Dipole moments of amidines and electron distribution within their functional group

Abstract

Two series of N,N,N′-trisubstituted amidines were prepared, their dipole moments measured in benzene solution, and their E configuration determined from the nuclear Overhauser effect. When the configuration is established, the dipole moments can be analyzed with respect to their direction, exploiting the substituents in the para position. The vector difference between the actual dipole moment and that calculated for the classical structural formula is called the mesomeric dipole moment and serves as a measure of the n-π conjugation within the amidine group. Compared to amides and thioamides this conjugation is weaker and is also strongly affected by the substituents on nitrogen. Additional proof of the conjugation was obtained from a search in the Cambridge Structural Database which revealed a lengthened CN bond, shortened CN bond, and a significant correlation between the two bond lengths.