On the Enthalpy of Formation of Hydroxyl Radical and Gas-Phase Bond Dissociation Energies of Water and Hydroxyl

Abstract

In a recent letter (J. Phys. Chem. A, 2001, 105,1), we argued that, although all major thermochemical tables recommend a value of ∆H°f 0 (OH) based on a spectroscopic approach, the correct value is 0.5 kcal/mol lower as determined from an ion cycle. In this paper, we expand upon and augment both the experimental and theoretical arguments presented in the letter. In particular, three separate experiments (mass-selected photoionization measurements, pulsed-field-ionization photoelectron spectroscopy measurements, and photoelectron-photoion coincidence measurements) utilizing the positive ion cycle to derive the O-H bond energy are shown to converge to a consensus value of the appearance energy AE 0 (OH(18.116 2 ( 0.003 0 eV). With the most accurate currently available zero kinetic energy photoionization value for the ionization energy IE(OH) ) 104989 ( 2 cm -1 , corroborated by a number of photoelectron measurements, this leads to D 0 (H-OH) ) 41128 ( 24 cm -1 ) 117.59 ( 0.07 kcal/mol. This corresponds to ∆H f0 (OH) ) 8.85 ( 0.07 kcal/mol and implies D 0 (OH) ) 35593 ( 24 cm -1 ) 101.76 ( 0.07 kcal/mol. These results are completely supported by the most sophisticated theoretical calculations ever performed on the H x O 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. These calculations have an estimated theoretical error of e0.2 kcal/mol based on basis set convergence properties. They reproduce the experimental results for dissociation energies, atomization energies, and ionization energies for the H x O system to within 0.0-0.2 kcal/mol. In contrast, the previously accepted values of the two successive bond dissociation energies of water differ from the current values by 0.5 kcal/mol. These values were derived from the spectroscopic determinations of D 0 (OH) using a very short Birge-Sponer extrapolation on OH/OD A 1 Σ + . However, on the basis of a calculation of the A state potential energy curve (with a multireference single and double excitation wave function and an augcc-pV5Z basis set) and an exhaustive reanalyzis of the original measured data on both the A and B states of OH, the Birge-Sponer extrapolation can be demonstrated to significantly underestimate the bond dissociation energy, although only the last vibrational level was not observed experimentally. The recommended values of this paper affect a large number of other thermochemical quantities which directly or indirectly rely on or refer to D 0 (H-OH), D 0 (OH), or ∆H°f(OH). This is illustrated by an analysis of several reaction enthalpies, deprotonation enthalpies, and proton affinities.

Anion photoelectron spectroscopy of deprotonated ortho-, meta-, and para-methylphenol

“…The dominant anion product observed corresponds to deprotonation of the most acidic site, with trace amounts of other products. Both OH¯ and NH 2¯ are energetically capable 42,43 of deprotonating either the hydroxyl or the methyl sites for all three isomers, although the hydroxyl site is the most acidic site by over 30 kcal/mol (according to calculations at the B3LYP/aug-cc-pVQZ level, listed in the Supplementary Material). The NH 2¯ anion is also energetically capable of deprotonating directly from the benzene ring, but this anion product channel is not observed.…”

Mechanistic investigations on the antioxidant action of a neuroprotective estrogen derivative

“…1) and the isomeric 10α-hydroxy-17β-butoxy-1,4-estradien-3-one were −155.7 kcal·mol −1 and −151.1 kcal·mol −1 , respectively, indicating the preferential formation of the β-isomer. Reaction trajectory calculations from the radical intermediate for the recombination with • OH (ΔH f = 8.9 kcal·mol −1 [41]), an exothermic reaction (ΔH = −60.2 kcal·mol −1 ) based on the ΔH f values of the reactants and products according to Hess's Law [42], revealed no transition states. The conclusions that could be drawn from a computational study for the formation of 10β, 17β-dihydroxy-1,4-estradien-3-one from E2 by the mechanism shown for 2 in Fig.…”

Effects of Electron Correlation and Scalar Relativistic Corrections on the Thermochemical and Spectroscopic Properties of HOF

“…Since the ab initio calculations do not include spin-orbit coupling, the experimental values used for comparison have been adjusted using experimental atomic 41 and molecular 28,29 spin-orbit splittings, and therefore, the comparisons are meaningful.…”

“…For IE-(OH), Pople and Curtiss used the Katsumata-Lloyd value of 13.01 eV 45 , and for the 0 K enthalpies of formation of OH and F( 2 P), they used the JANAF 29 value for the ∆H°f ,0K of OH, and adopting the JANAF value for the ∆H°f ,0K for F( 2 P), we obtain the "best" experimental value for the ∆H°f ,0K of HOF to be -20.02 ( 0.25 kcal/ mol. This is the basis for the experimental D e values reported for the dissociation of the HOF molecule reported in Table 1.…”