Amidines. Part 37. Comparison of substituent effects in cis and trans formamidines by ab initio 3-21G optimization of molecular structures of fluoro derivatives and their protonation products

The influence of the configuration at the CN double bond and of substitution at each site of an amidino group on the effect of substituents at the other two sites has been studied by ab initio 3‐21G optimization for eight derivatives of formamidine (all in the cis configuration at the CN double bond) with up to three fluorine substituents in different combinations, and the corresponding formamidinium cations (protonated at the imino nitrogen). The results were compared with those obtained previously for an identical set of compounds but in the trans configuration. The differences between the influence of substitution at various sites in cis and trans isomers of amidines on basicity, tautomerization, and the geometry of the molecules are discussed.