Article pubs.acs.org/JPCB
Special Pairs Are Decisive in the Autoionization and Recombination of Water Chen Bai and Judith Herzfeld* Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02453, United States ABSTRACT: Although water’s chemical properties are no less important than its exceptional physical properties, its acid−base behavior is relatively poorly understood. In fact, the Grotthus trajectories for ion recombination predicted by density functional theory do not comport well with the almost 100-fold slower diffusive trajectories observed in time-resolved spectroscopy. And, in the reverse reaction, the barrier to autoionization is not well characterized. Here we develop a self-consistent picture of both processes based on the occurrence and role of ultrashort hydrogen bonds. The predicted populations of these special pairs in bulk water are consistent with the high frequency electrodynamics of water and its pressure dependence. The rate-limiting role of the special pairs manifests in autoionization as a two-stage barrier, first to form a contact ion pair and then to separate it by one water molecule. From this configuration, similar frequencies are observed for further separation vs recombination. The requirement of ultrashort hydrogen bonds for proton transfer in autoionization is consistent with the rise in Kw with increasing pressure and points to a role for density fluctuations in autoionization events. In neutralization, the manifestation of the role of special pairs is the prolonged diffusional process observed in time-resolved spectroscopy experiments. The requirement of special pairs as transition states for proton transfer is less obvious for neutralization in isolated water chains than in the bulk liquid only because an unbroken sequence of ultrashort H-bonds is more easily formed in a 1D H-bonded chain than in a 3D H-bonded network.
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INTRODUCTION Water is a common solvent that plays essential roles in chemistry, biology, and material science. Its most important chemical property is its amphiproticity, and proton transfer processes have received commensurate attention. However, the physical complexity of bulk water makes these fundamental behaviors difficult to interpret on the molecular scale. With theoretical tools, some simplification can be had by considering the dynamics of individual proton defects, whether an excess proton or a missing proton (otherwise known as a “proton hole”). In this case, radically different approaches1,2 have led to a common picture. According to this picture, the default environment for individual defects is an Eigen-type complex3 in which the defect resides primarily on one oxygen (forming a hydronium in the case of the excess proton and a hydroxide in the case of the proton hole) which has three hydrogen bonding partners (three acceptors in the case of the hydronium and three donors in the case of the hydroxide). Proton transfer occurs when the 3-fold symmetry of this complex is broken, such that a pair of water molecules shares the defect in a Zundel-type complex (H5O2+ in the case of the excess proton and H3O2− in the case of the proton hole)4 with an ultrashort hydrogen bond (
