Hey, this is my hundreth post! Hard to believe MGW is still going strong after all this time. To celebrate, let’s take a little walk down memory lane, shall we?
In this post I gushed about circular dichroism spectroscopy, a method used to distinguish between optically active compounds, measure intramolecular distances, determine absolute configurations, and do all kinds of other crazy stuff. I also promised a post on how to calculate CD spectra, and although this by all means isn’t the whole story, here’s a little something on the subject.
An interesting situation occurs when we bathe chiral molecules in light they can almost absorb via electronic transitions. They’re able to absorb and rotate the light at the same time! Inducing absorption ends up having a very predictable effect on the CD spectra of the compound (only in the vicinity of absorption maxima, of course). And when you throw two identical chromophores into a molecule, oriented by stereochemical constraints at a particular angle, the behavior of the CD spectra at the relevant absorption wavelengths can tell you the absolute configuration of the chromophores! Check out this paper for a very math-intensive explanation of the effect.
The basic idea is actually pretty simple. Let’s say the two chromophores (more specifically, the two transition moment vectors of the chromophores, which may or may not line up with bond vectors) are oriented such that when looking down the line connecting them, the back one is clockwise from the front one. On the picture above, that’s case I, and it’s called positive chirality. When incident light is exactly at the absorption wavelength, the observed CD is zero (because any unabsorbed light just passes through empty space between the molecules…ideally). But at energies just below the absorption maximum, light rotating clockwise (one half of the plane-polarized beam fired at the compound during the experiment) has a much higher chance of being absorbed than light rotating the other way, because of the arrangement of the chromophores! The light that makes it out is almost all left-circularly polarized, which leads to a positive result for degrees of ellipticity, CD’s dependent variable.
At energies greater than the absorption maximum, the situation changes. With an increase in frequency the circular components of the incident beam rotate ever-so-slightly faster, which knocks the clockwise component out of the alignment it needs to hit both the front and back chromophores. The left-circularly-polarized (counterclockwise) component, on the other hand, can now swing around fast enough that it reaches the back chromophore more efficiently than the clockwise light can, leading ultimately to absorption of the counterclockwise component at energies slightly higher than the absorption maximum, leading to negative values for DOE (more right-circularly polarized light escapes this time).
The key is the speed of rotation, and the fact that, clockwise, the light has to cover less rotational ground due solely to the spatial arrangement of the chromophores. For case II (negative chirality), the situation is exactly the opposite, negative values at low energies, passes through zero, then positive values. Polonski and cohorts exploit this to identify the absolute configurations of enantiomeric dithiolactams, which, interestingly, don’t have transition moment vectors that pass through bonds (see below). Still works like a charm though. They figured out how to efficiently separate them using silver camphorsulfonate and did some X-ray work as well.