The singlet-triplet energy gap in divalent three, five and seven-membered cyclic C2H2M, C4H4M and C6H6M (M = C, Si, Ge, Sn AND Pb)
Total energy gaps, ΔEt–s, enthalpy gaps, ΔHt–s, and Gibbs free energy gaps, ΔGt–s, between singlet (s) and triplet (t) states were calculated for three, five and seven-membered cyclic C2H2M, C4H4M and C6H6M (M = C, Si, Ge, Sn and Pb) at B3LYP/6-311++G**. The singlet-triplet free energy gaps, ΔGt–s, for C2H2M (M = C, Si, Ge, Sn and Pb) are found to be increased in the order: C2H2Si > C2H2C > C2H2Ge > C2H2Sn > C2H2Pb. The ΔGt–s of C4H4M are found to be increased in the order: C4H4Pb > C4H4Sn > C4H4Ge > C4H4Si > C4H4C. Also, the ΔGt-s of C6H6M are determined in the order: C6H6Pb > C6H6Ge ≥ C6H6Sn > C6H6Si > C6H6C. The most stable conformers of C2H2M, C4H4M and C6H6M are proposed for both the singlet and triplet states. Nuclear independent chemical shifts (NICS) calculations were carried out for determination of aromatic character. The geometrical parameters are calculated and discussed.