Computational and experimental studies using absorption spectroscopy and vibrational circular dichroism

Abstract

Listen

Translate article

Chapter one provides an introduction to the general area of study and the general basis for the research to be performed. It concludes with a brief mathematical description of the computational equations used to calculate and measure vibrational circular dichroism (VCD).Chapter two evaluates four chemically related compounds and provides detailed analyses of the vibrational spectra obtained by performing a combination of experimental and theoretical chemical techniques. For 1,4-dithiane the ground state was found to possess C2h symmetry. A high energy form of 1,4-dithiane possessing D2 was also discovered. At the ground state, 1,4-thioxane was found to posses C2 symmetry. Two high energy forms of 1,4-thioxane possessing C1 and C2 symmetries were found using quantum chemical techniques. For di-vinyl sulfoxide, the ground state was found to posses CS symmetry. Three high energy forms of di-vinyl sulfoxide were found possessing CS, C’1 and C’S symmetries using quantum chemical techniques. The theory predicts di-vinyl sulfone to posses C2 symmetry in the ground state. This compound was found to possess two high energy forms with CS and C1 symmetries using quantum chemical techniques.For all compounds, a detailed thermodynamic analysis was performed. Enthalpies, entropies and free energy were derived and compared. It is concluded, both MP2 and DFT/B3LYP yield a good description of the vibrational modes and thermodynamics for the compounds researched.For chapter three, a detailed analysis of α-d-alanine, α-l-alanine, β-l-mannose and α-l-mannose were performed using a combination of experimental and theoretical techniques. Assignments for alanine were compared to the purely theoretical study performed by Tajkhorshid and coworkers. It was determined that alanine as a zwitterion hydrogen bounded to four water molecules is the lowest energy conformer. In water (pH 6.5) amide I bands are not observed. Potassium bromide presses (KBr-pellets) for α-dalanine, α-l-alanine and α-l-mannose compared favorably with literature. Computational VCD spectra differed with theoretical results suggesting a need for the proper damping function to treat the results thus improving agreement between computational and experimental data. It is concluded, DFT/B3LYP yields a good description of the VCD for α-d-alanine, α-l-alanine, β-l-mannose and α-l-mannose.Chapter four presents, the first time, VCD for several compounds. Five of the compounds have never before been synthesized. These compounds were measured to determine environmental effects at and around the chiral (stereo) center. It was determined for the first time that the electron influence on the chiral center can be recorded in the VCD. Results warranted the measurement of 25+ additional compounds in an effort to better understand observations from the initial measurements.Experimental data supports the general conclusion “electron-donating groups intensify the VCD signal by forcing conjugation or electron cloud contribution towards the chiral center.” Electron withdrawing groups weaken the…