Newer Applications of Circular Dichroism in Natural Products Chemistry

Abstract

Optical activity of a substance leads at least to three spectroscopic phenomena which are, however, related: Optical Rotatory Dispersion (ORD), i.e. the rotation of the plane of linearly polarized light by traversing the substance, Circular Dichroism, expressed as Δε ≡ ε_L - ε_R (if the relative molar mass of the molecules is known, or - in case of biopolymers - the mean residual relative molar mass), and Molar Ellipticity, [Θ] = 3300 × Δε. A necessary condition for optical activity of a substance is the Chirality of its molecules, i.e. the property of being not superposable onto its mirror image. This allows still the presence of (proper) axes of rotation C_p, but not improper ones (S_p, including S₁ ≡ mirror plane, and S₂ ≡ centre of inversion). Theoretically, the area under a CD-band is given by the scalar (or dot) product of the electric times the magnetic transition moments associated with the transition and this value is proportional to the Rotational strength

$$\text{R}_{\text{0i}} = 22.97 \times 10^{ - 40} \int {\left( {{{\Delta \varepsilon } \mathord{\left/ {\vphantom {{\Delta \varepsilon } \lambda }} \right.} \lambda }} \right)\text{d}\lambda } = \mu _{0\text{i}} \times \text{m}_{\text{i0}} \times \cos \left( \varphi \right)$$

(1)

(φ is the angle between the two transition moments, index 0 refers to the ground, and i to the excited state). Thus the CD is positive if is acute (including parallel moments), and negative if this angle is oblique (including antiparallel moments). R_0i. has been defined in analogy to the Dipol Strength D_0i, given by

$$\text{D}_{\text{0i}} = 91.88\; \times \;10^{ - 38} \int {\left( {{\varepsilon \mathord{\left/ {\vphantom {\varepsilon \lambda }} \right.} \lambda }} \right)\text{d}} \lambda = \mu _{0\text{i}}^2$$

(2)