This entry provides an introduction to the basic concepts of Electronic Circular Dichroism (ECD) spectroscopy. It describes the fundamental principles, instrumentation, and different approaches for interpreting and predicting ECD spectra. It surveys the most popular modern applications of ECD for the structural analysis of organic compounds.
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1. Introduction
Circular dichroism (CD) is defined as the differential absorption of left- & right-circularly polarized electromagnetic radiation by a sample. In particular, electronic circular dichroism (ECD, often shortened itself as CD) occurs in correspondence with electronic transitions and represents the chiroptical counterpart of the more common UV-vis absorption spectroscopy. Similarly, other chiroptical spectroscopies such as vibrational CD (VCD), Raman optical activity (ROA), circularly polarized luminescence (CPL), and so on, all have their counterparts in infrared (IR), Raman và fluorescence spectroscopies, respectively. The main advantage of using chiroptical spectroscopies instead of their non-chiral counterparts is that the former, but not the latter, are sensitive lớn the absolute configuration of a sample. In simple words, while the two enantiomers of a chiral substance, for instance (+) & (-)-camphor, will have identical UV-vis, IR và Raman spectra, they will have distinguishable, mirror-image looking, ECD, VCD & ROA spectra (Figure 1). Moreover, chiroptical spectroscopies are more sensitive sầu toward the overall molecular structure than their non-chiral counterpart. Any ECD or VCD or ROA spectrum contains a lot of structural information about the molecular structure, in terms of both conformation & configuration, although it not always easy to find robust & practical means to extract such the information from the spectra.<1>
Δε is now independent of the concentration c, expressed in M, & of the cell path-length b, expressed in centimet.
Non-negligible CD signals (a CD band is also called a Cotton effect) can be measured only in correspondence khổng lồ absorption bands, i.e., when resonance occurs in light/matter interaction. This usually involves either electronic or vibrational transitions, therefore the useful ranges for CD measurements are the UV-vis (ECD) và the IR one (VCD và ROA). The ratio between CD & absorption intensity is defined through the so-called anisotropy or dissymmetry g-factor
Symmetry restrictions imply that R may be non-negligible only for molecules belonging lớn symmetry groups devoid of roto-reflection elements, i.e., of inversion centers and symmetry planes. Therefore, only chiral systems may exhibit CD. The integral of a CD bvà allied with a single electronic transition is related to the rotational strength through:
where Γi is the appropriate band-shape function (for instance, a Gaussian or Lorentzian band)<2> centered at νi (it also contains a band-width quantity σi).
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2.2 Instrumentation & sample preparation
A standard benchtop ECD spectropolarimeter has the same instrumental thiết kế as a comtháng spectrophotometer with a few differences (Figure 2).<3> The incident radiation coming from a source (S) must be circularly polarized, which is achieved through a photoelastic modulator (PEM) placed after the monochromator (M). The PEM produces L- and R-CPL alternately with a frequency of 50 KHz. The transmitted light is thus modulated at the same frequency, the average of the two signals being proportional khổng lồ A và their difference lớn CD. This latter must be amplified & the two signals recombined (through a lock-in amplifier) to output the true ECD signal collected by the photomultiplier (PMT). Similarly to lớn UV-vis spectroscopy, the usual range for ECD measurements is between 185 & 700 nm. While extension to lớn longer wavelengths (toward the near IR region) is easily accessible, that khổng lồ shorter wavelengths requires a very special apparatus (vacuum và synchrotron sources). Samples are usually solutions contained in cylindrical quartz cells with path-lengths ranging from 0.01 khổng lồ 1 centimet. Suitable solvents must be transpartent in the near UV-vis region, that is, have relatively low wavelength cut-off, such as water, methanol, acetonitrile, dioxane and hexane. Sample concentrations must be chosen, similarly khổng lồ absorption spectroscopy, so not lớn exceed 0.8-1 absorbance units in the observed range. For a compound with a maximum extinction coefficient ε ≈ 104, one would need lớn prepare a solution with concentration ≈ 1 mM, to be measured with a 0.1 cm cell. Normally, solutions with ≈ 1 mg/mL concentration or less can be measured by ECD, with a proper choice of the cell. ECD spectra can also be recorded on solid samples, for examples microcrystals disperded in an inert salternative text matrix, thin films deposited on quartz windows, & gels.
Figure 6. Steps for absolute configuration assignment via ECD calculations. (a) from a stick-plot into lớn a spectrum; (b) from conformer spactra to lớn a weighted-average spectrum; (c) final comparison.