The [CH5NO]+ ˙ potential energy surface: Distonic ions, lon‐dipole complexes and hydrogen‐bridged radical cations
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AbstractBy combining results from a variety of mass spectrometric techniques (metastable ion, collisional activation, collision‐induced dissociative ionization, neutralization‐reionization spectrometry, 2H, 13C and 18O isotopic labelling and appearance energy measurements) and high‐level ab initio molecular orbital calculations, the potential energy surface of the [CH5NO]+ ˙ system has been explored. The calculations show that at least nine stable isomers exist. These include the conventional species [CH3ONH2]+ ˙ and [HOCH2NH2]+ ˙, the distonic ions [OCH2NH3]+ ˙, [ONH2CH3]+ ˙, [CH2O(H)NH2]+ ˙, [HONH2CH2]+ ˙, and the ion‐dipole complex CH2NH2+ …︁ OH˙. Surprisingly the distonic ion [CH2ONH3]+ ˙ was found not to be a stable species but to dissociate spontaneously to CH2O + NH3+ ˙. The most stable isomer is the hydrogen‐bridged radical cation [HCO …︁ H …︁ NH3]+ ˙ which is best viewed as an immonium cation interacting with the formyl dipole. The related species [CH2O …︁ H …︁ NH2]+ ˙, in which an ammonium radical cation interacts with the formaldehyde dipole is also a very stable ion. It is generated by loss of CO from ionized methyl carbamate, H2NC(O)OCH3 and the proposed mechanism involves a 1,4‐H shift followed by intramolecular ‘dictation’ and CO extrusion. The [CH2O …︁ H …︁ NH2]+ ˙ product ions fragment exothermically, but via a barrier, to NH4+ ˙ HCO…︁ and to H3NC(H)O+ ˙ H˙. Metastable ions [CH3ONH2]+…︁ dissociate, via a large barrier, to CH2O + NH3+ + and to [CH2NH2]+ + OH˙ but not to CH2O+ ˙ + NH3. The former reaction proceeds via a 1,3‐H shift after which dissociation takes place immediately. Loss of OH˙ proceeds formally via a 1,2‐CH3 shift to produce excited [ONH2CH3]+ ˙, which rearranges to excited [HONH2CH2]+ ˙ via a 1,3‐H shift after which dissociation follows.
