Electronic Circular Dichroism
ECD is a spectroscopic technique for chiral molecules in the UV/Visible region of the spectrum
On the left are computer data for twisted butadiene: From bottom to top:the potential energy profile for rigid twisting from anti to cis conformation; the corresponding oscillator strength (f) of the lowest pi -> pi* transition; the corresponding optical rotatory strength [R] of the lowest pi -> pi* transition
On the right Simulated CD spectra of 1,3-butadiene for different torsion angles 4 (defined on the left). The top and bottom spectra bear marked similarities to the vacuum uv circular dichroism spectra of beta- and alpha-phellandrene in solution.
A.Rauk and H A .Peoples, J. Comp. Chem. 1980, 1, 240-256.
Theory of ECD
The fundamental property of optical activity is the rotatory strength [R] (eqn 1), proportional to the area under an electronic circular dichroism (ECD) band in the UV/Vis part of the spectrum. The transition from ground state, 0, to an excited state, n, requires descriptions of the excited and ground state wave functions of comparable accuracy. This requirement poses a problem since electronic structure methods are designed to optimize the ground state wave function in order to minimize the ground state energy.. A comparable accuracy for an excited state wave function requires a configuration interaction (CI) expansion including doubly excited configurations, a formidable task back in early '70s when my group was working on the problem. Our first [paper on the subject did not have correlated excited state wave functions:
Arvi Rauk,* John 0. Jarvie, Hideo Ichimura, and Jose M. Barriel, J. Am. Chem. Soc., 1975, 97, 5656-5664
Doubly excited configurations were subsequently added by Jose Barriel, whose career was tragically cut short by an accident in 1976.
Arvi Rauk and Jose M. Barriel, Chem. Phys. 1977, 25, 409 - 424..
For an explanation of optical activity, click the button below: It downloads a nice powerpoint presentation.