Photodissociation Studies of HNCO: Heat of Formation and Product Branching Ratios

Article abstract of DOI:10.1021/j100281a025

The heat of formation (ΔH_f(298 K)) of HNCO is determined to be -24.9^+0.7_-2.8 kcal/mol (based on ΔH_f(NH) = 85.2 kcal/mol).This value is obtained by measuring the threshold for the production of NH(a¹Δ) and by determining the energy contents of the NH fragment and the CO cofragment produced by photolysis of HNCO at wavelengths near the threshold.Saturated laser-induced fluorescence is used to determine the internal state distribution of NH(a¹Δ), and multiphoton ionization is used to measure the internal state distribution of CO.An upper limit for the branching ratio of NCO/NH production from photodissociation of HNCO at 193 nm is determined from an analysis of kinetic experiments to be 0.10.To clarify the mechanism of photodissociation, HNCO fluorescence-excitation and NH(a¹Δ) action spectra are also measured.They imply that two excited states of HNCO are present where only one had previously been considered.