H. Ghoudarzi Afshar - Department of Chemistry, University of Ilam, Ilam, Iran
E. Vessally - Payame Noor University (PNU), Zanjan, Iran
M. Nikoorazm - Department of Chemistry, University of Ilam, Ilam, Iran
F. Esmaeili - Department of Chemistry, University of Ilam and Petrochemical company, Ilam , Iran

Since 1991, when the first stable carbene was prepared and described [1], the chemistry of carbenes has greatly been developed. Carbenes are important intermediates in a variety of the chemical reactions. They have been extensively studied both experimentally and theoretically [2–4]. In this work, full geometry optimizations were carried out on singlet and triplet states of halogen substituted of divalent five membered rings XC4H3M (X = H, F, Cl and Br; M = C, Si and Ge) by B3LYP method using 6-311++G** basis set. Thermal energy gaps, ΔEs–t; enthalpy gaps, ΔHs–t; Gibbs free energy gaps, ΔGs–t, between singlet (s) and triplet (t) states of above structures were calculated using the GAUSSIAN 98 program. DFT calculations indicated that increase electro negativity of substituents cause to intensify ΔGs–t and decrease electro negativity of substituents lead to reduce the ΔGs–t of XC4H3M. (X = -F > -Cl > -Br > -H). Also, for example when X = F the order of values ΔGs–t is: (FC4H3C < FC4H3Si < FC4H3Ge). Studying the relationship between all parameters (different energy types, electron energies of the frontier molecular orbitals (HOMO and LUMO), chemical hardness (η), chemical potential (μ), Dipole Moment (D), electrophilicity (ω), geometry parameters (Non-dihedral) and total charge on atom) are studied. The details would be presented and discussed.