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

Gas phase reactions of ammonia with the oxonium ions, ROCH2+ (R = H, CH3, C 2H5, n-C3H7, i-C3H7) have been investigated with Fourier transform ion cycl otron resonance mass spectrometry and theoretical methods (ab initio quantu m chemistry, Rice-Ramsperger-Kassel-Marcus theory). In all instances two re actions are observed to occur in competition: addition/elimination, which g ives CH2NH2+ + ROH, and substitution, which gives RNH3+ + CH2O. With the ex ception of R = H, the rare of CH2NH2+ formation is by far faster than RNH3 formation. The experimental observations are rationalised by the model cal culations, which also show that the more exothermic the overall reaction is , the lower is the barrier. This clear trend in reactivity is dictated by t he electronic properties of the R groups. For R = i-C3H7 an additional rout e for loss of formaldehyde was found. For the first time the prototype reac tion CH2OH+ + NH3 --> CH2NH2+ + H2O has been observed. (C) 2000 Elsevier Sc ience B.V.