Mechanisms of reactions between ammonia and methylene oxonium ions: immonium ion formation versus transfer of the oxygen substituent

Abstract

Gas phase reactions of ammonia with the oxonium ions, ROCH 2 + (R = H, CH 3, C 2H 5, n-C 3H 7, i-C 3H 7) have been investigated with Fourier transform ion cyclotron resonance mass spectrometry and theoretical methods (ab initio quantum chemistry, Rice-Ramsperger-Kassel-Marcus theory). In all instances two reactions are observed to occur in competition: addition/elimination, which gives CH 2NH 2 + + ROH, and substitution, which gives RNH 3 + + CH 2O. With the exception of R = H, the rate of CH 2NH 2 + formation is by far faster than RNH 3 + formation. The experimental observations are rationalised by the model calculations, which also show that the more exothermic the overall reaction is, the lower is the barrier. This clear trend in reactivity is dictated by the electronic properties of the R groups. For R = i-C 3H 7 an additional route for loss of formaldehyde was found. For the first time the prototype reaction CH 2OH + + NH 3 → CH 2NH 2 + + H 2O has been observed.