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..