Vibrational circular dichroism spectrometer

For our purpose, it is convenient to classify the measurements according to the format of the data produced. Sensors provide scalar valued quantities of the bulk fluid i. e. density p(t), refractive index n(t), viscosity dielectric constant e(t) and speed of sound Vj(t). Spectrometers provide vector valued quantities of the bulk fluid. Good examples include absorption spectra A t) associated with (1) far-, mid- and near-infrared FIR, MIR, NIR, (2) ultraviolet and visible UV-VIS, (3) nuclear magnetic resonance NMR, (4) electron paramagnetic resonance EPR, (5) vibrational circular dichroism VCD and (6) electronic circular dichroism ECD. Vector valued quantities are also obtained from fluorescence I t) and the Raman effect /(t). Some spectrometers produce matrix valued quantities M(t) of the bulk fluid. Here 2D-NMR spectra, 2D-EPR and 2D-flourescence spectra are noteworthy. A schematic representation of a very general experimental configuration is shown in Figure 4.1 where r is the recycle time for the system. 

Bonmarin M, Helbing J (2008) A picosecond time-resolved vibrational circular dichroism spectrometer. Optics Lett 33 2086-2088... 

In the years to follow the key to the measurement of vibrational circular dichroism was the development of photoelastic modulators suitable for work in the infrared spectral region. The first successful measurements of circular dichroism originating from vibrational transitions in the infrared were done by Hsu and Holzwarth (1973) on thin slices of monocrystalline a-NiS04 6 H2O and a-ZnSe04 6 H2O. For this measurements the authors used a normal dispersive IR spectrometer supplemented by a linear polarizer and a photoelastic modulator made from Germanium. 

This chapter concentrates on CD studies in the visible and ultraviolet, regions in which CD results from electronic excitations. There has been outstanding progress in extending CD measurements into the infrared, thus providing information about vibrational excitations in the form of vibrational circular dichroism (VCD). VCD instrumentation is currently available only in a few laboratories which have constructed custom-made VCD spectrometers. For a detailed discussion of VCD, the reader is referred to several reviews. " ... 

Vibrational circular dichroism (VCD) is defined as circular dichroism (CD) in vibrational transitions in molecules. These transitions typically occur in the infrared (IR) region of the spectrum and hence a VCD spectrometer is an infrared spectrometer that can measure the circular dichroism associated with infrared vibrational absorption bands. CD is defined as the difference in the absorption of a sample for left versus right circularly polarized radiation. This difference is zero unless the sample possesses molecular chirality, either through its constituent chiral molecules or through a chiral spatial arrangement of non-chiral molecules. 

Figure 1 Various types of CD measuranents and their applicability. The conventional CD can be called BCD (electronic circular dichroism) to distinguish it from otha types of CDs such as VUV-CD (vacuum-ultraviolet CD), NIR-CD (near-infrared CD), and VCD (vibrational CD). This list represents one of the typical classifications, although there is no clear boundary between each type of CDs. At the bottom, the wavelength coverage of normal CD/VCD spectrometers is shown (black lines). The coverage may be wida for some instruments or can be extended by an optional apparatus (gray lines).

See also Biomacromolecular Applications of Circular Dichroism and ORD Chiroptical Spectroscopy, Emission Theory Chiroptical Spectroscopy, General Theory Chiroptical Spectroscopy, Oriented Molecules and Anisotropic Systems Circularly Polarized Luminescence and Fluorescence Detected Circular Dichroism Light Sources and Optics Luminescence, Theory Nonlinear Optical Properties Vibrational CD Spectrometers Vibrational CD, Applications Vibrational CD, Theory. 

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