Electronically excited and ionized states of the CH2CH2OH radical: A theoretical study
The low lying excited electronic states of the 2-hydroxyethyl radical, CH2CH2OH, have been investigated theoretically in the range 5-7 eV by using coupled-cluster and equation-of-motion coupled-cluster methods. Both dissociation and isomerization pathways are identified. On the ground electronic potential energy surface, two stable conformers and six saddle points at energies below ~ 900 cm-1 are characterized. Vertical excitation energies and oscillator strengths for the lowest-lying excited valence state and the 3s, 3px, 3py and 3pz Rydberg states have been calculated and it is predicted that the absorption spectrum at ~ 270-200 nm should be featureless. The stable conformers and saddle points differ primarily in their two dihedral coordinates, labeled dHOCC (OH torsion around CO), and dOCCH (CH2 torsion around CC). Vertical ionization from the ground-state conformers and saddle points leads to an unstable structure of the open-chain CH2CH2OH+, cation. The ion isomerizes promptly either to the 1-hydroxyethyl ion, CH3CHOH+, or to the cyclic oxirane ion, CH2(OH)CH2+, and the Rydberg states are expected to display a similar behavior. The isomerization pathway depends on the dOCCH angle in the ground state. The lowest valence state is repulsive and its dissociation along the CC, CO and CH bonds, which leads to CH2 + CH2OH, CH2CH2 + OH, and H + CH2CHOH should be prompt. The branching ratio among these channels depends sensitively on the dihedral angles. Surface crossings among Rydberg and valence states and with the ground state are likely to affect dissociation as well. It is concluded that the proximity of several low-lying excited electronic states, which can either dissociate directly or via isomerization and predissociation pathways, would give rise to prompt dissociation leading to several simultaneous dissociation channels.