Hye Jin Chun

Raman Spectroscopy Analysis of Botryococcene Hydrocarbons from the Green Microalga Botryococcus braunii

Botryococcus braunii, B race is a unique green microalga that produces large amounts of liquid hydrocarbons known as botryococcenes that can be used as a fuel for internal combustion engines. The simplest botryococcene (C30) is metabolized by methylation to give intermediates of C31, C32, C33, and C34, with C34 being the predominant botryococcene in some strains. In the present work we have used Raman spectroscopy to characterize the structure of botryococcenes in an attempt to identify and localize botryococcenes within B. braunii cells. The spectral region from 1600–1700 cm−1 showed ν(C=C) stretching bands specific for botryococcenes. Distinct botryococcene Raman bands at 1640 and 1647 cm−1 were assigned to the stretching of the C=C bond in the botryococcene branch and the exomethylene C=C bonds produced by the methylations, respectively. A Raman band at 1670 cm−1 was assigned to the backbone C=C bond stretching. Density function theory calculations were used to determine the Raman spectra of all botryococcenes to compare computed theoretical values with those observed. The analysis showed that the ν(C=C) stretching bands at 1647 and 1670 cm−1 are actually composed of several closely spaced bands arising from the six individual C=C bonds in the molecule. We also used confocal Raman microspectroscopy to map the presence and location of methylated botryococcenes within a colony of B. braunii cells based on the methylation-specific 1647 cm−1 botryococcene Raman shift.

Vibrational Spectra, Theoretical Calculations, and Two-Dimensional Potential Energy Surface for the Ring-Puckering Vibrations of 2,4,7-Trioxa[3.3.0]octane

2,4,7-Trioxa[3.3.0]octane (247TOO) is an unusual bicyclic molecule which can exist in four different conformational forms which are determined by the directions of the two ring- puckering motions. The vibrational assignments of 247TOO have been made based on its infrared and Raman spectra and theoretical density functional theory (DFT) calculations. The two ring-puckering motions (in-phase and out-of-phase) were observed in the Raman spectra 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 was utilized to generate a two-dimensional potential energy surface (PES) for the two ring-puckering motions. The resulting quantum state energies for this PES were then calculated in order to better understand the patterns that are produced when the PES has four energy minima at different energy values. The wave functions corresponding to the different quantum states were also calculated. The NMR spectrum of 247TOO showed the presence of the two lowest energy conformations, consistent with the results of the ab initio calculations.

Ring-Puckering Potential Energy Functions for Trimethylene Sulfide and Its Monovalent Cation

The spectra and ring-puckering potential energy function for trimethylene sulfide cation (TMS+) from vacuum ultraviolet mass-analyzed threshold ionization spectra have recently been reported. To provide an in-depth comparison of the potential function with that of trimethylene sulfide (TMS) itself, we have used ab initio MP2/cc-pVTZ calculations and DFT B3LYP/cc-pVTZ calculations to predict the structures of both TMS and TMS+ and then used these to calculate coordinate-dependent ring-puckering kinetic energy functions for both species. These kinetic energy functions allowed us to calculate refined potential energy functions of the puckering for both molecules based on the previously published spectra. TMS has an experimental barrier of 271 cm-1 and energy minima at ring-puckering angles of ±29°. For TMS+ the barrier is 60 cm-1 and the energy minima correspond to ring-puckering angles of ±21°. The lower barrier for the cation reflects the smaller amount of angle strain in the ring angles for TMS+.

Vibrational Potential Energy Surfaces in Ground and Excited Electronic States

Abstract Infrared and Raman spectra of molecules, often of vapor samples at high temperatures and/or utilizing special sample cells, have been utilized to investigate molecular structures, conformations, and vibrational potential energy surfaces (PESs) in electronic ground states. Similarly, laser-induced fluorescence (LIF) and ultraviolet spectra have provided the data for investigating electronic excited states. Theoretical ab initio and density functional theory (DFT) calculations have complemented the experimental work. The focus of the work has been on cyclic and bicyclic molecules that possess large-amplitude motions such as the ring-puckering and ring-twisting vibrations. We first discuss molecules with intramolecular π-type hydrogen bonding in cyclic and bicyclic alcohols and amines. This type of bonding is achieved only when the ring puckering or ring twisting and OH or NH 2 torsional coordinates are at their optimal positions. Second, we review the Raman spectra of 1,3-butadiene that allowed us to calculate the torsional potential energy function for this molecule. Third, we examine the ultraviolet electronic spectra of pyridine and several fluoropyridines and discuss their potential energy functions in electronic excited states. Fourth, we present our experimental data and theoretical calculations for the two-dimensional PESs of three unusual molecules. In each case, the characteristics of the energy levels and their corresponding wave functions are discussed in detail. Fifth, the PESs for the torsional vibrations of stilbenes are reviewed. Sixth, we summarize our results for the ground and excited states of several bicyclic aromatics including the remarkable 1,3-benzodioxole molecule, which possesses the anomeric effect. Lastly, we describe our results on the LIF spectra of cyclic ketones that generally possess double-minimum PESs for the carbonyl inversion vibrations in their S 1 (n, π⁎) excited states.