Solvent, Temperature and Concentration Effects on the Optical Activity of Chiral Five- and Six-Membered Ring Ketones Conformers   Watheq Al-Basheer King Fahd University of Petroleum & Minerals, Dhahran 31261, KSA 72nd International Symposium of Molecular Spectroscopy 19-23 June 2017, Champaign-Urbana, IL, USA

Examples of chiral molecules conformers R-(+)-3-methylcyclopentanone Al-Basheer et al, JPCA,111,12, 2007 Eq 90% Ax 10% R-3-methylcyclohexanone Population of Eq + Ax conformers at 298 K > 99% Alenaizan, Al-Basheer and Musa, J Mol. Struct.,1130, 2017 S-(+)-carvone Population of Eq conformers at 298 K > 97% Moreno et al, Vib. Spect., 51,2, 2009

Chromophores Circular Dichroism Carbonyl chromophores (C=O), exist in most living substances. C -- 1s2 2s2 2py1 2px1 2pz0 O -- 1s2 2s2 2py2 2px1 2pz1 n→π* transition has weak UV/VIS absorption. Circular dichroism of R-(+)-3-methylcyclopenatnone (C6H10O) n→π* n→σ*

Circular Dichroism of carvone enantiomers: solvent effect π→π* Gas phase In Isopropyl CD spectra for R and S enantiomers show equal magnitude and opposite sign. Gas phase CD spectra (Left) have more vibronic fine structure. blue shift due to solvent (isopropyl alcohol) polarity.

Solvents effects on R-3-methylcyclopentanone Circular dichroism spectra Wavelength blue and red shifts can be explained by Franck-Condon principle. Increasing solvent polarity W. Al-Bashee et al, JPCA 2007

Temperature variation of R3MCP CD spectra in solvation ET(30) (a) cyclohexane 30.9 (b) acetonitrile 45.6 (c) acetic acid 55.2 (d) water 63.1 (a) (b) (d) (c) W. Al-Bashee et al, JPCA 2007

Temperature effect of R3MCP conformers population

Temperature effects of R3MCP conformers population

Concentration effects on Optical Rotation of R-3-methylcyclohexanone In cyclohexane A switch in the sign of the OR is observed in both solvents for the lowest concentration (0.002519 g/mL) at wavelengths above 580 nm.. In methanol Alenaizan, Al-Basheer and Musa, J Mol. Struct.,1130, 2017

Temperature effects on Optical Rotation of R-3-methylcyclohexanone As the temperature increases, OR decreases, which can be explained by the increase in the population of AX on the expense of the EQ conformer. EXP in toluene   CAL in toluene T (°C) EQ AX 0.917 0.083 25 0.901 0.099 35 0.896 0.104 50 0.887 0.113 65 0.878 0.122 Predicted population of EQ and AX conformers of R-3MCH in toluene using G4 scheme

Solvent effects on Optical Rotation of R-3-methylcyclohexanone The specific OR for R-3MCH in solvents with high polarity are observed to be lower than those in low polar solvents, ORD for R-3MCH in ten organic solvents. All measurements were performed at 0.005038 g/mL (at which the solute-solute interactions are significantly reduced) and at room temperature Alenaizan, Al-Basheer and Musa, J Mol. Struct.,1130, 2017

Solvent effects on Optical Rotation of R-3-methylcyclohexanone Another approach to studying solvent effect is the multi-parametric linear fitting, which relates the observed specific OR to empirical solvent parameters. hydrogen bond donation Intrinsic rotation at temperature T hydrogen bond acceptance dipolarity/polarizability Hydrogen bonding donation and dipolarity/polarizability terms are significant. In contrast, the hydrogen bond acceptance coefficient is low, which might be attributed to the lack of active hydrogen in R-3MCH. Fitting of OR data of R-3MCH in various solvents to Kamlet's and Taft's parameters by employing equation

Solvent effects on Optical Rotation of R-3-methylcyclohexanone Equatorial conformer Axial conformer Net OR obtained by the weighted sum of the AX and EQ contributions

Summary & Conclusions The solvent, temperature and concentration effects on the OR and CD of five-and-six membered ring ketones were useful to determine energy differences and conformers population. Quantum chemical calculations were used to obtain OR and CD spectra of individual dominant conformers and consequently used to obtain net spectra after taking the conformers populations into account. A good agreement was observed upon comparing experimental and computational results.

Acknowledgements Robert Compton – The University of Tennessee, Knoxville Richard Pagni – The University of Tennessee, Knoxville Deanship of Scientific Research at King Fahd University for generous support.

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