Dipole moments of para-substituted benzoyl chlorides and bromides, partly measured in benzene and partly recalculated from elderly literature, were analysed in terms of bond moments and additional contributions expressing the conjugation. Although the gross moments are similar, their decomposition yields different pictures for acyl chlorides and bromides. Only in the latter can significant electron transfer from the halogen be observed. On the other hand, conjugation with the benzene nucleus is strong in both aromatic acyl chlorides and bromides, but stronger in chlorides. There is agreement with the evidence from some other physical quantities but not from all.Conjugation of a double bond with a lone electron pair has been studied most thoroughly on systems X=C(R)Y. Experimental proof was based on lengthening of the formally double bond C=X, on their stretching frequencies5 and on the barriers to rotation around the C-Y An independent line of evidence was obtained from dipole moment^:^"-'^ the vector difference ( p m ) of the actual dipole moment (pexp) and that calculated for the non-polar structure l a (pcalc) may be interpreted as a charge redistribution originating from a contribution from the polar structure l b [equation (l)] .(1)shortening of the formally single bond C-Y,The absolute values of p m revealed439 that conjugation generally increases with increasing basicity of Y (dialkylamides' > amides' > esters'), and less regularly also with the electronattracting ability of X (amides > a m i d i n e~,~ but imidates = esters4). To extend these 0894-3230/89/060476-08$05 .OO 0 1989