The singlet-triplet energy gap in divalent three, five and seven-membered cyclic C2H2M, C4H4M and C6H6M (M = C, Si, Ge, Sn AND Pb)

Abstract

Total energy gaps, ΔE_t–s, enthalpy gaps, ΔH_t–s, and Gibbs free energy gaps, ΔG_t–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, ΔG_t–s, for C₂H₂M (M = C, Si, Ge, Sn and Pb) are found to be increased in the order: C₂H₂Si > C₂H₂C > C₂H₂Ge > C₂H₂Sn > C₂H₂Pb. The ΔG_t–s of C₄H₄M are found to be increased in the order: C₄H₄Pb > C₄H₄Sn > C₄H₄Ge > C₄H₄Si > C₄H₄C. Also, the ΔG_t-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.