The HNCO heat of formation and the N{endash}H and C{endash}N bond enthalpies from initial state selected photodissociation
- Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706 (United States)
We measure upper limits for the bond enthalpies of the N{endash}H and C{endash}N bonds in HNCO by observation of photodissociation appearance thresholds for the NCO ({ital X}{sup 2}{Pi}) and NH ({ital a}{sup 1}{Delta}) fragments from initially selected HNCO vibrational states. The upper limit of the dissociation energy of the H{endash}N bond is {ital D}{sub 0}(H{endash}NCO){le}109.6{plus_minus}0.4 kcal/mol and that of the N{endash}C bond is {ital D}{sub 0} (HN{endash}CO){le}122.1{plus_minus}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{sub {ital f}{sub 0}}{sup 0} (HNCO){ge}{minus}27.7{plus_minus}1.1 kcal/mol. The consistency of the two methods for calculating {Delta}H{sub {ital f}{sub 0}}{sup 0} (HNCO) suggests that the actual bond enthalpies for the N{endash}H and C{endash}N bonds are close to the upper limits from the measurement. {copyright} {ital 1996 American Institute of Physics.}
- OSTI ID:
- 434713
- Journal Information:
- Journal of Chemical Physics, Journal Name: Journal of Chemical Physics Journal Issue: 18 Vol. 105; ISSN JCPSA6; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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