Theoretical Study of the Gauche and Trans Conformers of SiH2X-CH2X, SiH2F-CH2Y and SiH2Y-CH2F (X = F, Cl, Br, I and Y = Cl, Br, I) in the Gas and Solution Phases

The gauche and trans rotamers of halogeno(halogenomethyl) silane (XSiH2CH2X; X = F, Cl, Br, I), fluoro(halogenomethyl) silane and halogeno(fluoromethyl) silane (SiH2F-CH2Y and SiH2Y-CH2F; Y = Cl, Br, I) have been studied in the gas phase using theoretical methods. The transition state arising from gauche-trans isomerization has also been modeled. The methods used are density functional theory (DFT) and second-order Moller-Plesset theory (MP2). B3LYP is the functional used for the DFT method. The basis set used is 6-311++G(d,p) for all atoms except that 6-311G(d,p) is used for the iodine atom only. The results indicate that the trans conformers are preferred in the gas phase and both energy difference and rotational barrier height increase as the size of the halogen increases. This study has been extended to include the solvent effect with the dielectric constant of the solvents varying from 2 to 80. The solvent effect was explored using Self-Consistent Reaction Field and the conformers have been fully optimized at the DFT/B3LYP level of theory. The net effect of a solvent is that energy difference decreases but the rotational barrier is not much affected. The findings from this work are explained in terms of different interactions and these are supported by a Natural Bond Orbital analysis.