Development and Application of Computational Tools to Study Optical Rotation of Chiral Molecules in Isotropic Media
University of Kansas
Copyright held by the author.
MetadataShow full item record
The study of optically active molecules has been continuous for over two centuries, due to the role chiral molecules play as both the building blocks of life, and therefore their importance in developing pharmaceuticals. Theoretical calculations to study chiroptical properties, such as optical rotation (OR), electronic circular dichroism (ECD), vibrational circular dichroism (VCD), and Raman optical activity (ROA), have only been developed in the last two decades. Since then, two fundamental questions have dominated research of chiral molecules. The first question lies in successfully relating the magnitude and sign of chiral properties to a structure, as there is no chemically intuitive way to determine, without performing measurements or calculations, if the OR of a given chiral molecule will be small or large, positive or negative. The second relates to the effect of solvation, as we currently do not know the root causes of the changes induced in the OR by a change in the environment. We investigate the structure-property relationship by examining helicenes, an interesting class of chiral molecules due to their intrinsic structural chirality (an unfunctualized helicene has no chiral centers) and their large OR. We used a method that decomposes the calculated OR into occupied-virtual MO pairs, and allows us to investigate what drives the large chiroptical response of helicenes. We examined the effect of the pitch, which is the distance between one full turn of a helix, and the length of helicenes. We then functionalized them with electron withdrawing and electron donating groups to investigate how this changed the OR. We find that helicene OR is driven by large magnetic dipoles and that we can characterize the type of contributions to OR into three subgroups by the general angle of the magnetic dipole with respect to the helical axis. We also extend our method to localized orbitals, which present the orbitals in the more chemically intuitive form of bonds and lone pairs. Next, we investigated solvation effects in concert with the Vaccaro group at Yale University. They measured gas and solution phase OR values for a series of rigid chiral molecules. We wanted to investigate how capable the polarizable continuum model (PCM) was at reproducing solvent effects and trends on OR. We performed calculations at a series of wavelengths and in a series of solvents, and compared our results to the calculated values. Gas phase calculations agreed very well with experimental values; however, PCM was unable to reproduce not only the solvent shifts, but also the trends in solvent shifts. We then calculated zero point vibrational corrections (ZPVCs) to the OR in an effort to improve our results, with mixed success. We also worked to reduce the cost of computational calculations of OR, as cheaper calculations would allow us to investigate larger or more complicated systems, including better descriptions of solvent effects. Our efforts were two-fold. First, we developed a method to reduce the cost of the linear response equations by removing a portion of the orbitals prior to the solution of the coupled perturbed equations. We tested a variety of cutoff types and magnitudes with two basis sets and two functionals, and found that despite discarding large portions of the MOs, we maintained high accuracy and showed large speedups. Next, we optimized the exponents of a small basis set for OR calculations by taking the diffuse functions of large basis sets and appending them to a smaller one. We performed multiple tests with our new basis sets on a variety of molecules with OR ranging from 4,000 deg dm^(-1) (g/mL)^(-1). The resulting basis sets were highly accurate, capable of producing values similar to the larger basis sets at a reduced cost. Finally, we collaborated with the Avarvari group at the Université d'Angers, who study functionalized helicenes. They were interested in synthesizing helicenes that could act as circularly polarized light (CPL) emitters. They began by synthesizing for the first time helicenes with a strong electron withdrawing group (benzothiadiazole, BTD), which they found were not emissive. We performed calculations of electronic and chiroptical spectra to characterize the transitions, and were able to show that the fluorescence quenching was likely due to intersystem crossing in their helicenes. Their next series of helicenes were highly emissive, making them likely candidates for CPL emitters, and due to the agreement with our calculated spectra, we were again able to characterize the transitions.
- Chemistry Dissertations and Theses 
- Dissertations 
Items in KU ScholarWorks are protected by copyright, with all rights reserved, unless otherwise indicated.
We want to hear from you! Please share your stories about how Open Access to this item benefits YOU.