Structural Origin and Environmental Mediation of Dispersive Optical Activity in the Vapor and Solution Phases.

摘要

Chiral molecules are ubiquitous in nature, with examples ranging from the building blocks of life such as amino acids and DNA to larger macromolecular systems. The interactions of chiral molecules with polarized light, known as optical activity, have been studied for over two centuries largely in the condensed phases, however analogous studies of the properties of isolated chiral molecules are comparatively limited. This dissertation presents a synergistic experimental and computational investigation of dispersive optical activity (i.e., circular birefringence or CB) in diverse chiral species, with particular emphasis directed towards elucidating the structural provenance of intrinsic chiroptical response and demonstrating the effect of solvation on isolated-molecule behavior. The wavelength-resolved specific rotation of several cyclic molecules has been explored under complementary solvated and vapor-phase conditions, where the latter studies have exploited the ultrasensitive technique of cavity ring-down polarimetry (CRDP) to measure specific rotation parameters at excitation wavelengths of 355 and 633 nm. Linear response calculations at analogous density-functional and coupled-cluster levels of theory have been enlisted to predict the chiroptical response of the isolated species and unravel the structural causes of observed behavior while an implicit polarizable continuum model (PCM) has been utilized to explore the influence of non-specific solute-solvent interactions on intrinsic optical activity. Targeted species include the isomeric terpenes, (S)-(+)-3-carene and ( S)-(+)-2 carene, wherein the transposition of a double bond within the bicyclic carbon skeleton imbues the latter with pronounced flexibility and an attendant enhancement of chiroptical response. Other molecules of interest include the bicyclic ketone (IR, 4R)-norbornenone, where the spatial arrangement of distal alkene and carbonyl moieties gives rise to unusually large optical rotation parameters and marked solvent effects, as well as the model cycloketones (R)-3-methylcyclopentanone and (R)-3-methylcyclohexanone, which exhibit distinct chiroptical properties stemming from differences in their ring morphology, and a conformationally flexible cyclic ether, (R)-(--)-glycidyl methyl ether. Comparison of the solvated and isolated-molecule response for these varied species has revealed the pronounced effect of solvation on the magnitude and sign of intrinsic optical activity, highlighting the importance of the solvent-free measurements as appropriate benchmarks for assessing the reliability of ab initio predictions of dispersive optical activity. The dependence of optical activity on bulk properties of the solvent environment such as dielectric constant, refractive index and polarizability has also been explored in an attempt to bridge the gap between isolated and solvated chiroptical properties.