Structure and conformational stability of CH2CHCH2X (X = F, Cl and Br) molecules-post Hartree-Fock and density functional theory methods

The molecular structure and conformational stability of CH2CHCH2X (X = F, C l and Br) molecules were studied using ab initio and density functional the ory (DFT) methods. The molecular geometries of 3-fluoropropene were optimiz ed employing BLYP and B3LYP levels of theory of DFT method implementing 6-3 11+G(d,p) basis set. The MP2/6-31G*, BLYP and B3LYP levels of theory of ab initio and DFT methods were used to optimize the 3-chloropropene and 3-brom opropene molecules. The structural and physical parameters of the molecules are discussed with the available experimental values. The rotational poten tial energy surface of the above molecules were obtained at MP2/6-31G* and B3LYP/6-311+G(d,p) levels of theory. The Fourier decomposition of the rotat ional potentials were analyzed. The HF/6-31G* and MP2/6-31G* levels of theo ry have predicted the cis conformer as the minimum energy structure for 3-f luoropropene, which is in agreement with the experimental values, whereas t he BLYP/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory reverses the o rder of conformation. The Delta E values calculated for 3-chloropropene at MP2/6-31G*, BLYP/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory show that the gauche form is more stable than the cis form, which is in agreemen t with the experimental value. The same levels of theory have also predicte d that the gauche form is stable than cis for 3-bromopropene molecule. The maximum hardness principle has been able to predict the stable conformer of 3-fluoropropene at HF/6-31G* level of theory, but the same level of theory reverses the conformational stability of 3-chloropropene and 3-bromopropen e molecules and MP2/6-31G* level of theory predicted the stable conformer c orrectly. (C) 2000 Elsevier Science B.V. All rights reserved.