The formation and destruction of H3O−

Abstract

We report the first measurements of rate constants for formation and reaction of the hydrated-hydride ion H3O−. We studied the Kleingeld–Nibbering reaction [Int. J. Mass Spectrom. Ion Phys. 49, 311 (1983)], namely, dehydrogenation of formaldehyde by hydroxide to form hydrated-hydride ion and carbon monoxide. The OD−+H2CO reaction is about 35% efficient at 298 K, with OD−/OH− exchange occurring in about half the reactions. H3O− was observed to undergo thermal dissociation in a helium carrier gas at room temperature with a rate constant of 1.6×10−12 cm3 s−1. We also studied a new reaction in which H3O− is formed: The association of OH− with H2 in a He carrier gas at low temperatures. The rate coefficient for this ternary reaction is 1×10−30 cm6 s−1 at 88 K. Rate coefficients and product branching fractions were determined for H3O− reactions with 19 neutral species at low temperatures (88–194 K) in an H2 carrier. The results of ion-beam studies, negative-ion photoelectron spectroscopy, and ion-molecule reaction data allow us to specify the hydride–water bond energy D0298(H−−H2O)=14.4±1.0 kcal mol−1 (0.62±0.04 eV). The heat of formation of H3O−, −37.5±1.0 kcal mol−1, and the proton affinity of H3O−, 386.0±1.0 kcal mol−1, are derived from these results. Dissociation of H3O− into OH− and H2 requires 4.5±1.0 kcal mol−1 energy.