Vibrational dynamics and structure of organosilanes

Movre Šapić, Iva (2011) Vibrational dynamics and structure of organosilanes. Doctoral thesis, Faculty of Science > Department of Physics.

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Abstract

Organosilanes are widely used in many industrial processes. The goal of this thesis is to make a complete vibrational and conformational analysis of aminopropylsilanetriol (APST) and 3-glycidoxypropylmethoxisilane (GPTMS), by using Raman and infrared spectra in combination with theoretical calculations. Reliable assignment enables us to follow and understand changes influenced by different variables like temperature, ultraviolet radiation and substrate. Chapter one is an introductory chapter, describing the importance of organosilanes through some examples from the literature. It briefly describes what is the experimental part of work and where was it done, and which theory was used for numerical calculations. Density functional theory is the basic theoretical background for numerical calculations done in this work. Development and new achievements of the theory are described in chapter two, along with the choice of basis functions and functionals best suited for calculations of molecular structure and vibrations. Those basis functions and functionals were used in GAUSSIAN03 program package. Experimental setup is described in chapter three. Water solutions of the molecules were purchased from a chemical company and we performed polymerization processes under different conditions. Second part of this chapter describes the instrumental setup for collecting the Raman and infrared spectra. Vibrational dynamics and structure of APST, GPTMS and aminopropylsiloxane (APS) were described in the fourth chapter. First part describes the analysis of aminopropylsilanetriol. Potential energy scanning was done for the H-O-Si-C dihedral angle. Structure with the lowest potential energy was optimized by all geometrical parameters and trans conformer was obtained. Trans conformer was then taken as the basic structure for the potential energy scanning for the Si-C-C-C dihedral angle and gauche, one more stabile conformer, was obtained. Potential energy scanning was also done for the dimer made of two trans conformers, for the Si-O-Si-C dihedral angle. Structure with the lowest energy was optimized. For both trans and gauche conformers and for dimer vibrational frequencies, Raman activities and infrared intensities were calculated. Scaled calculated frequencies were compared with the measured infrared and Raman spectra of aqueous solution in order to make spectral assignation of APST molecule. Characteristic bands were found in the Raman spectra for trans (1050 cm-1 and 453 cm-1) and gauche (1070 cm-1 and 490 cm-1) conformers and for dimer ( in IR 1025 cm-1 and 534 cm-1). Second part of the fourth chapter describes analysis of the 3-glycidoxypropilmethoxysilane. Potential energy scanning was done for three most sensitive dihedral angles: Si-C-C-C, C-C-C-O and O-C-C-O. Seven stabile conformers were found (1-ttg, 2-gtg, 3-gtg, 4-tgg, 5-tgg, 6-ttg and 7-ttt) named after the orientation of scanned dihedral angles. Conformer with the lowest energy was 6-ttg. Assignation of vibrational spectra of GPTMS molecule was done, revealing several bands characteristic for groups of conformers. Comparison of potential energy distribution for all conformers shows that changes in molecular structure influence normal modes. Monitoring the temperature dependence of the integrated Raman intensities of liquid sample enabled us to assign the Raman bands at 642 cm−1 and 1466 cm−1 to the lowest energy conformer 6-ttg, and the bands at 612 cm−1 and 1456 cm−1 to the 3-gtg conformer. The population ratio between these two conformers in the liquid phase rises from 3.29 at room temperature to 5.56 at 203 K. Third part of the chapter four analyses APST polymer, aminopropysiloxane (APS). Raman and infrared spectra of APS polymerized at PVC substrate, at room temperature are studied. Assignation of vibrational spectra was made by comparison with vibrational frequencies calculated using DFT theory. Calculations were performed for two different conformations created from silicon-oxygen ring-like structures: ladder-like structure and cubic structure. Two-ring structure was used for structural and vibrational calculations of APS. Dependence of geometrical parameters and vibrational frequency on the number of subunits in the ladder structure was also investigated. It was found that the width of the vibrational bands is increasing with the number of the subunits. Raman band at 1145 cm-1 was found to be the characteristic band for ladder structure. Influence of different conditions on polymerization of APST molecule is described in chapter five. In order to investigate influence of the temperature, polymerization was performed at three different temperatures (8°C, 23°C and 60°C). Influence of electromagnetic radiation was studied at sample polymerized at dark, and two samples exposed to wavelengths of 365 nm and 254 nm. For investigation of substrate influence solidification was performed on teflon, PVC, glass, brass and silica. Raman spectrum was taken for each polymerized sample. Differences in spectra were observed only at characteristic spectral lines of conformers. Best ordered structure was found to be the one polymerized at PVC substrate, at room temperature and in dark. Those samples only have bands characteristic for trans conformer, and Si-O-Si band has the highest intensity. Chapter six describes temperature dependence of polarized low wavenumber Raman spectra of APST polymer. That part of the spectrum contains information of macromolecular structure of amorphous materials. Spectra were recorded at different temperatures, from 300 K to 78 K, and in spectral range from 650 cm-1 to 10 cm-1. In low wavenumbers region (20 cm-1 to 200 cm-1) spectra contain broad, asymmetric continuous band, characteristic boson peak, revealing the persistence of medium range order in disordered APST polymer. On the other hand, optical anisotropy of the sample and polarized vibrational bands at 454 cm-1 and 522 cm-1 would suggest ladder-like layered structure oriented parallel to the plane of growth. Chapter seven gives conclusions of the work followed by the reference list.

Item Type: Thesis (Doctoral thesis)
Keywords: aminopropylsilanetriol ; 3-glycidoxypropylmethoxisilane ; polymerization ; DFT ; vibrational spectra ; conformations
Supervisor: Volovšek, Vesna
Date: 6 July 2011
Number of Pages: 85
Subjects: NATURAL SCIENCES > Physics
Divisions: Faculty of Science > Department of Physics
Depositing User: Gordana Stubičan Ladešić
Date Deposited: 21 Dec 2013 23:33
Last Modified: 23 Dec 2013 11:16
URI: http://digre.pmf.unizg.hr/id/eprint/43

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