Near-UV photodissociation dynamics of formic acid

Abstract

H (Rydberg) atom photofragment translational spectroscopy has been used to study the photodissociation dynamics of jet-cooled formic acid molecules following excitation to their first excited singlet (S₁) state at numerous wavelengths in the range 216–241 nm. Analysis of the resulting H-atom time-of-flight spectra indicates contributions from three H-atom formation channels, which we identify as the primary C–H and O–H bond fission processes and the secondary photolysis of HCO() fragments resulting from primary C–O bond fission. It also allows determination of the bond dissociation energies: D₀(H–CO₂H)≈30000 cm^-1 and D₀(HCOO–H)=39080±100 cm^-1. The former bond fission is deduced to occur after intersystem crossing to the neighbouring ã³A″ state, and to involve passage over (or tunnelling through) a barrier in the C–H dissociation coordinate on the triplet potential-energy surface. O–H bond fission, in contrast, is shown to occur predominantly on the S₁ surface but it, too, must overcome an activation barrier, the magnitude of which we can estimate at ca. 5400 cm^-1, measured relative to the asymptotic products H+HCOO(and/or Ã). The latter assignment affords a refined value for the 0 K heat of formation of the formyloxyl radical: Δ_fH₀^o (HCOO)=-119.5±3 kJ mol^-1.