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

We measure upper limits for the bond enthalpies of the N-H and C-N bonds in HNCO by observation of photodissociation appearance thresholds for the NCO (X (2) Pi) and NH (a (1) Delta) fragments from initially selected HNCO vibrational states. The upper Limit of the dissociation energy of the H-N bond is D-0(H-NCO)less than or equal to 109.6+/-0.4 kcal/mol and that of the N-C bond is D-0 (HN-CO)less than or equal to 122.1+/-0.3 kcal/mol. Observation of unrelaxed fragment quantum state distributions at fixed energies supports the bond enthalpy measurement. The two appearance thresholds, together with known heats of formation of NH, NCO, H, and CO, provide two independent methods of calculating the HNCO heat of formation. Both methods give a value of Delta H-f0(0) (HNCO)greater than or equal to-27.7+/-1.1 kcal/mol. The consistency of the two methods for calculating Delta H-f0(0) (HNCO) suggests that the actual bond enthalpies for the N-H and C-N bonds are close to the upper limits from the measurement. (C) 1996 American Institute of Physics.