SEMIEMPIRICAL (AM1) CALCULATIONS OF THE PROTON AFFINITIES FOR N1, N1-DIMETHYLFORMAMIDINES - SUCCESS AND FAILURES IN THE DESCRIPTION OF SUBSTITUENT EFFECTS

Proton affinities (PAs) for a series of N-1,N-1-dimethylformamidines, substituted on the imino (N-2) atom by simple alkyls or by substituents with heteroatoms directly linked to the N-2 atom or separated from it by a CH2 group are calculated by means of the semiempirical AMT method. Results are compared with those studied previously for a series of N-1,N-1-dimethyl-N-2-pheylformamidines. Optimized geometries of the neutrals, N-1 N-2- and X-protonated forms are proposed. AM1 calculations predict the N-2 atom as the favored site of protonation in the gas phase. Comparison between theoretical and experimental PA values shows that the AM1 method reproduces better the experimental proton affinities in the series of amidines with the substituent separated by a methylene or a phenyl group than in the series with the substituent linked directly to the N-2 atom. Attention is drawn on previous problems mentioned in the literature concerning the reproduction of the substitution effect on nitrogen bases by AM1.