Isolated Se₆Rings in the Voids of a Weakly Interacting, Electroneutral, and Crystalline SiO₂Matrix:  Combined Experimental and Theoretical Study

For the first time a selenium species has been successfully trapped in the voids of an electroneutral SiO2 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 Se6 rings with D3d symmetry. The A1g(r) and Eg(r) mode frequencies of the matrix-isolated Se6 rings are considerably higher than those of bulk Se6, 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 SiO2 framework and the embedded Se6 rings are weak, resulting in a novel host−guest compound with nearly unperturbed Se6 guest molecules. The crystalline electroneutral host thus acts similarly to a noble gas matrix at low temperatures.