Electronic effects at 2 and 7 α–position of divalent unsaturated seven membered ring R2C6H6M (M=C, Si, Ge, Sn, Pb)

Electronic effects were investigated on the singlet–triplet energy gaps of divalent unsaturated seven- membered ring R2C6H6M (M=C, Si, Ge, Sn, Pb, R= –H, -CH3, i-Pr , t-Bu) at B3LYP/6-311++G** level. All the triplet states of R2C6H6C were more stable than the related the singlet states while all the singlet states of R2C6H6M (M= Si, Ge, Sn, Pb, R= –H, -CH3, i-Pr , t-Bu) were more stable than the related triplet states. The ∠C2-M1-C7 angle of all the triplet states of R2C6H6M is larger than their singlet states. Calculated dihedral angles indicated a nonplanar structure for both singlet and triplet states of R2C6H6M. The NBO charge at heteroatom center (M) of R2C6H6M (M=C, Si, Ge, Sn, Pb, R= –H, -CH3, i-Pr , t-Bu) for both singlet and triplet states were generally increased when the bulky substituent (R = t-Bu) were placed at 2 and 7 (α–position) of seven membered ring. The chemical hardness (η) value for triplet states of R2C6H6M (M=C, Si, Ge, Sn R= –H, -CH3, i-Pr , t-Bu) were larger than the corresponding singlet states. A linear correlation was found between the LUMO–HOMO energy gaps of the singlet species, and their corresponding ΔGt–s..