I have recorded the VCD (vibrational circular dichroism) spectra of two enantiomers of a chiral molecule. One enantiomer showed all negative bands within the fingerprint region while the other enantiomer showed all positive bands in the fingerprint region. However, VCD spectra obtained from electronic structure calculations using Gaussian software showed a mixture of positive and negative bands for both enantiomers. This discrepancy has led me to the question that whether it is possible for a chiral molecule to show all positive or all negative bands in its experimental VCD spectrum.
VCD is due to coupling which leads in the simplest case of two coupled vibrations to one VCD band going up and another one going down. It is therefore not possible that all bands of one enantiomer are either negative or positive.
It is possible for a chiral molecule to show all positive or all negative bands in its experimental VCD spectrum. The VCD spectrum of a chiral molecule is sensitive to its conformation, orientation, and environment. Therefore, small differences in experimental conditions, such as temperature, solvent, or sample preparation, can lead to different VCD spectra.
Additionally, experimental VCD spectra are usually obtained by averaging the signals from many molecular conformations, which can lead to a suppression of certain bands. This could potentially result in an experimental VCD spectrum that appears to have only positive or only negative bands.
In contrast, electronic structure calculations using Gaussian software provide a theoretical prediction of the VCD spectrum based on the optimized molecular geometry and the specific computational method used. This can result in different spectra compared to experimental results, especially if the computed spectrum is not averaged over multiple conformations.
In summary, it is possible for a chiral molecule to show all positive or all negative bands in its experimental VCD spectrum, but it is important to consider the experimental conditions and the molecular conformations involved when interpreting these results.
Ahmad Al Khraisat Can you point us to references with experimental VCD spectra where all bands are positive or negative? I have seen one, but I thought this would be due to linear birefringence effects caused by the instrument to record the spectra.
Actually, the book VCD Spectroscopy for Organic Chemists, by Philip J. Stephens, Frank J. Devlin, and James R. Cheeseman addresses the problems of conformers in chapter 5, but while a number of compounds and there VCD spectra are presented, none of it has only negative or only positive bands.
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