Direct estimate of the strength of conjugation and hyperconjugation by the energy decomposition analysis method

The intrinsic strength of pi interactions in conjugated and hyperconjugated molecules has been calculated using density functional theory by energy decomposition analysis (EDA) of the interaction energy between the conjugating fragments. The results of the EDA of the trans-polyenes H2C=CH-(HC=CH)(n)-CH=CH2 (n = 1-3) show that the strength of pi conjugation for each C=C moiety is higher than in trans-1,3-butadiene. The absolute values for the conjugation between Si= Si 71 bonds are around two-thirds of the conjugation between C=C bonds but the relative contributions of Delta E-pi to Delta E-orb in the all-silicon systems are higher than in the carbon compounds. The pi conjugation between C=C and C=O or C=NH bonds in H2C=CH-C(H)=O and H2C=CH-C(H)=NH is comparable to the strength of the conjugation between C=C bonds. The pi conjugation in H2C=CH-C(R)=O decreases when R=Me, OH, and NH2 while it increases when R=halogen.