THE DECARBONYLATION OF IONIZED FORMAMIDE, H-C(=O)-NH2.-C-NH2.+ - DECAY VIA AN EXCITED-STATE - A QUANTUM-CHEMICAL INVESTIGATION(, AND AMINOHYDROXYCARBENE, HO)

The unimolecular chemistry of the title [H-3,C,N,O](.+) ions has been investigated by ab initio molecular orbital calculations executed at t he MR-SDCI//CASSCF/DZPP(+) f level of theory. Metastable formamide rad ical cations, H2NCHO.+, 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 no t proceed via a classical 1,2-hydrogen shift but rather via ion-dipole complexes. The ground state of H2NCHO.+ is the (2)A' state which can be represented as the structure (NH2)-N-+=C(H)-O-.; to lose CO via a c lassical 1,2-hydrogen shift the NH2, group has to be rotated about the double bond and this requires considerable energy. However in the (2) A '' state, which lies only 5 kcalmol(-1) above the 2A' state, the CN pi molecular orbital is singly occupied and now the NH2 group may free ly rotate. Decarbonylation proceeds as follows: after surface crossing (2)A' --> (2)A '' the single C-N bond stretches, but this does not im mediately lead to dissociation to H2N. + HCO+; rather at 2.6 Angstrom a fast and irreversible proton transfer takes place in the transient H 2N.... HCO+ complex, leading to NH3.+ CO. An appearance energy of 11.2 8 eV for the decarbonylation of (ionized) formamide is predicted. Amin ohydroxycarbene, HO-C-NH2.+, rearranges to 2A' H2NCHO.+ prior to the l oss of CO and H-.. Our calculations are in good agreement with experim ent and in addition they provide a rationalization for a large isotope effect associated with the decay of D-C(=O)-NH2.+.