D0(H2O) = 118.08 ? 0.05 kcal/mol, which appears to be the best currently available experimental bond dissociation energy of water, is based on DHf 0?(OH) = 9.347 ? 0.048 kcal/mol given by Gurvich et al., and accepted2, , , values for DHf 0?(H2O) and DHf 0?(H). Gurvich et al.2 use D0(OH, X2P3/2) = 35420 ? 15 cm-1 of Carlone and Dalby, which is based on a short extrapolation (~1.5 vib. levels) of DGv+1/2 of the A2S+ state, yielding D0(OH, A2S+) = 18847 ? 15 cm-1 to O 1D2. Similar treatment of OD produces a congruent D0(OD), and broadening of rotational lines in OH and OD, attributed to predissociation, seems to further corroborate the derived dissociation energy. In principle, DHf 0?(OH) by Gurvich et al.2 is to be preferred over JANAF,4 since the latter ignores Carlone and Dalby6 and focuses on the older (and longer) extrapolation of the same state by Barrow [D0(OH) = 35450 ? 100 cm-1], producing DHf 0?(OH) = 9.26?0.29 kcal/mol. However, instead of this value, DHf 0?(OH) = 9.175 ? 0.29 kcal/mol [corresponding to D0(OH) = 35480 ? 100 cm?1] appears without explanation in the table.4 Even the latest NIST-JANAF tables do not consider Carlone and Dalby, nor attempt to correct the puzzling discrepancy.
Revised: February 1, 2001 |
Published: January 11, 2001
Citation
Ruscic B., D.F. Feller, D.A. Dixon, K.A. Peterson, L.B. Harding, R.L. Asher, and A.F. Wagner. 2001.Evidence for a Lower Enthalpy of Formation of Hydroxyl Radical and a Lower Gas-Phase Bond Dissociation Energy of Water.Journal of Physical Chemistry A 105, no. 1:1-4.PNNL-SA-33690.