Abstract
Amides and amidines contain an NCY group which can be regarded as a resonance hybrid between a classical and a zwitterion valence structure (I and II), whose calculated dipole moments (as vectors) are M → ( I) and M → ( II) . The mesomeric moment of these compounds ( m → ) can be obtained from their their dipole moment μ → ) in two ways: (a) from M → ( I) assuming the direction of m → , indicated by the angle τ between NC and m → ; (b) by using M → ( I) and M → ( II) , with no assumption about the value of τ. Methods (a) and (b) were applied to the amides quoted in Tables 1 and 3, whilst first procedures have been used for two aliphatic amidines and for N, N-dimethyl-benzamide, -thiobenzamide and -benzamidine. The conformations in solution of sterically crowded tetramethyl-urea and -thiourea were predicted in the light of their mesomeric moments by application of an equation relating the m-values with the shape of the nitrogen pyramids (not quite flattened as in uncrowded amides and amidines), and their rotation about the relevant NC axes. The relative contribution of II was determined for all the amides quoted in Table 3 and for four amides, by taking into account Proutiére's induction terms. Finally the relation between the π-moments and mesomeric moments is outlined, and the method used for the calculation of M → ( I) and M → ( II) is presented in the Appendix.
Published Version
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