PNNL

Abstract

Several photoionization experiments utilizing the positive ion cycle to derive the O-H bond energy converge to a consensus value of AE0(OH+/H2O) = 146117 ? 24 cm-1 (18.1162 ? 0.0030 eV). With the most accurate currently available ZEKE valueError! Bookmark not defined. IE(OH) = 104989 ? 2 cm-1, corroborated by a number of photoelectron measurements,Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not defined. this leads to D0(H?OH) = 41128 ? 24 cm-1 = 117.59 ? 0.07 kcal/mol. This corresponds to DHf 0(OH) = 8.85 ? 0.07 kcal/mol, and implies D0(OH) = 35593 ? 24 cm-1 = 101.76 ? 0.07 kcal/mol. The most sophisticated theoretical calculations performed so far on the HxO system, CCSD(T)/aug-cc-pVnZ, n=Q, 5, 6, and 7, extrapolated to the CBS limit and including corrections for core-valence effects, scalar relativistic effects, incomplete correlation recovery, and diagonal Born-Oppenheimer corrections reproduce the experimental results to within 0.0 - 0.2 kcal/mol. The new values of the two successive bond dissociation energies of water supersede the previously accepted values,Error! Bookmark not defined.,Error! Bookmark not defined. which were based on spectroscopic determinationsError! Bookmark not defined.,Error! Bookmark not defined. of D0(OH) using a very short Birge-Sponer extrapolation on OH/OD A1S+. An exhaustive analysis of the latter approach, combined with the application of the same procedure on a calculated potential energy curve for the state in question, demonstrates that the Birge-Sponer extrapolation underestimates the bond dissociation energy, in spite of the fact that only the last vibrational level was not observed experimentally. The new values affect a large number of other thermochemical quantities which directly or indirectly rely on or refer to D0(H-OH), D0(OH), or DHf?(OH).