Gas phase study of the trans and gauche rotamers of 1,2-dicyanoethane, novel 1,2-dicyanodisilane and cyano(cyanomethyl)silane by ab initio and density functional methods

The gauche and trans rotamers of 1,2-dicyanoethane, novel 1,2-dicyanodisilane and cyano(cyanomethyl)silane have been studied theoretically in the gas phase. The methods used are second order Møller-Plesset theory (MP2) and density functional theory (DFT). The basis set used is 6-311++G(d,p) for all atoms. B3LYP is the functional used for the DFT method and G2/MP2 calculation has also carried out using the MP2 optimised structure. All calculations have been done using Gaussian 03W. All 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 (ΔE) between the trans and gauche rotamers. The optimised structures have been used for calculations of vibrational frequencies and these frequencies are reported with appropriate assignments. The computed parameters for 1,2-dicyanoethane compare satisfactorily with experimental literature values. However, the literature for 1,2-dicyanodisilane and cyano(cyanomethyl)silane, in terms of conformational studies, is limited and therefore the data of this work should also be appropriate for them. The results indicate that in general, the energy difference for these molecules is in the order 1,2-dicyanoethane > cyano(cyanomethyl)silane > 1,2-dicyanodisilane.