Chirality is an important aspect of several industrial processes along with the aspects of life. For instance in pharmaceutical applications, the enantiomers of a chiral drug molecule can result in different pharmacology. For this reason, the validity of enantiomers’ selective preparation method is of high importance.
Asymmetric catalysis is one of the preparation methods for chiral molecules. However, in a bid to adequately understand and monitor the enantio-selectivity of these reactions, sensitive analytical methods are required, which will allow for enantiomeric excess measurements of their products under divers conditions. The most common methods for this task are based on circular dichroism and optical rotation dispersion. The circular dichroism effect, which is the difference in absorption of the right- and left-circularly polarized light for the enantiomers, can be integrated with several other forms of spectroscopy.
Circular dichroism resonance-enhanced multiphoton ionization has been applied for circular dichroism measurements on a number of single vibronic transitions of the supersonic beam-cooled (R)-(+)-1-phenylethanol. However, owing to the low molecular densities within the beam and the small anisotropy factor of 1-phenylethanol in the permille region, the accuracy of the method has to be improved significantly.
Researchers led by Dr Aras Kartouzian from the Technical University of Munich, Germany, presented a single laser pulse evaluation integrated with a twin-peak method enabled within the supersonic beam setup. They investigated with the (1+1) Circular dichroism resonance-enhanced multiphoton ionization, the anisotropy factor of the 0-0 transition as well as other ten vibrational modes of (R)-(+)-1-phenylethanol. Their research work is published in journal, Physical Chemistry Chemical Physics.
The effect of the isolated single vibronic transitions on the anisotropy factor has been exhibited for the carbonyl 3-methylcyclopentanone for now. The circular dichroism analysis of the π-π* transition of the supersonic beam with the proposed method indicated that this effect could also be found for other forms of molecules other than carbonyls. The results of the study also indicated that the anisotropy factor could also vary up to a factor of forty for varying vibronic transitions.
The research team observed the highest g-value for ring bending vibration with 1.785±0.004% and the lowest value for the ring bending with 0.041±0.003%. A comparison of the anisotropy factors for effusive and supersonic beam measurements exhibited a strong enhancement of the g-values in the former case.
Single vibronic excitation of the ring torsion mode of 3-methylcyclopentanone indicated a unique strong improvement of the anisotropy factor. No such impact was initiated by the excitation of the ring torsion mode of the (R)-(+)-1-phenylethanol. This was based on the fact that the effect of a unique vibrational mode on the molecular anisotropy is dictated by the molecular structure and the configuration of the magnetic and electric transition dipole moments.
A change of the angle between the two transition moments was of great interest. The authors observed a change of angle from more than 90˚ to less than 90˚ for the π-π* transition of the (R)-(+)-1-phenylethanol when various vibrational modes were excited. This theoretically predictable chiroptical inversion will be of great molecular research interest.
“While analysing the enantiomeric excess in an extremely dilute gas of chiral products, it is crucial to choose the vibronic transition with highest anisotropy factor” said Dr. Aras Kartouzian lead author on the paper.
Jörn Lepelmeier, José Lorenzo Alonso-Gómez, Farinaz Mortaheb, Ulrich Boesl, Ulrich Heiz and Aras Kartouzian. Chiroptical inversion for isolated vibronic transitions of supersonic beam-cooled molecules. Physical Chemistry Chemical Physics, volume 19 (2017), 21297.
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