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 bon ds in HNCO by observation of photodissociation appearance thresholds f or the NCO (X (2) Pi) and NH (a (1) Delta) fragments from initially se lected HNCO vibrational states. The upper Limit of the dissociation en ergy of the H-N bond is D-0(H-NCO)less than or equal to 109.6+/-0.4 kc al/mol and that of the N-C bond is D-0 (HN-CO)less than or equal to 12 2.1+/-0.3 kcal/mol. Observation of unrelaxed fragment quantum state di stributions 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 o f the two methods for calculating Delta H-f0(0) (HNCO) suggests that t he actual bond enthalpies for the N-H and C-N bonds are close to the u pper limits from the measurement. (C) 1996 American Institute of Physi cs.