The structures and relative energies of formamide (H2NCHO) and radical ions H2NCHO·+, H2NCOH·+ and H3NCO·+ Original Research Article

The structures of the electronic states of H2NCHO·+, its parent molecule H2NCHO and the isomers H2NCOH·+ and H3NCO·+ were optimized at the HF, MP2 and DFT levels with the 6-31G(D) basis sets, and at the CASSCF level with generally contracted basis sets of atomic natural orbital type: [8s4p]/[3s2p] for hydrogen atoms and [14s9p4d]/[4s3p2d] for carbon, nitrogen and oxygen atoms. Vibrational frequencies were calculated at the HF and hybrid HF-DFT levels to verify the characteristics of the optimized stationary points, and to provide an estimate of the vibrational zero-point energies. The energies of these species were calculated at high levels of theory including QCISD(T), G2(MP2) and CASPT2. The calculated ionization potential for H2NCHO is in reasonable agreement with the experimental value. The lowest electronic state of structure H2NCHO·+ is a σ radical (2A′) with the unpaired spin primarily centered on the oxygen atom. The electronic energy of the lowest π state of H2NCOH·+ (2A″) is 37.6 kJ/mol above as evaluated at the G2(MP2) level. The two tautomeric forms, H2NCOH·+ and H3NCO·+, are more stable than H2NCHO·+ at all correlated levels. At the G2(MP2) level, the relative stabilities in kJ/mol are: H2NCHO·+, 0.0; H2NCOH·+, −22; H3NCO·+, −14.