Substituent effects in chain and ring π-systems studied by core-electron binding energies calculated by density functional theory

The substituent effects of X- in 1-X-hexatrienes, which are chain π-systems with conjugated double bonds, was compared with those in PhX, which are ring π-systems, using core electron binding energy shifts (ΔCEBE) of the carbon atoms in the molecules. The ΔCEBE of C1C4 in 1-X-hexatrienes are generally close to ΔCEBE of C1C4 in PhX. The ΔCEBE of carbon atoms in the 1-X-hexatrienes are not only highly correlated to the Hammett σ substituent constants, but their numerical values are also close to each other. The core electron binding energies (CEBE) of the six carbons in 1-X-hexatriynes, which are the chain π-systems with conjugated triple bonds, were calculated by density functional theory (DFT) and the substituent effect was investigated. Average ΔCEBE of the five carbon atoms between C2 and C6 in 1-X-hexatriyne were calculated. The magnitude and sign of the average ΔCEBE provides quantitative strength and direction, electron withdrawing or donating, of a substituent X.