Photodissociation studies of isocyanic acid: heat of formation and product branching ratios

Abstract

The heat of formation (AH¡(29& K)) of HNCO is determined to be -24.9J2°87 kcal/mol (based on Afff(NH) = 85.2 kcal/mol). This value is obtained by measuring the threshold for the production of NH(a'A) and by determining the energy contents of the NH fragment and the CO cofragment produced by photolysis of HNCO it wavelengths near the threshold. Saturated laser-induced fluorescence is used to determine the internal state distribution of NH(a'A), 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(a'A) action spectra are also measured. They imply that two excited states of HNCO are present where only one had previously been considered.

The HNCO heat of formation and the N–H and C–N bond enthalpies from initial state selected photodissociation

“…There is clearly a very large shift between the two curves in Fig. 5 that allows us to improve the previous measurement of the C-N dissociation threshold significantly, 48 which came from data similar to the single-photon curve on the left-hand side. A recent measurement of the appearance threshold for NH fragments using a molecular beam to cool the parent HNCO obtains a threshold curve that is similar to the doubleresonance curve on the right side of Fig.…”

“…A recent experimental measurement of the C-N singlet dissociation energy in HNCO by Spiglanin et al 48 used photofragment yield spectroscopy to measure the appearance threshold of NH (a 1 ⌬) fragments from photolysis of a 300-K sample of HNCO. The observed appearance threshold gave a value of D 0 ͑HN-CO͒ϭ41 530Ϯ150 cm Ϫ1 ͑118.7 Ϯ0.4 kcal/mol͒.…”

Mass spectrometric fragmentation pathways of isotope labeled 2,5-disubstituted-1,3,4-oxadiazoles and thiadiazoles

“…Although the isothiocyanic acid is thermodynamically more stable than isocyanic acid [11][12][13], the rearrangement discussed is less efficient for thiadiazoles than for oxadiazoles. It seems to be reasonable to assume, that other directions of fragmentation pathway observed for 3, shown in Fig.…”

Thermochemical properties from G3MP2B3 calculations, Set‐2: Free radicals with special consideration of CH2CHC•CH2, cyclo−•C5H5, •CH2OOH, HO•CO, and HC(O)O•

“…In this instance, computations of the thermochemistry of the cyanato free radical helped to sort out its energetics and enabled the choice of thermochemical values that could be used in flame codes. The recommended JANAF [22] value for f H o (0) of the NCO radical was given as 159.0 ± 10.5 kJ mol −1 based on photoionization threshold experiments by Okabe [50] which was later revised to 151 (+3, −12) kJ mol −1 based on photodissociation experiments of HNCO by Spiglanin et al [51]. Direct photolytic studies of NCO performed by Liu and Coombe [52] resulted in a lower bound of f H o (0) > 154.8 kJ mol −1 for the NCO radical.…”