A b initio calculations at several basis set levels were used to examine the rotational potential energy function around the C-C bond of l,Pdisilylethane, also known as disilabutane (DSB). The best basis set for this system was found to be the 3-21G(*) basis, which was also used to verify the potential function around the CH2-SiHz segment in ethylmethylsilane (EMS). The torsional parameters developed by Frierson and Allinger for the C-Si-C-C fragments were shown to yield a potential in excellent agreement with those from the 3-21G(*) basis. Full 3-21G(*) geometry optimizations were performed on the O", gauche (60Β°), and 180" conformers of DSB and EMS. Data derived from the ab initio calculations on DSB were used to establish MM2 torsional and bond length parameters for the Si-C-C-Si fragment. The MM2 and ab initio structures agree well. A detailed comparison of rotational potentials and the steric energy components is presented. We also report the torsional potential of propylsilane (PSI and compare the MM2 calculated structure with experiment. Our findings suggest that the similarity of the torsional potentials of butane and DSB and the dissimilarity between butane and EMS arise from the same phenomenonvan der Waals interactions that are attractive rather than repulsive dominant.