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Intrinsic Optical Activity and Conformational Flexibility: The Role of Size-Dependent Ring Morphology in Model Cycloketones

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journal contribution
posted on 27.11.2013, 00:00 by Priyanka Lahiri, Kenneth B. Wiberg, Patrick H. Vaccaro
The optical rotatory dispersion of two monocyclic ketones, (R)-3-methylcyclopentanone [R-3MCP] and (R)-3-methylcyclohexanone [R-3MCH], has been investigated under isolated and solvated conditions to explore the role of ring size/morphology and to elucidate the impact of environmental perturbations. Vapor-phase measurements of specific rotation, [α]λT, were performed at 355/633 nm by means of cavity ring-down polarimetry while complementary solution-phase work employed a canonical discrete-wavelength polarimeter to probe five distinct solvents. The magnitude of [α]λT was found to increase upon solvation, albeit to different extents for the two species of interest, with the attendant sign switching between the solution and vapor phases for λ ≥ 510.7 nm in the case of R-3MCH. Quantum-chemical analyses suggest two low-lying conformers to exist for each ketone, distinguished by an equatorial or axial arrangement of the methyl substituent. Linear-response calculations built upon density-functional [DFT­(B3LYP)/aug-cc-pVTZ] and coupled-cluster [CCSD/aug-cc-pVDZ] frameworks gave antagonistic chiroptical parameters for these isomers, which were combined with various energy metrics in a conformer-averaging ansatz to simulate the response for a thermally equilibrated ensemble. The intrinsic behavior of R-3MCP was reproduced best by averaging DFT optical-activity predictions according to relative populations deduced from free-energy differences; however, less satisfactory agreement was realized for isolated R-3MCH molecules. The sizable circular birefringence of R-3MCP can be attributed to inherent chirality of its twisted carbon ring whereas the more modest response of R-3MCH stems mainly from the lone stereogenic center. The implicit polarizable continuum model treated solvation effects in R-3MCP with moderate success, but failed to replicate solvent-dependent trends in R-3MCH. The relationship of dispersive optical activity to bulk characteristics of the surrounding medium, including dielectric constant, refractive index, and polarizability, is discussed with the goal of bridging the gap between isolated and solvated chiroptical properties.