Gauche, Ortho, and Anti Conformations of Saturated A₄X₁₀Chains:  When Will All Six Conformers Exist?

Geometries of A 4 X 10 molecules (A = C, Si; X = H, F, Cl, Br, CH 3, SiH 3 ) have been optimized at the HF/6-31G* level as a function of the AAAA dihedral angle ω. In addition to the generally known gauche and trans conformational minima, some have an additional (“ortho”) minimum near ω = 90°. This appears only within a certain critical range of sizes of substituents X. It is attributed to a splitting of the ordinary gauche minimum by 1,4 interactions between substituents, similarly as the twisting of the anti minimum from 180° is attributable to 1,3 interactions. A universal model is proposed to rationalize the appearance and subsequent disappearance of the ortho minimum as X increases in size. It contains intrinsic barriers described according to Weinhold, van der Waals interactions described by a Lennard-Jones 6−12 potential, and Coulomb charge−charge interactions.