CH437 CLASS 21

CHIROPTICAL METHODS FOR THE DETERMINATION OF ORGANIC STRUCTURES: CIRCULAR DICHROISM (CD) AND OPTICAL ROTATORY DISPERSION (ORD)

Synopsis. Definitions of CD and ORD, with examples. Origins of CD and ORD: interactions of electromagnetic radiation with chiral molecules.

Introduction and Definitions

Chiroptical properties are properties of chiral substances arising from their nondestructive interaction with anisotropic radiation (polarized light). These properties can differentiate between the two enantiomers of a chiral compound. Spectroscopic methods that can utilize these properties are called chiroptical methods. They are summarized below.

Method / Principle / Accessible chromophore
Polarimetry and optical rotatory dispersion (ORD) / Refraction / No
Circular dichroism (CD) / Absorption / Yes
Circular polarization of emission (CPE) / Emission / Yes

Circular dichroism (CD) describes the anisotropic absorption of circularly polarized light (cpl) by chiral samples that have an excess of one enantiomer.

This occurs only in the spectral regions in which absorption bands are found – here, focus will be on the UV-visible region (electronic transitions), but CD also occurs in the IR region (vibrational transitions).

CD spectra are usually plotted as molar ellipticity [q] against wavelength, as shown below. A comparison with the UV-visible absorption spectrum is shown on the same diagrams. The positive CD curve is known as a “positive Cotton Effect” and similarly for the negative CD curve. There is a parallel with ORD (see later).

Optical Rotatory Dispersion (ORD)

Optical activity (or optical rotatory power) results from refraction of left and right circularly polarized light (cpl) to different extents by chiral molecules.

This differential refraction is also known as circular birefringence or anisotropic refraction. As the light passes through the sample, nR ¹ nL, where n is the refractive index.

Optical rotatory dispersion describes the variation of optical rotatory power with wavelength.

Usually, specific molar rotation [F], rather than specific rotation [a], is plotted against wavelength.

Plain or Normal ORD Curves

When there is no chromophore present that absorbs strongly in the wavelength range of study, “normal” or “plain” ORD curves are obtained that obey Fresnel’s equation,

Plain ORD curves are illustrated for sterol enantiomers, below.

Anomolous ORD Curves

When a chromophore is present that absorbs somewhere in the wavelength range of study, “anomalous” ORD curves result, in which the plot passes through [F] = 0 (the “crossover point”) at a particular wavelength that corresponds to absorption spectrum lmax (emax). This effect is also known as the “Cotton Effect”. The ORD spectrum of (+)-camphor, along with the corresponding UV-visible absorption spectrum (with lmax for the n®p* transition at 292 nm) is shown below. Various features associated with ORD curves are also shown in the diagram.

The above is an example of a positive Cotton Effect, as the first peak encountered on going from high to low wavelength is a positive peak. (-)-Camphor would have a negative Cotton Effect ORD curve, which is virtually the mirror image of the one above.

One complicating factor regarding ORD curves is the fact that other chromophores that have absorptions near the range of wavelengths under study contribute to the overall shape of the spectrum: in particular causing it to be less symmetrical, as shown in the example below.

Diagram (a) shows the actual ORD spectrum of a steroid derivative, and (b) shows the superposition of a negative Cotton Effect (background) due to a chromophore (probably OH) at low wavelength, on a positive Cotton Effect due to the absorption at 264 nm (the sulfide group).

Nowadays, CD spectra are more widely used than ORD spectra, largely because the former are inherently easier to interpret, as illustrated for steroid ketone below.

Origins of CD: Anisotropic Absorption

An electronic (or vibrational) transition associated with a chiroptic chromophore causes left and right circularly polarized light (cpl) to be absorbed differentially. This is called anisotropic absorption and gives rise to the Cotton Effect. AL ¹ AR and DA = AL – AR ( = eL – eR) cl, is a measure of the CD, where eL and eR are the molar absorption coefficients for left and right cpl, respectively. If eL > eR, then a positive CD curve results, but if the reverse is true then a negative CD curve is observed. The signs of the CD curve and the corresponding ORD curve in the region of the anomaly are the same.

Elliptical polarization is the most general form of polarized light: linear (plane) polarized light and circularly polarized light are special cases. The relationships between these are shown below.

The eccentricity of the ellipse [(a – b)/a] is 1 for linear (plane) polarization and 0 for cpl (a = b). These points are illustrated in more detail below, showing elliptical polarization resulting from unequally absorbed left and right cpl. Diagram (a) shows displays polarization in a region of the electromagnetic spectrum where a = 0, and (b) shows the polarization in a region where a = a positive value, viewed toward the light source.

The molar ellipticity [q] is defined by the equations below.

For spectroscopic transitions to occur,

(1)  E (photon) = hn = E (excited state) – E (ground state)

(2)  The excitation must be accompanied by a migration of charge, thus setting up a transitory electric transition moment, m

Now, CD requires that charge rotation must accompany the translation of the excited electron: it must rotate or follow a helical path. Hence a magnetic transition moment m is set up. The relative magnitudes and directions of m and m can be estimated using MO theory, as illustrated for p®p* and n®p* transitions below.

The rotational strength R (a measure of the “allowedness”) of the CD absorption is given by

R = m.m = |m| |m| cos b,

Where b is the angle between the m and m vectors. The nature of the CD curve (sign and intensity) according to the angle b is summarized below.

The sense of chirality (the handedness or configuration) of an absorbing molecule influences the sign of the Cotton Effect (the sign of both CD and ORD curves) by influencing the angle between the electric and magnetic transition moments.

This is most easily illustrated by hexahelicene, a molecule whose whole chromophore is chiral (unlike simple alkenes or carbonyl compounds).

In Class 22, it will be shown that (semiempirical) sector and helicity rules are available for correlating absolute stereochemistry (configuration or conformation) of particular types of compounds with their chiroptical properties, especially the signs of individual Cotton Effects, as manifested by CD and ORD curves.

11