Electronic structure of halogen-substituted methyl radicals: Excited states of CH2Cl and CH2F
Electronically excited states in CH2Cl and CH2F radicals are studied at the EOM-CCSD/6-311(3+,3+)G(3df,3pd) level of theory. Excited states' characters and changes in the electronic spectrum in the CH3->CH2F->CH2Cl sequence are interpreted in terms of a simple molecular orbital picture. The key factors determining the electronic structure of these radicals are: (i) the presence of lone pairs on the halogen; and (ii) how strongly these lone pairs are bound to the halogen. In CH2Cl, the small energy gap between the unpaired electron on carbon and the lone pair on chlorine results in additional π-bonding between C and Cl. Moreover, the relatively weak binding energy of the chlorine's lone pairs is responsible for the presence of several low lying valence states in CH2Cl. In CH2F, where the lone pairs have a considerably lower energy, no additional bonding is found. The character of two lowest valence states in CH2F is similar to that of the lowest states in CH2Cl, but the excitation energies are considerably higher. The low lying Rydberg states appear to be similar in all three radicals.