Spectroscopic Characterization of the First and Second Excited States of the HOSO Radical.

The spectroscopic properties of the ground and first two excited states of the HOSO radical are investigated using the internally contracted multireference configuration interaction method, including the Davidson correction (MRCI+Q) and explicit treatment of the electron correlation (MRCI-F12). The vertical and adiabatic excitation energies are also determined. The results reveal that both the 1 2A and 2 2A electronic states contain minima in their potential energy surfaces. The first excited state 1 2A possesses a nonplanar structure and has an adiabatic excitation energy of 1.45 eV (855 nm), lying in the near-infrared region. The second excited state 2 2A has a planar geometry and an adiabatic excitation energy of 2.91 eV (426 nm) existing in the visible region. The calculated oscillator strengths for the vertical electronic excitations to the 1 2A (327 nm) and 2 2A (270 nm) states are 0.003 and 0.022, respectively, indicating experimental intensity should be observed. The small but non-negligible Franck-Condon factors for excitations ∼300 nm, and the broad and intense absorption feature in the 225-275 nm region suggest that detection of the HOSO radical with electronic spectroscopy may be feasible.