Transoid, ortho, and gauche conformers of n-Si4Cl10: Raman and mid-IR matrix-isolation spectra

Calculations at the HF16-31G*; MP2(FC)/6-31G*, and B3LYP/6-31G* levels of t heory predict the existence of three pairs of enantiomeric conformers on th e potential energy surface of n-Si4Cl10: transoid, ortho, and gauche. At th e fully optimized MP2(FC)/6-31G* level the SiSiSiSi dihedral angles (deg) a nd relative energies (kcal/mol) are 160.5 degrees and 0.0 (transoid), 93.0 degrees and 0.65 (ortho), and 48.7 degrees and 0.27 (gauche). Matrix anneal ing, spectral differencing, and singular value decomposition analyses of Ra man and mid-IR spectra of n-Si4Cl10 in nitrogen matrix demonstrate that thr ee distinct backbone conformers are indeed trapped in the matrices and inte rconvert upon annealing. Their initial relative concentrations and the dera ils of the annealing behavior depend on the matrix isolation ratio. Their v ibrational spectra have been assigned by comparison with results of MP2(FC) /6-31G* calculations and the nature of the normal modes has been analyzed i n terms of calculated potential energy distributions. Annealing in concentr ated matrices (N-2:n-Si4Cl10 similar to 200:1) indicates that at least in s ome environments, the relative stability of the conformers increases in the order gauche < ortho < transoid, in agreement with results of HF/6-31G* an d B3LYP/6-31G* calculations.