For the first time a selenium species has been successfully trapped in the voids of an electroneutral SiO 2 modification, namely, in the porosil decadodecasil 3R (DDR). Raman spectroscopy in combination with theoretical ab initio calculations at the SCF and MP2 levels of theory have been used to assign the observed spectral bands exclusively to Se 6 rings with D 3d symmetry. The A 1g (r) and E g (r) mode frequencies of the matrix-isolated Se 6 rings are considerably higher than those of bulk Se 6 , suggesting that intermolecular correlations are lost for the perfectly isolated molecules in the microporous host matrix. By an EXAFS analysis of Se K-spectra, Se-Se bond lengths of 2.34 Å and next-nearest Se‚‚‚Se distances of 3.61 Å are obtained, in very good agreement with the theoretical ab initio studies. EXAFS analysis in addition reveals that the interaction between the SiO 2 framework and the embedded Se 6 rings are weak, resulting in a novel hostguest compound with nearly unperturbed Se 6 guest molecules. The crystalline electroneutral host thus acts similarly to a noble gas matrix at low temperatures.
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-Si4Cl10: 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-Si4Cl10
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 (N2:n-Si4Cl10 ∼ 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.
The Raman vibrational spectra of the hexamethyltrisilanes with X = H, F, Cl, Br, I, X 2MeSiSiMe 2 SiMe 3 C 6 H 5 and recorded at various temperatures, prove the simultaneous presence of two rotamers, anti and gauche, OCH 3 , in the liquid state. Enthalpy di †erences between the rotamers were determined as described previously. Ab initio calculations of equilibrium geometries and harmonic force constants were performed for anti and gauche with X = H, F, Cl, Br, I and Fundamental vibrations were assigned with the help of X 2 MeSiSiMe 2 SiMe 3 OCH 3 . normal coordinate analyses and potential energy distributions were calculated. The preferred conformations of these trisilanes are gauche (CSiSiSi torsional angle near 180Ä) for X = H, Br, I, and and anti for OCH 3 C 6 H 5 , X = F and Cl with enthalpy di †erences ranging from 0.25 to 2.2 kJ mol-1.
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