Electron affinities and electronic structures of o-, m-, and p-hydroxyphenoxyl radicals: a combined low-temperature photoelectron spectroscopic and Ab initio calculation study.

Hydroxyl substituted phenoxides, o-, m-, p-HO(C(6)H(4))O(-), and the corresponding neutral radicals are important species; in particular, the p-isomer pair, i.e., p-HO(C(6)H(4))O(-) and p-HO(C(6)H(4))O*, is directly involved in the proton-coupled electron transfer in biological photosynthetic centers. Here we report the first spectroscopic study of these species in the gas phase by means of low-temperature photoelectron spectroscopy (PES) and ab initio calculations. Vibrationally resolved PES spectra were obtained at 70 K and at several photon energies for each anion, directly yielding electron affinity (EA) and electronic structure information for the corresponding hydroxyphenoxyl radical. The EAs are found to vary with OH positions, from 1.990 +/- 0.010 (p) to 2.315 +/- 0.010 (o) and 2.330 +/- 0.010 (m) eV. Theoretical calculations were carried out to identify the optimized molecular structures for both anions and neutral radicals. The electron binding energies and excited state energies were also calculated to compare with experimental data. Excellent agreement is found between calculations and experiments. Molecular orbital analyses indicate a strong OH antibonding interaction with the phenoxide moiety for the o- as well as the p-isomer, whereas such an interaction is largely missing for the m-anion. The variance of EAs among three isomers is interpreted primarily due to the interplay between two competing factors: the OH antibonding interaction and the H-bonding stabilization (existed only in the o-anion).