Electronic Theses and Dissertations

Date

2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemistry

Committee Chair

Gamil Guirgis

Committee Member

Paul Simone

Committee Member

Tomoko Fujiwara

Committee Member

Timothy Brewster

Committee Member

Charles Garner

Abstract

This dissertation describes the conformational preference and energetic relationships between conformers for five- and six-membered organosilicon compounds with silicon in the ring. Organosilicon compounds are particularly of interest due to their well-documented applications in medicinal chemistry. Some of the compounds studied include 1,1-dihalo-1-silacyclopent-2-enes, 1,1-dihalo-1-silacyclopent-3-enes, 1,1-dihalo-1-silacyclohex-2-enes and their hydrogen analogues. Additionally, monosubstituted derivatives with fluoro, chloro, bromo, and isocyanato functional groups with analogous carbon-silicon five- and six-membered unsaturated rings described above, were also investigated, but not all the studies have been published as of yet. One saturated compound, 1-chloromethyl-1-fluorosilacyclohexane was also investigated computationally and with vibrational spectroscopy. Most of the organosilicon compounds investigated have not been reported in the literature, and therefore their structures were unknown. These compounds may provide novel synthetic intermediates for the formation of larger organosilicon biomolecules. Additionally, their structural characterization will contribute to the scientific body in the field of conformational analysis. Rotational or vibrational spectroscopy were used to collect experimental data for the structure of the molecules under investigation. Quantum chemical calculations using DFT, MP2, and CCSD methods were used to evaluate spectra and to identify the corresponding structural features indicated by experimental spectral data. The combination of experimental data from rotational and vibrational spectroscopy, and insight gained from quantum chemical calculations allow for the accurate determination of molecular structure for these compounds. The first publication (Chapter 2), investigated the structures and ring-puckering potential energy functions (PEFs) for 1,1-difluorosilacyclopent-2-ene (2Si(5)F2) and 1,1-dichlorosilacyclopent-2-ene (2Si(5)Cl2) using high level theoretical CCSD/cc-pVTZ computations. The structure and PEF for 1,1-dibromosilacyclopent-2-ene (2Si(5)Br2) was obtained by ab initio MP2/cc-pVTZ computations. Microwave spectra of seven isotopic species of 2Si(5)F2 were recorded on a chirped pulse, Fourier transform microwave (CP-FTMW) spectrometer in the 6-18 GHz region. The A-type, B-type, and C-type transitions were observed. The rotational constants and three quartic centrifugal distortion constants were determined for the parent,29Si,30Si, and all singly-substituted 13C isotopologues in natural abundance. This allowed for the determination of the heavy-atom structure of the ring and showed the ring to be planar for 2Si(5)F2. A comparison of the PEFs for the silane and the three dihalides shows the silane to have the stiffest puckering motion and the dibromide to be the least rigid. The second publication (Chapter 3), reports the synthesis and the CP-FTMW spectra for the molecules 1,1-difluorosilacyclopent-3-ene (3Si(5)F2) and silacyclopent-3-ene (3Si(5)). For 3Si(5), this is the first ever microwave study of the molecule and, for 3Si(5)F2, the spectra reported in this work have been combined with that of previous work in a global fit. Analyses of the spectra provide evidence that the two title compounds are planar which agrees with the high-level calculations, but the source of some anomalous splitting in the spectra has not been determined. The third publication (Chapter 4), reports the first synthesis of 1-chloromethyl-1-fluorosilacyclohexane and its structural parameters and conformational stability using ab initio and DFT computational methods. Raman and infrared vibrational spectroscopy were also employed for the molecule’s analysis. IR spectra were recorded for both gas phase and liquid sample, whereas the Raman experiments were performed in the liquid state. Additionally, low temperature matrix isolation experiments that were monitored using infrared spectroscopy were recorded after isolating the molecule in argon and nitrogen matrices. For the assignment of the experimental spectral bands, theoretical DFT/B3LYP/aug-cc-pVDZ and MP2/aug-cc-pVDZ calculations were performed. From the calculations it was found that the four lowest energy, chair type conformers – axial trans, equatorial trans, equatorial gauche and axial gauche are stable enough to be observed in the experimental spectra. The author conducted the synthesis (except for in the 1-chloromethyl-1-fluoro-1-silacyclohexane paper), aided in quantum chemical calculations, aided in microwave and infrared spectroscopy assignment, and contributed to manuscript writing for all publications.

Comments

Data is provided by the student.

Library Comment

Dissertation or thesis originally submitted to ProQuest.

Notes

Open access

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