Overtone-induced chemistry of trifluoroacetic acid: An experimental and theoretical study

The effect of overtone-induced chemistry on the atmospheric fate of trifluoroacetic acid has been investigated. We report the absolute absorption intensities for the v_OH = 3, 4, and 5 overtone transitions as well as an ab initio calculation of the energetics of the dissociation pathways. Calculations at the MP2 level give the lowest dissociation barrier as 50.3 kcal mol^-1 for the elimination of HF. Integrated cross sections for v_OH = 3, 4, and 5 are 2.70 × 10^-20, 1.68 × 10^-21, and 1.5 × 10^-22 cm² molecule^-1 cm^-1, respectively. Dissociation may proceed after absorption into v = 5 or 6, giving an upper limit to the photodissociation rate constants of 3.7 × 10^-9 s^-1 or 2.5 × 10^-10 s^-1, respectively. These correspond to a tropospheric lifetime of between 8 and 127 years. The overtone-driven photodissociation is more important than the ultraviolet photodissociation in the troposphere but is insignificant in comparison to wet deposition and the reaction with OH radicais.