1. 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

2. 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

3. 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*

4. 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.

5. 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

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

7. Temperature effect of R3MCP conformers population

8. Temperature effects of R3MCP conformers population

9. 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 ( g/mL) at wavelengths above 580 nm..
In methanol
Alenaizan, Al-Basheer and Musa, J Mol. Struct.,1130, 2017

10. 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

11. 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 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

12. 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

13. 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

14. 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.

15. 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.

16. Thank you for your attention and Questions