Dissociation Dynamics of Difluoroacetic Acid from the Ground and Excited Electronic States

Abstract

The photodissociation dynamics of difluoroacetic acid (DFA) is investigated in both its ground and the first excited electronic states employing, respectively, pulsed CO2 and ArF (193 nm) lasers. DFA undergoes facile infrared multiphoton dissociation (IRMPD) on irradiation with a pulsed CO2 laser to open up various dissociation channels from the ground electronic state. Energetically the most preferred primary dissociation channel of DFA is 1,3-HF elimination, as demonstrated by ab initio molecular orbital (MO) calculations and the unimolecular rates for all molecular channels using Rice−Ramsperger−Kassel−Marcus theory. Contrary to an assumed dissociation mechanism for fluorinated carboxylic acids, the decarboxylation reaction is observed as a primary dissociation channel in the IRMPD of DFA. The vibrationally excited photoproducts CO, CO2, COFH, and HF have been detected by measurement of time-resolved infrared fluorescence. Photoexcitation of DFA to the electronically excited S1 state at 193 nm leads to the cleavage of the C−OH bonda higher energy channel absent in IRMPDproducing OH(v,J), which was detected state-selectively employing the laser-induced fluorescence technique. We measured partitioning of the available energy, and observed that the nascent OH(v,J) is generated vibrationally cold (i.e., in v = 0) with moderate rotational excitation (TR = 630 ± 80 K). But, a high fraction of energy goes to the relative translation (fT value of 0.37) of the photofragments. This observation is explained on the basis of the presence of an exit barrier for the C−OH bond cleavage reaction, and supported by modeling of the measured partitioning of energy and ab initio MO calculations. The transition-state structure for the C−OH bond cleavage producing OH could be calculated in the electronically excited state, T1.