First principle study of the trans and gauche rotamers of 1,2-dihalogenodisilanes (XSiH2SiH2X; X= F, Cl, Br and I) in vacuum.

The trans and gauche rotamers of 1,2-dihalogenodisilanes (XSiH2SiH2X; X= F, Cl, Br and I) have been studied in the gas phase using theoretical methods. The transition state arising from the trans-gauche isomerisation has also been modelled. The methods used are second order Moller-Plesset theory (MP2) and density functional theory (DFT). The basis set used is 6-311++G(d,p) for all atoms except that 6-311G(d,p) has been used for iodine atom only. B3LYP is the functional used for the DFT method and G2/MP2 and CCSD(T) computations have also carried out using the MP2 optimised structures. All the computations have been done using Gaussian 03W. All the structures have been fully optimised and the optimised geometries, dipole moments, moment of inertia and energies are reported. Energies of the optimised structures have been used to obtain the energy difference between the trans and gauche rotamers and the barrier of rotation. The optimised structures have been used for computations of vibrational frequencies and these frequencies are reported with appropriate assignments. It is found that the trans conformers are preferred and both the energy difference and rotational barrier increase as the size of the halogen increases. These observations have been interpreted in terms of steric or electrostatic repulsion and antiperiplanar hyperconjugation supported with NBO analysis. The findings from this work have also been compared with the analogous 1,2-dihalogenoethanes. Some of the predicted parameters compare satisfactorily with those reported in literature. The theoretical rate constant for the trans rotamer-transition state isomerisation is reported for the first time for this type of interconversion. The literature for these compounds is limited in terms of conformational investigations and therefore the data from this work should serve as a useful set for reference.