Proton affinities and heats of formation of the imines CH2NH, CH2NMe and PhCHNH

Abstract

The proton affinities of the imines CH₂ NH, CH₂ NMe 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_f ⁰ values for the protonated species, to ΔH_f ⁰(CH₂ NH)= 69 ± 8 kJ mol^–1, which is significantly lower than previously reported in the literature, and to ΔH_f ⁰(CH₂ NMe)= 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 CH₂ NH.