Isolated Se-6 rings in the voids of a weakly interacting, electroneutral, and crystalline SiO2 matrix: 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 decado decasil 3R (DDR). Raman spectroscopy in combination with theoretical ab ini tio calculations at the SCF and MP2 levels of theory have been used to assi gn 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 Ses rings are considerably higher than those of bulk Se-6, suggesting that intermole cular correlations are lost for the perfectly isolated molecules in the mic roporous host matrix. By an EXAFS analysis of Se K-spectra, Se-Se bond leng ths of 2.34 Angstrom and next-nearest Se Se distances of 3.61 Angstrom are obtained, in very good agreement with the theoretical ab initio studies. EX AFS analysis in addition reveals that the interaction between the SiO2 fram ework and the embedded Se-6 rings are weak, resulting in a novel host-guest compound with nearly unperturbed Se-6 guest molecules. The crystalline ele ctroneutral host thus acts similarly to a noble gas matrix at low temperatu res.