Circular Dichroism of Oriented Molecules

Abstract

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Electronic Circular Dichroism Imaging (ECDi) Casts a New Light on the Origin of Solid‐State Chiroptical Properties

“…However, for small n (possibly limited to a few units), then Δϵ from Equation (3) could be different than the isotropic average and be dominated by the anisotropic CD components. An important result of this ECD imaging study is that the isotropic ECD measured on the whole pellet is the average of ACD spectra for all possible orientations, meaning that the isotropic CD can be considered to be a true chirality measurement [10,14b] to be used for the assignment of the absolute configuration.…”

“…As a model compound, we have studied pellets prepared from fine ground crystals of form I of finasteride (Figure 1), a drug used for the treatment of male pattern hair loss and benign prostatic hypertrophy, [17] mixed with KCl, using a bench‐top CD instrument and the highly collimated microbeam of B23 beamline of the Diamond Light Source synchrotron for CD imaging [7a,8] . The aims of this ECD imaging study were the following: 1) to ascertain the role and possible impact of local anisotropies in the solid state by imaging the grid area of the pellet at high spatial resolution of about 100 μm; 2) to verify that the average over an ensemble representing all possible crystal orientations coincides with the conventional solid‐state ECD spectrum and is therefore a true chirality measurement, [10,14b] which can be used to assign the absolute configuration; and 3) to develop a new methodology to extract additional information from microcrystalline samples, without the need of orienting the sample specimens and of single large crystals (side≥2 mm). Our findings obtained from a synergy between experimental ECD i data obtained using the highly collimated microbeam of B23 beamline [7a] and a theoretical approach for calculating isotropic CD and ACD spectral components paved the way for the characterization of the chiroptical properties of chiral molecules in the crystal form.…”

“…The interaction of circularly polarized light (CPL) with chiral samples in isotropic media produces isotropic CD reported as a pseudoscalar quantity Δϵ. The interaction of CPL with oriented samples produces instead anisotropic CD (ACD) which is described by a second‐rank pseudotensor with elements Δϵ αβ (α,β=1, 2, 3) [10] . The isotropic CD is determined as follows: …”