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1
CHIRAL RECOGNITION IN NEUTRAL AND IONIC MOLECULAR COMPLEXES
Ananya Sen, Aude Bouchet, Valeria Lepere, Katia Le Barbu Debus, Anne Zehnacker Rentien
Institut des Sciences Moléculaires d’Orsay ISMO CNRS-Université Paris Sud Orsay-France
International Symposium on Molecular Spectroscopy - 68th Meeting (June 17-21 2013)
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INTRODUCTION
An object that cannot be superimposed on its mirror image is called chiral.
S R
Mirror plane
S(-)-limoneneOrange aroma
R(+)-limoneneTurpentine odour
QUININE : A CINCHONA ALKALOID
1N
H
H
OH
H
N
OMe
3 4
8 9
(-)-Quinine (Qn) (1S,3R,4S,8S,9R)
4’
* *
*
**
3’
Quinuclidine
Amino alcoholLinker.Flexibility.
Quinolinechromophore
• Antimalarial drug • Enantio-selective catalysts (hydrogenation of ketones)• Chiral stationary phase (chromatography)
3
*8
98
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QUININE: A PSEUDOENANTIOMER
(+)Quinidine (Qd)(1S,3R,4S,8R,9S)
N
H
HOH
H
N
MeO
(-)Quinine (Qn) (1S,3R,4S,8S,9R)
N
H
H
OH
H
N
OMe
PSEUDOENANTIOMERS
* *
*
**
**
**
• slightly different packing properties in the crystal.• slightly different efficiency against malaria interpreted in terms of different solubility.
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Protonated Quinine used as a fluorescence standard.(Qf =0.546 for exc=310 nm) h
OH/H 2
HO
H
h
Neutral quinine
n *
Protonated/H-bonded quinine
¹ Addition of water/ protonation no charge transfer. Locally Excited (high Qf) emission restored
*
¹Albert M Brouwer J. Phys. Chem. C 2009, 113, 11790-11795
PHOTOPHYSICS OF QUININE
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Nonpolar solvents Charge Transfer low Qf
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The spectroscopic properties of jet-cooled Quinine in gas phase.
Comparison with Vibrational Circular Dichroism studies of Quinine in solution.
OBJECTIVE
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TECHNIQUES
In the gas phase: Laser ablation and Supersonic Expansion
Resonance Enhanced Multiphoton Ionisation (REMPI)
Laser Induced Fluorescence Spectroscopy (LIF)
IR-UV Double Resonance Spectroscopy
In solution:
Vibrational Circular Dichroism
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CALCULATIONS
1. Global exploration of the potential energy surface.Molecular dynamics
2. Local optimization (B3lyp/6-31G(d,p))
3. Dispersion-corrected DFT calculations.
4. Simulations of vibrational spectrum in vacuum and in solution considering the solvation model (PCM)
Calculations performed by Dr Katia Le Barbu-Debus.
cis--open
Cis OMe
= no H bond
MOST STABLE GEOMETRY IN GAS PHASE
Identical for the two pseudo enantiomers
Open geometry with Quinuclidine N far away from the Quinoline ring.
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QUININE QUINIDINE
B3lyp-6-31G+(d,p)
LASER INDUCED FLUORESCENCE
• Same electronic spectroscopy• Same vibrational pattern AND same resonant emission
characteristic of the * transition of Quinoline (no n* transition)
• low QF
8b₁g
8a₁g
8b₁g 8a₁g5a₁g
EXCITATION EMISSION
QUININEQUINIDINE
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QUININE vs QUINIDINEProbe kept on 0-0
bendChiral linker
bend OMe
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Same (OH)Different (CH)
QUINIDINE: Probe kept on 0-0, +23 cm-1
and +45 cm-1 show no difference.
QUININE: +14 cm-1 and +5 cm-1 are hot bands or higher energy conformers ??
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QUININE vs QUINIDINE
2800 3000 3200 3400 3600 3800 4000
trans--open (8%) cis--closed (9%) cis--open (11%) cis--open (72%)
Rel
ativ
e In
tens
ity
QUININE
Wave number(cm-1)
CALCULATED VIBRATIONAL SPECTRUM
conformer not likelyShift in (OH) not observed experimentally
Trans conformer not likelyCalculated transition energies (TDDFT) does not match withexperimental value.
Hence the bands at +5 and +14 cm-1 are hot bands!
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G=1.24 kcal/molToo high for excitationwithin 20 cm-1.
VIBRATIONAL CIRCULAR DICHROISM
Difference in absorbance of left vs right circular polarized IR light
∆A=AL – AR Enantiomers have opposite spectra Sign and intensity of bands very sensitive to molecular
conformation
Calculated spectra:Contribution of different conformers
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Good mirror-image relation
Presence of higher-energy conformers, more for Quinidine at RT. Mirror-image relation lost for the delocalised modes
Exp
Calc
1
1 = aromatic CH in plane bend2 = OCH3 CH bend3 = CH bend on the linker
1 22 3
Quinine
Quinidine
RESULTS OF VCD
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STABLE GEOMETRY IN SOLUTION
cis--open trans--open
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Most abundant conformerSame in gas phase and solution.
QUININE
One additional open conformer populated at room temperature in solution. FOUR in case of Quinidine.
mol/kcal5.1G
The two pseudo-enantiomers of Quinine do fluoresce in gas phase. Pseudo-enantiomers have very similar properties in the gas phase (same structure). Difference between the two are more prominent in solution than in gas phase. Quinidine is more flexible than Quinine in solution.²
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GENERAL CONCLUSION
Role of higher energy conformers in Chiral Recognition.Role of higher energy conformers in Chiral Recognition.
² Sen et. al J. Phys. Chem. A 2012, 116(32), 8334-8344
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ACKNOWLEDGEMENTS
Dr Anne Zehnacker-Rentien
Dr Philippe Bréchignac
Dr Katia Le Barbu-Debus
Dr Valeria Lepère
Dr Debora Scuderi
Dr Aude Bouchet
And the helpful mechanics and electronic workshop team at ISMO.
THANK YOU FOR YOUR ATTENTION
Pellet
Supersonicexpansion
EXCITATION LASER
IONISATION LASER
Carrier gas Ne
Ions detected at the microchannel plates.
System of detection of ions
Axis of jet Axis of detection of ionsAxis of excitation
EXPERIMENTAL SET-UP : REMPI
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Ablation laser