The gas phase chemistry of the methyl carbamate radical cation H 2NCOOCH +3[rad]: isomerization into distonic ions, hydrogen-bridged radical cations and ion—dipole complexes

Abstract

The unimolecular chemistry of the methyl carbamate radical cation, H 2NCOOCH + 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 [H 2NCHO ⋯ H ⋯ OCH] + which is best viewed as the carbenium ion H 2NCHOH + interacting with the formly dipole. The related species [H 2NCO ⋯ H ⋯ OCH 2] + in which the hydroxyaminocarbene ion H 2NCOH + 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. H 2NCHOH + + HCO (the components of the most stable isomer) and [CH 2O ⋯ H ⋯ NH 2] + + CO. The minimum energy requirement paths have been located by ab initio calculations and the reactions follow multistep isomerizations. In the first step, H 2NCOOCH + 3 , 1, isomerizes via a 1,4-hydrogen shift to the distonic ion H 2NC(OH)OCH + 2 , 2, which then rearranges to the hydrogen-bridged radical ion [H 2NCO ⋯ H ⋯ OCH 2] + . The incipient formaldehyde molecule can then donate a hydrogen to the C atom of H 2NCOH, followed by loss of HCO or it can accept the hydroxyl hydrogen to form a CH 2OH radical; this radical then migrates within the electrostatic field of the H 2 N + CO ion towards the N atom to form the complex [H 2CO ⋯ H ⋯ NH 2CO + . This latter species, which can be viewed as a formaldehyde and a CO molecule interacting with NH + 3 lies in a shallow potential well only and sheds CO to produce [CH 2O ⋯ H ⋯ NH 2] , as observed experimentally. It is stressed that only with the aid of high level ab initio calculations could the above mechanisms be elucidated.