Proton affinities and heats of formation of the imines CH2NH, CH2NMe and PhCHNH

Abstract

The proton affinities of the imines CH₂NH, CH₂NMe and PhCHNH have been determined by Fourier transform ion cyclotron resonance (FT-ICR) mass Spectrometry to be 854 ± 8 kJ mol^–1, 880 ± 8 kJ mol^–1 and 908 ± 8 kJ mol^–1, respectively. These results lead, in conjunction with known ΔH_f⁰ values for the protonated species, to ΔH_f⁰(CH₂NH)= 69 ± 8 kJ mol^–1, which is significantly lower than previously reported in the literature, and to ΔH_f⁰(CH₂NMe)= 44 ± 8 kJ mol^–1. The proton affinities of the small imines are between the values for the corresponding amines and nitriles. The decrease in proton affinity with the change in hybridization of the nitrogen atom from sp³ to sp² to sp is discussed together with the increase in proton affinity upon introduction of a methyl group on either the carbon atom or the nitrogen atom in CH₂NH.