases by means of different mechanisms. Decarboxylation of gas phase PA has been seen to occur thermochemically, through IR multiphoton pyrolysis, as well as through UV and visible excitation (13,(25)(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36). The main removal pathway of gas phase PA from the troposphere is UV photolysis, with reaction with OH radical providing a minor contribution (31).…”

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

“…[1][2][3][4][5][6][7][8][9][10][11][12] Recently, we investigated dynamics of photodissociation of saturated [8][9][10] and unsaturated 11,12 carboxylic acids at 193 and 248 nm, and found OH to be a primary product, formed after the C-OH bond cleavage. We observed an appreciable amount of energy being channeled into the relative translation of OH and its cofragment.…”

4 Photodissociation in the gas phase

“…Those of the acetyl radical were 8.6, 15 and 23 ps for R ~C2 H 5 , C 3 H 7 and iso-C 4 H 9 , respectively, suggesting that for larger R more vibrational freedom compete for the excess energy so that less energy is partitioned into the internal energy of the acetyl radical. Photodissociation dynamics at 266, 248, and 193 nm of acetylacetone, which exists predominantly as an enolic form [H 3 CCOCHLC(OH)CH 3 ] in the gas phase, was explored by Upadhyaya et al 63 Dhanya et al 64 studied OH formation from pyruvic acid (CH 3 COCOOH) at 193 nm.…”