Amidines. Part 33. Full ab initio 3-21G optimization of the molecular structures of fluoro derivatives of formamidine and their protonation products. Quantitative structure–basicity relations of amidines
ab initio 3-21G geometry optimizations for eight derivatives of formamidine with up to three fluorine substituents, and all corresponding formamidinium cations protonated at the imino nitrogen, show strong conjugation in the amidine system between the CN double bond and the lone electron pair on the amino nitrogen atom. Substitution with fluorine at each atom of the NC–N group exerts considerable influence on this conjugation, and as a result the geometry of the molecule can be drastically changed. The introduction of the electron-withdrawing fluorine substituent at the amino nitrogen atom of formamidines leads to a tetrahedral bond arrangement at this atom. For amidines unsubstituted at the amino group and for all protonated systems, co-planarity is predicted by calculation, and all substituents lie in the plane of the NC–N group.
Calculations of energies of protonations as well as proton transfer between amidines and ammonia show that changes in basicities of amidines caused by substitution at certain sites of the amidine group depend on polar effects of substituents at other sites. This provides an explanation for the observed changes of ρ values caused by substituents at different sites in correlations of experimental pka values of amidines with Hammett σ constants. The influence of substitution at both nitrogen atoms on tautomeric equilibria in amidines is discussed.