Gauche, ortho, and anti conformations of saturated A4X10 chains:: When will all six conformers exist?

Geometries of A(4)X(10) molecules (A = C, Si; X = H, F, Cl, Br, CH3, SiH3) have been optimized at the HF/6-31G* level as a function of the AAAA dihedral angle omega. In addition to the generally known gauche and trans conformational minima, some have an additional ("ortho") minimum near omega = 90 degrees. 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 degrees 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.