Infrared and ultraviolet spectra of the gauche and anti conformers of matrix-isolated permethyl-n-tetrasilane have been obtained separately by taking advantage of thermally induced gauche-to-anti conversion and of wavelength-selective photochemical destruction of either conformer. The resolved UV spectrum of the gauche conformer provides the first piece of experimental evidence in favor of the recently proposed reinterpretation of conformational effects on tetrasilane electronic states. According to this, it is not the energy but the intensity of the lowest singlet excitation that changes dramatically as the SiSiSiSi dihedral angle is varied, as a result of an avoided crossing between s̀s̀* and s̀π* states. Implications for the general understanding of sigma conjugation in simple terms are discussed. Unconstrained MP2/6-31 G* optimization predicts the existence of a third backbone conformer (ortho), with a dihedral angle of about 90°. Its predicted (HF/3-21 G*) mid-IR spectrum is indistinguishable from that of the gauche conformer, and the matrix-isolation spectra thus provide no evidence for or against its presence.

Geometries of A4X10 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 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.

Matrix-isolation IR and UV spectra of Si 3 H 8 , i-Si 4 H 10 , and the two conformers of n-Si 4 H 10 have been recorded. A quantitative separation of the IR spectrum of n-Si 4 H 10 into contributions from the anti and gauche forms was accomplished by a combination of matrix annealing and selective monochromatic photodestruction experiments. A qualitative separation of their UV spectra was achieved as well. The IR spectra of Si 3 H 8 , i-Si 4 H 10 , and the two conformers of n-Si 4 H 10 have been assigned by comparison with results of ab initio calculations, which reproduce the frequencies and even the relative intensities quite well. The calculations predict dihedral angles ω of 180°and 57°for the anti and the gauche conformer of n-Si 4 H 10 , respectively, and confirm earlier predictions of nearly equal stability for an isolated molecule. In the matrix, the anti conformer is more stable. The conformational effects on the UV spectrum of n-Si 4 H 10 are not those anticipated from simple models of the Sandorfy or ladder C type, in that it is primarily not the energy but the intensity of the low-energy excited singlet states that depends strongly on the SiSiSiSi dihedral angle ω. This result is interpreted in terms of data from 6-in-8 CASSCF 6-31G* calculations, which predict an avoided crossing between a strongly allowed σσ* B state and a very weakly allowed σπ* B state as ω changes, with the former lower in energy at 180°and the latter lower at 0°. Consequences for attempts to understand the effects of conformation on optical spectra of polysilanes are noted.