Proton affinities and heats of formation of the imines CH 2 NH, CH 2 NMe and PhCHNH

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 ΔHf0 values for the protonated species, to ΔHf0(CH2NH)= 69 ± 8 kJ mol–1, which is significantly lower than previously reported in the literature, and to ΔHf0(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.