Abstracts Chemistry

Vibrational Spectra, Theoretical Calculations, and Structures of Cyclic Silanes, 2,4,7-Trioxa(3.3.0)Octane and Botryococcenes

by Hye Jin Chun

The vibrational spectra and structures of several cyclic silanes and a bicyclic molecule have been investigated with high-level ab initio and density function theory (DFT) calculations. In addition, the Raman spectra of botryococcene hydrocarbons have been studied to help with their identification. Infrared and Raman spectra and ab initio and DFT calculations have been utilized to study 1,3-disilacyclopent-3-ene, 1,3-disilacyclopentane, 1-silacyclopent-3-ene, silacyclopentane and their derivatives. In each case the agreement between observed and calculated infrared and Raman spectra was very good. Theoretical computations have also been used to calculate the potential energy surfaces (PES) for four cyclic silanes. The calculated ring-puckering potential energy functions of 1-silacyclopent-3-ene and 1,3-disilacyclopent-3-ene had barriers of 3.8 cm^-1 and 0 cm^-1, respectively, in good agreement with experimental results. The calculated results for and 1,3-disilacyclopentane predicted ring-twisting barriers of 2493 cm^-1 (vs. 2110 cm^-1 observed) and 1395 cm^-1, respectively. The conformational energies for the bent forms were calculated to be 1467 cm^-1 (vs. 1509 cm^-1 observed) for the former and 878 cm^-1 for the latter relative to the energy of the twist minima. The vibrational assignments of 2,4,7-trioxa(3.3.0)octane have been made based on its infrared and Raman spectra and theoretical DFT calculations. The two ring-puckering motions (in-phase and out-of-phase) were observed in the Raman spectrum of the liquid at 249 and 205 cm^-1 and these values correspond well to the DFT values of 247 and 198 cm^-1. Ab initio calculations were utilized to calculate the structures and conformational energies for the four energy minima and the barriers to interconversion and the data were utilized to generate a two-dimensional PES for the two ring-puckering motions. The Raman and infrared spectra of liquid squalene, which is a building block molecule for the production of essential cellular molecules, have been collected and assigned using DFT calculations. This was helpful for analyzing the Raman spectra of botryococcus braunii. DFT calculations also assisted in understanding the Raman spectra of the botryococcenes. The spectral region from 1600-1700 cm^-1 shows C=C stretching bands specific for botryococcenes, and this is of great value for identifying the specific molecules.