Mesomeric and pi-moments in some hetero-conjugated compounds

A comparison of the dipole moments of adequately chosen compounds with those calculated for their classical valence structures affords more significant values for the mesomeric moments of the N-C = Y mesomeric groups (with Y= C,N,O or S) when taking into account the pi-inductive effect of the nitrogen atom on the C=Y bond. In so doing, HMO pi-diagrams show a nil charge at the central carbon atom as predicted by the contributing valence structure Z (Fig. 1). The pi-molecular diagram was also constructed for acrolein, indicating a small pi-charge at the C(2) carbon atom. Ab initio electronic structures (a 6-31G basis set) are also determined for vinylamine, formamidine, formamide and thioformamide and interpreted according to the classical resonance concept. (C) 1998 Elsevier Science B.V.