Vibrational spectra of trimethylsilanol: The problem of the assignment of the SiOH group frequencies

Abstract

The assignment of the SiOH group vibrations of trimethylsilanol, which is still controversial, is proposed. This assignment is based on theoretical B3LYP force field scaled using the constants of the (CH 3) 3Si group optimized to fit experimental vibrational frequencies of (CH 3) 3SiF and (CD 3) 3SiF molecules as well as the OH stretching scale factor from methanol. The ab initio force field defined in this way gives a good agreement of the theoretical vibrational frequencies of trimethylsilanol with the positions of IR and Raman bands observed in the gas phase. This force field predicts the greatest contribution of the δ SiOH coordinates to the vibration with frequency of 804 cm −1. The elimination of the coupling of the SiOH deformation with methyl rocking modes by the normal coordinate treatment of (CD 3) 3SiOH gives 832 cm −1 for silanol deformation which is in a good agreement with the 834 cm −1 value proposed earlier for the bending mode of free silanol groups. The geometry and force field of the open chain H 3SiOH trimer is computed to model the change of the δ SiOH frequencies upon formation of the hydrogen-bonded polymers. This model predicts a significant shift of SiOH bending frequencies to the 1000–1200 cm −1 range while those of SiOD to the 800–850 cm −1 range. These predictions allow us to ascribe the 1087 cm −1 band observed in the IR spectrum of crystalline (CH 3) 3SiOH and the Raman 775 cm −1 band of the liquid (CH 3) 3SiOD to deformations of the hydrogen-bonded silanol groups.