A mechanistic investigation of the reactions between the tert-butyl methylene oxonium ion [CH2OC(CH3)3+] and ammonia:: competition between substitution, addition-elimination, and elimination

Fourier transform ion cyclotron resonance mass spectrometry and ab initio calculations have been used to investigate the reactions between CH2O(CH3)(3)(+) (1) and NH3 (2). The main primary reactions are formation of CH2O . . .H-NH+ (14) and NH4+ (15). In addition to this, Small and approximately equal amounts of CH2NH2+ (17) and (CH3)(3)CNH3+ (5) are formed. Rice-Ramsperger-Kassel-Marcus (RRKM) reaction rate calculations confirm that in spite of the substantial difference in the energy of the transition structures for CH2NH2+ and (CH3)(3)CNH3+ formation, the rate constants for these two reactions approach each other at thermal energies for the reactants. The proton bonded dimer between ammonia and formaldehyde formed upon i-butene loss undergoes a ligand-exchange reaction with ammonia forming (NE3)(2)H+ (9). The MP2/6-31G(d,p) calculations predict that all the reactions are exothermic with transition states below the energy of the reactants, except for the formation of the ammonium ion which is calculated to be slightly endothermic. The results of the more accurate G2-method calculations indicate that the latter process probably is slightly exothermic - in accordance with the experiments. (C) 2001 Elsevier Science B.V.