Transoid, Ortho, and Gauche Conformers of n-Si4Cl10: Raman and Mid-IR Matrix-Isolation Spectra

Calculations at the HF/6-31G*, MP2(FC)/6-31G*, and B3LYP/6-31G* levels of theory predict the existence of three pairs of enantiomeric conformers on the potential energy surface of n- Si 4 Cl 10: transoid, ortho, and gauche. At the fully optimized MP2(FC)/6-31G* level the SiSiSiSi dihedral angles (deg) and relative energies (kcal/mol) are 160.5° and 0.0 (transoid), 93.0° and 0.65 (ortho), and 48.7° and 0.27 (gauche). Matrix annealing, spectral differencing, and singular value decomposition analyses of Raman and mid-IR spectra of n- Si 4 Cl 10 in nitrogen matrix demonstrate that three distinct backbone conformers are indeed trapped in the matrices and interconvert upon annealing. Their initial relative concentrations and the details of the annealing behavior depend on the matrix isolation ratio. Their vibrational spectra have been assigned by comparison with results of MP2(FC)/6-31G* calculations and the nature of the normal modes has been analyzed in terms of calculated potential energy distributions. Annealing in concentrated matrices (N 2: n- Si 4 Cl 10 ∼ 200:1) indicates that at least in some environments, the relative stability of the conformers increases in the order gauche < ortho < transoid, in agreement with results of HF/6-31G* and B3LYP/6-31G* calculations.