Singlet-triplet splitting and stability of divalent five-membered ring C4H4M, C4H6M, and C4H8M (M = C, Si, Ge, Sn, and Pb)

The sum of electronic and thermal free energy differences between singlet and triplet states (Delta G(t-s)) is calculated for C4H4M, C4H6M, and C4H8M (M = C, Si, Ge, Sn, and Pb) at B3LYP/6-311++G (3df,2p) level. Singlet-triplet splitting (Delta G(t-s)) is compared for three analogs C4H4M, C4H6M, and C4H8M. The change order of A G, is (except for M = C) C4H6M > C4H8M > C4H4M. The results of homodesmotic reaction energies show the most stability for singlet state of C4H6M with respect to C4H4M and C4H8M. In contrast, the triplet state of C4H4M (except for M = C) is the most stable with respect to C4H6M and C4H8M. (c) 2008 Wiley Periodicals, Inc.