Matrix-Isolation IR and UV Spectra of Si₃H₈ and Si₄H₁₀:  Isomers and Conformers of Oligosilanes

Abstract

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.

Intuitive Understanding of σ Delocalization in Loose and σ Localization in Tight Helical Conformations of an Oligosilane Chain

“…Unlike the perfectly stable peralkylated oligosilanes forw hich much information is available, the parent oligosilanes are unstable, pyrophoric, and rarely studied. [26] (i) s Delocalization…”

IR laser-induced co-decomposition of gaseous trisilane and carbon disulfide

“…2) and carbon disulfide is transparent for the CO 2 laser radiation. The absorption in trisilane (5.8 Â 10 À2 cm À1 Torr À1 ) is due to d asym (SiH 3 ) [29,30] vibrational mode.…”

IR laser-induced co-decomposition of trisilane and thiirane for deposition of polycarbosilthiane films

“…10 0 0 transition (944.19 cm À1 ) of the CO 2 laser (Fig. 1) and the absorption of laser radiation in these compounds is due to d asym (SiH 3 ) [23,24] and d(CH 2 ) [25] vibrational modes (respective molar absorbances 6.8 Â 10 À3 cm À1 Torr À1 and 5.4 Â 10 À4 cm À1 Torr À1 ).…”

“…They lack a contribution at 411 cm À1 of elemental sulfur (S 8 , [31]) and show continuous absorptions within 600-1000 cm À1 . This region includes n(Si-C) ($800 cm À1 ), n(C-C-S) (650-900 cm À1 ) and n(S-C-S) (940-960 cm À1 ) bands [30,33] and also bands at 860 and 900 cm À1 (d(SiH n ) modes [24,32]) that are distinctly seen with deposits obtained with the trisilane-rich mixture (Fig. 5c) Table 2) in which case the n(Si-H) band is located at highest wavenumber.…”