RING-OPENING VIBRATIONS OF SPHEROSILOXANES

The ring-opening vibrations of the spherosilasesquioxanes of the gener al formula (HSiO3/2)(2n), n = 2, 3, 4, etc., are normal modes in which all Si-O stretching and/or O-Si-O bending displacements of the consid ered ring are in phase. We have investigated the vibrational structure of the experimentally well-known H8Si8O12 and H12Si12O15 and of I-g-H 20Si20O30 and O-h-H24Si24O36, which are not yet known as isolated mole cules. The energy range of the 11 IR active, the 15 Raman active, and the three inactive ring-opening modes which belong to the 4-, 5-, and 6-ring vibrations of O-h-H8Si8O12, D-5h-H10Si10O15, I-h-H20Si20O30, an d O-h-H24Si24O36 decreases from 490-390 to 440-250 to 340-219 cm(-1). The 4-, 5-, and 6-rings are in fact built of four, five, and six Si at oms plus four, five, and six O atoms, respectively. The totally symmet ric vibrations show predominantly stretching character with one except ion, the delta(O-Si-O) line at 451 cm(-1) of H24S24O36 They occur in s pecific regions, namely, at 460-440 cm(-1) for the 4-ring, at 340-250 cm(-1) for the 5-ring, and at 220-210 cm(-1) for the 6-ring. We show t hat the ring-opening vibrations of the hydrosilasesquioxanes suggest a new way to study the pore-opening vibrations of zeolites, which simpl ifies the problem remarkably and thus leads to a better understanding of the more complex extended structures. The vibrations of the investi gated spherosiloxanes can be divided into six distinct regions denoted as nu(Si-H), nu(as)(Si-O-Si), delta(O-Si-H), nu(s)(Si-O-Si), delta(O- Si-O), and delta(Si-O-Si). This means that the concept of group freque ncies makes sense. However 10 modes were observed which do not fit int o this general scheme, namely, the modes at 481 and 446 cm(-1) (H8Si8O 12), at 455, 450, and 334 cm(-1) (H10Si10O15), at 336, 284, and 251 cm (-1) (H20Si20O30), and at 219 and 214 cm(-1) (H24Si24O36). All of them can be identified as ring-opening vibrations.