Ring-Opening Vibrations of Spherosiloxanes

Abstract

The ring-opening vibrations of the spherosilasesquioxanes of the general 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 considered ring are in phase. We have investigated the vibrational structure of the experimentally well-known H8Si8O12 and H10Si10O15 and of Ih-H20Si20O30 and Oh-H24Si24O36, 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 Oh-H8Si8O12, D5h-H10Si10O15, Ih-H20Si20O30, and Oh-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 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 H24Si24O36. They occur in specific regions, namely, at 460−440 cm-1 for the 4-rin...