Accurate spectroscopic characterization of the HOC(O)O radical: A route toward its experimental identification.

A set of accurate spectroscopic parameters for the detection of the atmospherically important HOC(O)O radical has been obtained by means of state-of-the-art ab initio computations. These include advanced coupled cluster treatments, involving both standard and explicitly correlated approaches, to correctly account for basis set incompleteness and core-valence effects. Geometric parameters for the X̃2A' and Ã2A'' states and, for the ground state only, vibrationally corrected rotational constants including quartic and sextic centrifugal distortion terms are reported. The infrared spectrum of the X̃2A' state has been simulated in the 4000-400 cm-1 wavenumber interval with an approach based on second order vibrational perturbation theory that allows accounting for anharmonic effects in both energies and intensities. Finally, the vibronic spectrum for the Ã ← X̃ transition has been calculated at three different temperatures in the 9000-3000 cm-1 energy range with a time-independent technique based on the Franck-Condon approximation.