Comparison of theoretical (AM1) and experimental (Fourier transform ion cyclotron resonance) basicity of N1,N1,N2-trimethylamidines in the gas phase†

Abstract

Proton affinities (PA) for series of N¹,N¹,N²-trimethylamidines (TMA) substituted on the functional carbon atom by simple alkyl, heteroalkyl and aryl groups were calculated by means of the semiempirical AM1 method and compared with those obtained previously by experiment using Fourier transform ion cyclotron resonance spectrometry. Comparison shows that the AM1 method reproduces well the experimental PA (except for the NMe₂ group), but the errors are slightly larger than for series of N¹,N¹-dimethylformamidines (FDM) substituted on the imino nitrogen atom. Effects induced by substitution at the functional carbon atom on the n–π conjugation effect in TMA decrease in the same order as for the corresponding N,N-dimethylamides (DMA). However, three subfamilies can be distinguished (separately for alkyl, heteroalkyl and aryl groups) on the plot of the experimental PA(TMA) against the experimental PA(DMA). Comparison of substituent effects at the functional carbon atom in TMA with those at the imino nitrogen atom in FDM, studied previously, shows strong differences in the transmission of the alkyl, heteroalkyl and aryl effects. Substituent effects of the heteroalkyl (NMe₂) and aryl groups, which can be conjugated by resonance with the protonation site in TMA, are about 3–7 kcal mol^–1 stronger than those in FDM.