Electronic structure of para aminophenoxyl radical in water

摘要

The electronic structure of aqueous p-aminophenoxyl raidcal (H_2NPhO~centre dot) 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 (~2B_1) to increase by 8(+-)2 D and the difference between the CN and CO bond lengths to decrease by approx 0.05 A from gas phase to aqueous solution. Thi sprofound hydration effect converts the structure and chemical properties of H_2NPhO~centre dot 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 ~2A_2 symmetry in the 340-390 nm region, which borrow transition momen fromclose by strongly allowed electronic states of ~2B_1 symmetry at lower (approx 440 nm) and higher (approx 320 nm) energies. One of these transitions is parity forbidden (~2B_2_g<->~2B_1g) in p-benzosemiquinone radical anion (PhO_2~-centre dot) and p-phenylenediamine radical cation (Ph(NH_2)_2~+centre dot) and this is the first experiemtnal 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 H_2NPhO~centre dot as approx 1.263 and approx 1.34 A, respectively, in liquid water and approx 1.245 and approx 1.37 A in the gas phase.