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Conformational Stability and Vibrations of Aminopropylsilanetriol Molecule

Silanols are important in a variety of industrial applications such as the manufacturing of silicones, sol-gel processes, and as silane coupling agents for use in the functionalisation of surfaces. These compounds are also the intermediate step in the chemical reaction of forming different kinds of organosiloxanes. We investigated the conformational stability and vibrational spectra of aminopropylsilanetriol (APST) in 25% aqueous solution. The FT Raman spectra were recorded in 100-4000 cm-1 frequency range. The density functional theory calculations using Becke's three-parameter exchange functional in combination with the Lee-Young-Parr correlation functional (B3-LYP) and standard 6-311+G(d, p) basis set were carried out to study the conformations and vibrational spectra of APST. Calculations reveal the existence of two stable conformers trans and gauche. The predicted energy of the more stable trans conformer is 337 cm-1 less than the energy of gauche conformer. The calculated barriers to the conformation interchange are: 1094.5, 2845 and 437.8 cm-1 for the trans to gauche, gauche to gauche and trans to gauche conformers, respectively. For the trans conformer the potential energy curve for the Si(OH)3 groups torsion in APST has been calculated changing the HOSiC dihedral angle. The barrier for the internal rotation of 3064, 6 cm-1 has been obtained. A complete vibrational assignment is proposed for both conformers based on the normal coordinate analysis, calculated IR intensities as well as Raman activities. DFT/B3LYP Cartesian force-constants matrix was transformed into local symmetry coordinates and scaled to reproduce the experimental frequencies. The scale factors are calculated by a least squares fitting to the experimental spectra of propylsilane [1]. In the Raman spectra of APST aqueous solution both the trans and gauche conformers have been identified. The analysis of normal modes related to Si(OH)3 groups has implicated the formation of hydrogen bonds between APST molecules and between the APST and water molecules. [1] J. R. Durig, C. Pan, G. A. Guirgis, Spectrochim. Acta Part A 59 797 (2000).

aminopropylsilanetriol ; vibrations ; DFT potential

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