Ring-Opening Vibrations of Spherosiloxanes

The ring-opening vibrations of the spherosilasesquioxanes of the general formula (HSiO 3/2 ) 2 n, n = 2, 3, 4, etc., are normal modes in which all Si−O stretching and/or O−Si−O bending displacements of the considered ring are in phase. We have investigated the vibrational structure of the experimentally well-known H 8 Si 8 O 12 and H 10 Si 10 O 15 and of I h -H 20 Si 20 O 30 and O h -H 24 Si 24 O 36, which are not yet known as isolated molecules. 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 -H 8 Si 8 O 12, D 5 h -H 10 Si 10 O 15, I h -H 20 Si 20 O 30, and O h -H 24 Si 24 O 36 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 atoms plus four, five, and six O atoms, respectively. The totally symmetric vibrations show predominantly stretching character with one exception, the δ(O−Si−O) line at 451 cm -1 of H 24 Si 24 O 36 . They occur in specific 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 that the ring-opening vibrations of the hydrosilasesquioxanes suggest a new way to study the pore-opening vibrations of zeolites, which simplifies the problem remarkably and thus leads to a better understanding of the more complex extended structures. The vibrations of the investigated spherosiloxanes can be divided into six distinct regions denoted as ν(Si−H), ν as (Si−O−Si), δ(O−Si−H), ν s (Si−O−Si), δ(O−Si−O), and δ(Si−O−Si). This means that the concept of group frequencies makes sense. However 10 modes were observed which do not fit into this general scheme, namely, the modes at 481 and 446 cm -1 (H 8 Si 8 O 12 ), at 455, 450, and 334 cm -1 (H 10 Si 10 O 15 ), at 336, 284, and 251 cm -1 (H 20 Si 20 O 30 ), and at 219 and 214 cm -1 (H 24 Si 24 O 36 ). All of them can be identified as ring-opening vibrations.