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 c alculations 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 equ al amounts of CH2NH2+ (17) and (CH3)(3)CNH3+ (5) are formed. Rice-Ramsperge r-Kassel-Marcus (RRKM) reaction rate calculations confirm that in spite of the substantial difference in the energy of the transition structures for C H2NH2+ and (CH3)(3)CNH3+ formation, the rate constants for these two reacti ons approach each other at thermal energies for the reactants. The proton b onded dimer between ammonia and formaldehyde formed upon i-butene loss unde rgoes a ligand-exchange reaction with ammonia forming (NE3)(2)H+ (9). The M P2/6-31G(d,p) calculations predict that all the reactions are exothermic wi th transition states below the energy of the reactants, except for the form ation of the ammonium ion which is calculated to be slightly endothermic. T he results of the more accurate G2-method calculations indicate that the la tter process probably is slightly exothermic - in accordance with the exper iments. (C) 2001 Elsevier Science B.V.