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 substituent s with heteroatoms directly linked to the N-2 atom or separated from i t by a CH2 group are calculated by means of the semiempirical AMT meth od. Results are compared with those studied previously for a series of N-1,N-1-dimethyl-N-2-pheylformamidines. Optimized geometries of the n eutrals, N-1 N-2- and X-protonated forms are proposed. AM1 calculation s predict the N-2 atom as the favored site of protonation in the gas p hase. Comparison between theoretical and experimental PA values shows that the AM1 method reproduces better the experimental proton affiniti es in the series of amidines with the substituent separated by a methy lene or a phenyl group than in the series with the substituent linked directly to the N-2 atom. Attention is drawn on previous problems ment ioned in the literature concerning the reproduction of the substitutio n effect on nitrogen bases by AM1.