The results of CNDO/2 and PMO calculation of polyenals MeO(CH:CH)3CHO and RCH:CHCHO (R = furyl, Ph) indicate a distinction between conjugation and conjugated stem, i.e., the conjugation includes an overall change of mol. properties resulting from interaction among conjugated groups; the conjugated stem reflects only the effect of the interaction on the frontier MO's.

Key words: BENZENE, CONJUGATION, METHOXY GROUP, ALKENE P, CNDO APPROXIMATION, ALDEHYDES, MOLECULAR ORBITAL THEORY, PERTURBATION THEORY, FURAN