Matrix isolation infrared and density functional theoretical studies of organic silanones, (CH3O)2Si=O and (C6H5)2Si=O

Transient organic silanones (CH3O)(2)Si=O (3) and (C6H5)(2)Si=O (4) were generated by vacuum pyrolysis from 3,3-dimethoxy-6-oxa-3-silabicyclo[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 studied by IR spectroscopy. Vibrational assignments in the observed IR spectra of 3 and 4 have been made by comparison with the density functional theory B3LYP/6-311G(d,p) calculated harmonic frequencies and infrared intensities for these molecules and for the other silanones, H2Si=O (1), (CH3)(2)Si=O (2), and CH3(CH3O)Si=O (5), studied earlier by matrix 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 frequency shifts as nu (M=O) in ketones, phosphinoxides, and sulfoxides. Under the conditions of vacuum pyrolysis studied the diphenylsilanone 4 was found to be more thermodynamically stable than the dimethoxy derivative 3, while the latter indicated a higher kinetic stability towards cyclooligomerization than the silanones 2 and 4. (C) 1998 Elsevier Science S.A. All rights reserved.