UV Photochemistry of Peroxyformic Acid (HC(O)OOH): An Experimental and Computational Study Investigating 355 nm Photolysis

The photochemistry of peroxyformic acid (PFA), a molecule of atmospheric interest exhibiting internal hydrogen bonding, is examined by exciting the molecule at 355 nm and detecting the nascent OH fragments using laser-induced fluorescence. The OH radicals are found to be formed in their ground electronic state with the vast majority of available energy appearing in fragment translation. The OH fragments are vibrationally cold (v″ = 0) with only modest rotational excitation. The average rotational energy is determined to be 0.35 kcal/mol. Further, the degree of OH rotational excitation from PFA is found to be significantly less than that arising from the dissociation of H₂O₂ as well as other hydroperoxides over the same wavelength. Ab initio calculation at the EOM-CCSD level is used to investigate the first few electronic excited states of PFA. Differences in the computed torsional potential between PFA and H₂O₂ help rationalize the observed variation in their respective OH fragment rotational excitation. The calculations also establish that the electronic excited state of PFA accessed in the near UV is of ¹A″ symmetry and involves a σ*_(O–O) ← n_(O) excitation. Additionally, the UV absorption cross section of PFA at 355 and 282 nm is estimated by comparing the yield of OH from PFA at these wavelengths to that from hydrogen peroxide for which the absorption cross sections is known