Hydration and mobility of HO-(aq)

Abstract

The hydroxide anion plays an essential role in many chemical and biochemical reactions. But a molecular-scale description of its hydration state, and hence also its transport, in water is currently controversial. The statistical mechanical quasichemical theory of solutions suggests that HO.[H2O]3(-) is the predominant species in the aqueous phase under standard conditions. This result agrees with recent spectroscopic studies on hydroxide water clusters and with the available thermodynamic hydration free energies. In contrast, a recent ab initio molecular dynamics simulation has suggested that HO.[H2O]4(-) is the only dominant aqueous solution species. We apply adiabatic ab initio molecular dynamics simulations and find good agreement with both the quasichemical theoretical predictions and experimental results. The present results suggest a picture that is simpler, more traditional, but with additional subtlety. These coordination structures are labile but the tricoordinate species is the prominent case. This conclusion is unaltered with changes in the electronic density functional. No evidence is found for rate-determining activated interconversion of a HO.[H2O]4(-) trap structure to HO.[H2O]3(-) mediating hydroxide transport. The view of HO- diffusion as the hopping of a proton hole has substantial validity, the rate depending largely on the dynamic disorder of the water hydrogen-bond network.

Fig. 1.

R is the radius of the observation volume centered on the hydroxide oxygen. θ and ϕ identify the angles that specify the directionality of the hydrogen bond to water. The hydroxide hydrogen, uppermost here, is not included in the coordination number counts or in the radial distribution functions shown later.

Fig. 2.

RUN1 coordination number and temperature versus time. R = 2.5 Å. The block-averaged temperature is shown with the solid line. The mean temperature is 332 ± 22 K. The short vertical bars at the n = 3.5 level flag hydrogen exchange events, which also change the identity of the hydroxyl. Note that many hydrogen exchange events occur without intercession of the n = 4 configuration.

Fig. 3.

RUN3 coordination number and temperature versus time. The mean temperature is 313 ± 21 K. Other conditions are as in Fig. 2. The dashed line applies to the selection criterion involving R ≤ 2.5, θ ≥ 80°, ϕ ≥ 150°. Note that many hydrogen exchange events occur without intercession of the n = 4 configuration.

Fig. 4.

Density distribution of water oxygen and proton around the hydroxide oxygen for RUN1, which used PW91. The distributions of the neighboring atoms are also separated into contributions according to distance-order. The hydrogen of the nominal HO chemical bond, otherwise the nearest H, is not included in this distance-ordering.

Fig. 5.

Density distribution of water oxygen and proton around the hydroxide oxygen for RUN3, which used rPBE; otherwise as in Fig. 4.