Valdemaras Aleksa

The conformations of bromomethyl dimethyl silane and bromomethyl dimethyl silane-d1 as studied by vibrational spectroscopy and by ab initio calculations

Abstract Bromomethyl dimethyl silane, CH 2 Br(CH 3 ) 2 SiH (BDS), and its deuterated analogue, CH 2 Br(CH 3 ) 2 SiD (BDSD), were synthesised for the first time. Raman spectra of both liquids were recorded at various temperatures between 295 and 160 K and spectra of the amorphous and crystalline solids were obtained. Infrared spectra were recorded in the vapour phase and in the amorphous and crystalline states in the range 4000–60 cm −1 . Additional infrared spectra of the compounds isolated in argon and nitrogen matrices were recorded at 5 K before and after annealing to temperatures in the range 15–35 K. The compounds exist in anti and gauche conformers due to restricted rotation of the CH 2 Br group. Raman temperature studies gave an average Δ H conf 0 ( gauche–anti ) of 0.6±0.3 kJ mol −1 , anti being the low energy conformer in the liquid and in the matrices. Two different crystals were observed for these molecules after careful annealing of the amorphous solid in the range 110–150 K: one crystal (I) containing molecules in the gauche conformation and one crystal (II) with molecules in the anti conformation. For the parent molecule, both of these crystals were repeatedly investigated in the infrared and in the Raman spectra, while crystallisation was much more difficult to achieve for the deuterated compound. However, two different crystals (I, III) both containing the gauche conformer, was observed in the Raman spectra and one with anti (II) in the infrared spectrum of the deuterated compound. Ab initio calculations at the HF/6-311G* level gave vibrational frequencies and Raman and infrared intensities for both conformers. Complete assignments were made for the anti and gauche conformer spectra of both molecules.

NMR and Raman Spectroscopy Monitoring of Proton/Deuteron Exchange in Aqueous Solutions of Ionic Liquids Forming Hydrogen Bond: A Role of Anions, Self-Aggregation, and Mesophase Formation

The H/D exchange process in the imidazolium-based room temperature ionic liquids (RTILs) 1-decyl-3-methyl-imidazolium bromide- and chloride ([C10mim][Br] and [C10mim][Cl]) in D2O solutions of various concentrations was studied applying (1)H, (13)C NMR, and Raman spectroscopy. The time dependencies of integral intensities in NMR spectra indicate that the H/D exchange in [C10mim][Br] at very high dilution (10(-4) mole fraction of RTIL) runs only slightly faster than in [C10mim][Cl]. The kinetics of this process drastically changes above critical aggregation concentration (CAC). The time required to reach the apparent reaction saturation regime in the solutions of 0.01 mole fraction of RTIL was less 10 h for [C10mim][Br], whereas no such features were seen for [C10mim][Cl] even tens of days after the sample was prepared. The H/D exchange was not observed in the liquid crystalline gel mesophase. The role of anions, self-aggregation (micellization), and mesophase formation has been discussed. Crucial influence of Br(-) and Cl(-) anions on the H/D exchange rates above CAC could be related to the short-range ordering and molecular microdynamics, in particular that of water molecules. The concept of the conformational changes coupled with the H/D exchange in imidazolium-based ionic liquids with longer hydrocarbon chains can be rejected in the light of (13)C NMR experiment. The revealed changes in (13)C NMR spectra are caused by the secondary ((13)C) isotope effects not being the signal shifts due to the conformational trans-gauche transition.

The conformers of chloromethylmethyldifluorosilane studied by vibrational spectroscopy and ab initio methods

Chloromethylmethyldifluorosilane (CH2ClCH3SiF2) was synthesized for the first time. Raman spectra of the liquid were recorded at various temperatures between 298 and 175 K and spectra of the amorphous and crystalline solids were obtained. Infrared spectra were recorded in the vapour, amorphous and crystalline solid phases in the MIR and FIR regions. Additional MIR spectra of the compound isolated in argon and nitrogen matrices were obtained at 10 K. The spectra of chloromethylmethyldifluorosilane showed the existence of two conformers, anti and gauche, present in the vapour and in the liquid. By careful annealing the amorphous solid formed by depositing the vapour on a cold Cu finger (Raman) or on CsI or silicon windows (infrared) at 80 K, two different crystals were formed. One of these, obtained after annealing to 125 K (crystal I), contained the anti conformer; the other, achieved after annealing to ca 170 K (crystal II), contained molecules in the gauche conformation. If the liquid was cooled, crystal II was formed, suggesting this crystal to be stable, whereas crystal I appeared to be metastable. The anti and gauche conformers had approximately the same enthalpy in the liquid, giving a negligible ΔH (gauche–anti) equal to 0.2 ± 0.4 kJ mol−1, but gauche was the conformer of the stable crystal II. Intensity variations in the matrix spectra after annealing indicated that gauche was the low-enthalpy conformer in both matrices and probably also in the vapour. These experimental data make a very reliable assignment of the conformer bands possible. Ab initio calculations were performed at the HF/6–311G* level, from which optimized geometries, infrared and Raman intensities and scaled vibrational wavenumbers for the anti and gauche conformers were obtained. The conformational energy derived was 5.9 kJ mol−1, with gauche being the low-energy conformer. Copyright

Conformational Stability, Raman and Infrared Spectra, Vibrational Assignment, and Ab Initio Calculations of Allyltrifluorosilane

The Raman spectra (3500 to 30 cm−1) of allyltrifluorosilane, CH2CHCH2SiF3, in the liquid with quantitative depolarization ratios and solid states, and the infrared spectra (3500 to 30 cm−1) of the gas and solid have been recorded. Additionally, the mid-infrared spectra of the sample dissolved in liquified xenon as a function of temperature (−100° to −55°C) have been recorded. All of these data indicate there are two conformers, the more stable gauche rotamer and a very small amount of the cis conformer in the fluid states, but only the gauche form remains in the polycrystalline solid. The variable temperature studies of the infrared spectrum of the xenon solution indicate a relatively large enthalpy difference of 354±30 cm−1 (4.23±0.36 kJ/mol) between the conformers. The fundamental frequencies for the asymmetric (54 cm−1) and SiF3 (48 cm−1) torsions for the gauche conformer were observed in the far infrared spectrum, and from the SiF3 torsional frequency the barrier to internal rotation is calculated to have a value of 525 cm−1 (6.28 kJ/mol). A complete vibrational assignment is presented for the gauche conformer that is consistent with the predicted wavenumbers utilizing the force constants from ab initio MP2/6-31G* calculations. The optimized geometries, conformational stabilities, harmonic force fields, infrared intensities, Raman activities, depolarization ratios, and vibrational wavenumbers have been obtained from RHF/6-31G* and/or MP2/6-31G* ab initio calculations. These quantities are compared to the corresponding experimental quantities when appropriate as well as with corresponding results for some similar molecules.

Structural Studies of 1,1-Dimethyl-2-oxy-1-silacyclohexane by Means of Matrix Isolation Infrared Absorption Spectroscopy

Infrared spectra of gaseous, liquid, and matrix-isolated samples of newly synthesized 1,1-dimethyl-2-oxy-1-silacyclohexane were recorded. Raman spectra of 1,1-dimethyl-2-oxy-1-silacyclohexane in liquid and solid states were obtained in the temperature range from 170 to 340 K. Ab initio HF and DFT B3LYP calculations were performed in order to determine the possible conformations of 1,1-dimethyl-2-oxy-1-silacyclohexane and to make accurate assignment of the vibrational spectral bands. The study confirms the existence of only one chair-type conformer of 1,1-dimethyl-2-oxy-1-silacyclohexane.

The vibrational spectra, including matrix isolation, conformations and ab initio calculations of bromomethyl dimethyl chlorosilane

Abstract A vibrational spectroscopic study of bromomethyl dimethyl chlorosilane (CH 2 Br(CH 3 ) 2 SiCl) was carried out. Infrared spectra of the vapour, the amorphous and crystalline solids at liquid nitrogen temperature, and spectra of argon and nitrogen matrices (1:1000) at about 5 K were recorded. Raman spectra of the liquid were obtained at five temperatures between 295 and 190 K, and spectra of the crystalline solid were recorded. Owing to restricted rotation around the CSi bond, the compound apparently exists as anti and gauche conformers. Approximately five IR and Raman bands present in the fluid phases vanished upon crystallization. From intensity variations with temperature in the Raman spectra of the liquid, a ΔH o value of 1.0 ± 0.4 kJ mol −1 was obtained. The high energy conformer bands did not vanish in the matrix spectra after annealing to approximately 39 K in the argon matrix, suggesting a barrier higher than 10 kJ mol −1 . Ab initio calculations were carried out with the gaussian 94 program using the basis sets HF/3-21G∗ and HF/6-311G∗; optimized geometries, IR and Raman intensities, and the vibrational frequencies for the anti and gauche conformers were calculated. After appropriate scaling, reasonably good agreement was obtained between the experimental and calculated wavenumbers for the anti and gauche conformers, suggesting the anti conformer to be the more stable and present in the crystal.