Electronic structure of para aminophenoxyl radical in water

Abstract

The electronic structure of aqueous p-aminophenoxyl radical (H2NPhO•) has been examined by time-resolved resonance Raman spectroscopy and ab initio and density functional theories. The effects of hydrogen bonding and solvent reaction field on polarity of the radical have been visualized in terms of simple models. Calculations predict the dipole moment of the radical in its ground electronic state (2B1) to increase by 8(±2) D and the difference between the CN and CO bond lengths to decrease by ∼0.05 Å from gas phase to aqueous solution. This profound hydration effect converts the structure and chemical properties of H2NPhO• from a substituted phenoxyl radical in the gas phase to a semiquinone-like radical in water. The observation of vibrational modes enhanced in Raman by a non-Franck–Condon vibronic coupling mechanism has led to the identification of two very weakly absorbing electronic states of A22 symmetry in the 340–390 nm region, which borrow transition moment from close by strongly allowed electronic states of B12 symmetry at lower (∼440 nm) and higher (∼320 nm) energies. One of these transitions is parity forbidden (2B2g↔2B1g) in p-benzosemiquinone radical anion (PhO2−•) and p-phenylenediamine radical cation (Ph(NH2)2+•) and this is the first experimental evidence on energy location (3.44 eV) of this transition in an isoelectronic radical. The experiment and theory are combined to estimate the CO and CN bond lengths in H2NPhO• as ∼1.263 and ∼1.34 Å, respectively, in liquid water and ∼1.245 and ∼1.37 Å in the gas phase.