THE GAS-PHASE CHEMISTRY OF THE METHYL CARBAMATE RADICAL CATION H2NCOOCH3+. - ISOMERIZATION INTO DISTONIC IONS, HYDROGEN-BRIDGED RADICAL CATIONS AND ION-DIPOLE COMPLEXES

The unimolecular chemistry of the methyl carbamate radical cation, H2NCOOCH+3, 1, has been investigated by a combination of mass spectrometry based experiments (metastable ion (MI), collisional activation (CA), colision-induced dissociative ionization (CIDI), neutralization-reionization (NR) spectrometry, 2H, 13C and 18O isotopic labeling, appearance energy (AE) measurements), and ab initio molecular orbital calculations, executed at the SDCI/G-31G**//4-31G level of theory and corrected for zero-point energies. These calculations indicate that besides ionized methyl carbamate there are at least seven other equilibrium structures inlcuding distonic ions and hydrogen-bridged radical cations. The most stable isomer is the hydrogen-bridged species [H2NCHO ⋯ H ⋯ OCH]+{radical dot} which is best viewed as the carbenium ion H2NCHOH+ interacting with the formly dipole. The related species [H2NCO ⋯ H ⋯ OCH2]+{radical dot} in which the hydroxyaminocarbene ion H2NCOH+{radical dot} interacts with the formaldehyde dipole is also a stable species. This hydrogen-bridged radical cation is the key intermediate in the spontaneous unimolecular dissociations of methyl carbamate ions. Experimentally, the metastable molecular ions form two sets of products, namely. H2NCHOH+ + HCO{radical dot} (the components of the most stable isomer) and [CH2O ⋯ H ⋯ NH2]+{radical dot} + CO. The minimum energy requirement paths have been located by ab initio calculations and the reactions follow multistep isomerizations. In the first step, H2NCOOCH+3{radical dot}, 1, isomerizes via a 1,4-hydrogen shift to the distonic ion H2NC(OH)OCH+2{radical dot}, 2, which then rearranges to the hydrogen-bridged radical ion [H2NCO ⋯ H ⋯ OCH2]+{radical dot}. The incipient formaldehyde molecule can then donate a hydrogen to the C atom of H2NCOH, followed by loss of HCO{radical dot} or it can accept the hydroxyl hydrogen to form a {radical dot}CH2OH radical; this radical then migrates within the electrostatic field of the H2N +CO ion towards the N atom to form the complex [H2CO ⋯ H ⋯ NH2CO+{radical dot}. This latter species, which can be viewed as a formaldehyde and a CO molecule interacting with NH+3{radical dot} lies in a shallow potential well only and sheds CO to produce [CH2O ⋯ H ⋯ NH2]{radical dot}, as observed experimentally. It is stressed that only with the aid of high level ab initio calculations could the above mechanisms be elucidated. © 1990.