Stabilization Studies on Divalent Five‐Membered Rings C4H4C, C4H4Si, C4H4Ge, C4H4Sn, and C4H4Pb

Energy differences, ΔX s-t (X = E, H and G) (AX s-t = X (singiet) -X (triplet) ) between singlet (s) and triplet (t) states are calculated at B3LYP/6-311++G (3df,2p). The DFT calculations show that the triplet state of C 4 H 4 C is a ground state with planar conformer respect to its corresponding nonplanar singlet state. Both singlet and triplet states of C 4 H 4 M (M = Si, Ge, Sn and Pb) have a planar conformer with the singlet ground state. Four isodesmic reactions are presented for determining the stability energies, SE. NICS calculations are carried out for C 4 H 4 M to determine the aromatic character.