Absorption spectrum of OH radical in water

The influence of water on the ultraviolet absorption spectrum of OH radical is investigated with electronic structure calculations. One purpose of the work is to benchmark computational methods for their ability to treat this problem. That is done by applying a number of methods to characterization of the excited states of a variety of arrangements having OH interacting with one H2O molecule. In high-level coupled-cluster approaches, it is found that triple excitations are of considerable importance. Two promising methods based on highly efficient time-dependent density functional theory are identified that may provide qualitatively useful results, but no method is found that is both efficient and capable of providing quantitative accuracy. Another purpose of the work is to suggest a plausible interpretation of the experimental absorption spectrum of aqueous OH radical. For this purpose an accurate coupled cluster approach is applied to the various OH·H2O structures considered, along with a dielectric continuum representation of the further effects of additional bulk water. The valence transition localized on OH that is found at ∼300 nm in gas is found to be considerably broadened by hydrogen bonding interactions with water. These transitions are assigned to the very broad shoulder on the experimental aqueous spectrum that extends from ∼300 to 400 nm. The main experimental aqueous absorption band peaking at ∼230 nm is found to arise instead mainly from rare hemibonded structures, which contribute out of proportion to their relative populations by virtue of having large oscillator strengths. The region near the experimental peak and on its blue side is primarily due to charge transfer transitions that move an electron to OH from hemibonded water, while the region on the near red side of the peak is primarily due to valence transitions localized on OH that is interacting with hemibonded water.