Role of hemibonding in the structure and ultraviolet spectroscopy of the aqueous hydroxyl radical

Abstract

The presence of a two-center, three-electron hemibond in the solvation structure of the aqueous hydroxl radical has long been debated, as its appearance can be sensitive to self-interaction error in density functional theory.

Hidden Hemibonding in the Aqueous Hydroxyl Radical

“…In an effort to avoid artifacts arising from ai MD in small simulation cells, we recently reported mixed quantum mechanics/molecular mechanics (QM/MM) simulations of OH(aq) in very large unit cells. 39…”

“…On the other hand, while our QM/MM simulations used a very large periodic simulation cell (1,024 water molecules), the QM region was limited to OH(H 2 O) 31 and the simulation timescale was restricted to < 5 ps, because exchange of QM and MM water molecules in the cybotactic region takes place on a timescale of only a few picoseconds. 39 While the prevailing viewpoint in the literature is that the hemibonded solvation motif is an artifact, calculations on small OH(H 2 O) n clusters suggest that that hemibonded configurations are needed to explain the electronic absorption spectrum of OH(aq). [41][42][43][44] The band maximum of that spectrum of appears at 230 nm (5.4 eV), [45][46][47][48][49][50] which constitutes an enormous blueshift compared to the à 2 Σ + ← X 2 Π transition of gasphase OH, which appears at 307 nm (4.0 eV).…”

“…Our QM/MM simulations seemed to confirm this mechanism. 39 In summary, the situation is that scant attention has been paid to hemibonded configurations in condensed-phase ai MD simulations, because they were thought to be artifacts, yet these "artifacts" (or perhaps rare events or a minority population) seem to be necessary to reproduce the ultraviolet (UV) spectroscopy of OH(aq).…”

“…57 If the hemibonded configurations are indeed rare, and ), the same level of theory used to determine spin leakage in the previous QM/MM calculations. 39 especially if the use of a hybrid functional reduces the O * • • • O distance relative to that obtained using semilocal functionals, then it is easy to see how a distinct hemibonding feature in g(r O * O ) might be hidden beneath the much larger hydrogen-bonded feature, especially in warmer water.…”

“…S3), using standard geometrydependent cutoff parameters. 39,58 The average value is n HB ≈ 2, including both the hydrogen bonds in which OH is the donor moiety and also those in which it is the acceptor; this represents under-coordination relative to the canonical hydrogen-bonded structure that is shown in Fig. 1b.…”