The decarbonylation of ionized formamide, HC(O)NH2+, and aminohydroxycarbene, HOCNH2+: decay via an excited state. A quantum chemical investigation

The unimolecular chemistry of the title [H3,C,N,O]+ ions has been investigated by ab initio molecular orbital calculations executed at the MR-SDCI//CASSCF/DZPP+f level of theory. Metastable formamide radical cations, H2NCH0+, abundantly lose the carbon-bonded hydrogen to produce H2NCO+. At slightly higher energies decarbonylation to NH3+ takes effect and our calculations indicate that this reaction does not proceed via a classical 1,2-hydrogen shift but rather via ion-dipole complexes. The ground state of H2NCHO+ is the 2A′ state which can be represented as the structure +NH2C(H)O′; to lose CO via a classical 1,2-hydrogen shift the NH2 group has to be rotated about the double bond and this requires considerable energy. However in the 2A″ state, which lies only 5 kcal mol−1 above the 2A′ state, the CN π molecular orbital is singly occupied and now the NH2 group may freely rotate. Decarbonylation proceeds as follows: after surface crossing 2A′ → 2A″ the single CN bond stretches, but this does not immediately lead to dissociation to H2N′+HCO+; rather at 2.6Å a fast and irreversible proton transfer takes place in the transient H2N⋯HCO+ complex, leading to NH3++ CO. An appearance energy of 11.28 eV for the decarbonylation of (ionized) formamide is predicted. Aminohydroxycarbene, HOCNH2+, rearranges to 2A′ H2NCHO+ prior to the loss of CO and H+. Our calculations are in good agreement with experiment and in addition they provide a rationalization for a large isotope effect associated with the decay of DC(O)NH2+.