EXPERIMENTAL (FT-ICR) VS CALCULATED (AM1) GAS-PHASE BASICITIES OF N1,N1-DIMETHYL-N2-PHENYLFORMAMIDINES - SUBSTITUENT AND SOLVENT EFFECTS

Gas-phase basicities (as GBs) for a series of 11 N1,N1-dimethyl-N2-phenylformamidines are calculated by means of the semiempirical AM1 method and compared with those experimentally obtained from proton transfer equilibria measurements using FT-ICR mass spectrometry. Geometries of the neutral, N1-, N2- and X-protonated forms are discussed. AM1 calculations confirm the earlier conclusion based on experimental results that the N2 atom is the favoured site of protonation in the gas-phase. Its basicity is higher by ca. 25 kcal/mol than that of the N1 atom. The AM1 method reproduces satisfactorily the experimental gas-phase basicities in the series under study. Calculated and measured GB of formamidines are linearly correlated to the substituent polarizability, field and resonance effects. As compared to the aza group in pyridines the formamidine group is more sensitive to the transmission of the field, than the resonance effect. Sensitivity of the formamidine group to substituent effects is very close to that of the amino group in N,N-dimethylanilines. Substituent effects in the gas phase are compared with those found previously in solution.