Contribution of the study of the properties of carbenes with advanced quantum chemistry calculations: geometries

The purpose of this work is to determine the geometric parameters of methylene and six (6) analogues in the first electronic states: singlet state (S0) and triplet state (T1) with advanced quantum chemistry methods for a better knowledge of their structures. For this reason, we used Hartree-Fock (HF), MP2, B3LYP, G3, CBS Q and CBS APNO levels of theory associated with correlated Dunning basis set (cc-pVTZ), using the Gaussian 03 suite of programs. According to this study, we observe that C H bond lengths are always larger in the S0 state than in the T1 state for the methylene; according to the results of all computational methods used. For C X (X = Cl) bond, we have the same behavior as for C H bond in the methylene. When X = F, we have an opposite behavior. Indeed, C X bond is always longer in the state T1 than in the state S0. Concerning bond angles, we observe that, the angle XCY is always smaller (100 ° 110°) in the S0 state than the T1 one (118° 135°). The valence angle increases from F to Cl; this may be explained both by the decrease of electronegativity and the steric hindrance of atoms becoming increasingly large. In the particular case of the monohydroxycarbene and dihydroxycarbene, we have a slightly more complex geometry than methylene and group of the halogenocarbenes because here, a dihedral angle is in addition to other internal drawcords.