The influence of temperature on VCD

1
The influence of temperature on VCD
Debie Elke1, Bultinck Patrick1, Herrebout Wouter2

• Department of Inorganic and Physical Chemistry,
  Ghent University, Krijgslaan 281 S-3, B-9000
  Ghent, Belgium.
• Department of Chemistry, University of Antwerp,
  Groenenborgerlaan 171, B 2020 Antwerp, Belgium

VCD is a spectroscopic technique to determine
the absolute configuration of chiral molecules in
which the differential absorption of left and
right circularly polarized is measured. In order
to do the assignment, the experimental spectra
are to be compared with the calculated ones. To
simulate spectra, using ab initio calculations,
it is very important to take into account every
stable conformer of the molecule. The weighting
factors are based on the Boltzmann
distribution. Doing VCD experiments at lower
temperature would facilitate the calculations.
The weighting factors of the least stable
conformers are supposed to decrease and for the
construction of the theoretical spectra fewer
conformers have to be taken into account.
Therefore the experimental setup has to be
adapted. A cryostatic cell has to be introduced
into the sample room of the PMA 37 module. The
cell is cooled, using liquid nitrogen reheating
is done by a heating component. The temperature
is controlled by a thermocouple. Varying the
solvents, different minimal temperatures can be
reached. Until now 3 solvents are of common use
in our research group

• INTRODUCTION

Experimental set up for recoring VCD spectra at
different temperatures.
In order to reduce loss of heat the cell is
placed under vacuum, at temperatures lower than
213K.

• Theoretical
• B3LYP/6-31G
• 2 conformers (equatorial and axial)

• EXAMPLE

Calculated VCD spectra of each conformer of R 2
methylcyclohexanone together with its weighted
spectrum
Experimental
Experimental VCD spectra of () 2
methylcyclohexanone in CDCl3.
Experimental VCD spectra of () 2
methylcyclohexanone in CD2Cl2.
Experimental VCD spectrum of () 2
methylcyclohexanone in CS2.
ResultsBecause of the small contribution of the
axial conformer, only a few small bands
originating from it are visible. Two trends can
be seen by recording VCD spectra at lower
temperatures. These phenomenons are reproducible
for each solvent and can be explained by the
decrease of the share of the axial (the least
stable) conformer.

• FURTHER INVESTIGATION

Other molecules (borneol, bornylacetate,
bornylamine, limonene and menthene) have been
investigated. Not every trend can be explained by
the change of the weighting factors. The possible
interactions arising by decrease of temperature
also have to be taken into account.
Acknowledgement Debie Elke is a Research
Assistant of the Fund for Scientific
Research-Flanders (Belgium) (F.W.O. Flanders)