MATRIX-ISOLATION INFRARED AND DENSITY-FUNCTIONAL THEORETICAL-STUDIES OF ORGANIC SILANONES, (CH3O)(2)SI=O AND (C6H5)(2)SI=O

Transient organic silanones (CH3O)(2)Si=O (3) and (C6H5)(2)Si=O (4) we re generated by vacuum pyrolysis from 3,3-dimethoxy-6-oxa-3-silabicycl o[3.1.0]hexane (6) and its 3,3-diphenyl derivative (7), respectively, and after being trapped in argon cryogenic matrices at 12 K directly s tudied by IR spectroscopy. Vibrational assignments in the observed IR spectra of 3 and 4 have been made by comparison with the density funct ional theory B3LYP/6-311G(d,p) calculated harmonic frequencies and inf rared intensities for these molecules and for the other silanones, H2S i=O (1), (CH3)(2)Si=O (2), and CH3(CH3O)Si=O (5), studied earlier by m atrix isolation techniques. The observed bands at 1247 cm(-1) in 3 and at 1205 cm(-1) in 4 were assigned to the Si=O stretching modes, which under the influence of the same substituents show the similar frequen cy shifts as nu (M=O) in ketones, phosphinoxides, and sulfoxides. Unde r the conditions of vacuum pyrolysis studied the diphenylsilanone 4 wa s found to be more thermodynamically stable than the dimethoxy derivat ive 3, while the latter indicated a higher kinetic stability towards c yclooligomerization than the silanones 2 and 4. (C) 1998 Elsevier Scie nce S.A. All rights reserved.