Vibrational circular dichroism instrumentation

As seen in Table 7, the halogen atoms vary widely in atomic weight and this is reflected in the location of their infrared stretching bands. These bands were outside the range of the first generation if vibrational circular dichroism instruments and only the C—F bands are... 

However, the improved sensitivity of FT-IR allows one to obtain better sensitivity using the conventional sampling accessories and expand the range of sampling techniques. Emission, diffuse reflectance and photoacoustic spectroscopy represent new areas where FT-IR reduces the difficulty of the techniques considerably. Greatly improved results are also achievable from reflection spectroscopy. Special effects such as vibrational circular dichroism can be observed using FT-IR instrumentation. 

We present the basic concepts and methods for the measurement of infrared and Raman vibrational optical activity (VOA). These two forms of VOA are referred to as infrared vibrational circular dichroism (VCD) and Raman optical activity (ROA), respectively The principal aim of the article is to provide detailed descriptions of the instrumentation and measurement methods associated with VCD and ROA in general, and Fourier transform VCD and multichannel CCD ROA, in particular. Although VCD and ROA are closely related spectroscopic techniques, the instrumentation and measurement techniques differ markedly. These two forms of VOA will be compared and the reasons behinds their differences, now and in the future, will be explored. 

Current instruments allow CD measurements not only to be performed in the vacuum-ultraviolet (vacuum-UV) region X < 190 nm), but also in the infrared (IR) spectral region. This means that not only chiral absorption effects related to excitations of molecular electronic subsystems are amenable to experimental observations, but also effects involving excitations of the nuclear subsystems of molecules ( vibrational circular dichroism VCD) Recently, results of VCD experiments with cyclopropanes were published. Therefore, in the present chapter the discussion of chiroptical properties of cyclopropanes can include vibrational circular dichroism. Hence, the discussions of chiroptical properties of cyclopropanes will cover the spectral range extending from the vacuum-ultraviolet to the infrared region. 

The routine use of VCD spectroscopy is limited to liquid solutions. Vibrational circular dichroism is an intrinsically weak phenomenon (g values are very small) and its measurement requires optimum experimental conditions, in addition to state-of-the-art instrumentation. In general, VCD spectra are measured at fairly high concentrations—in the range 0.01-1.0M—in solvents with good mid-IR transmission at fairly short pathlengths (< 1 mm). The accessibility of such conditions depends on the solubilities of the molecules to be studied in available solvents. Compounds only soluble to significant extent in water are generally not easily studied. 

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. " ... 

Abstract The Vibrational Circular Dichroism (VCD) spectroscopy has been developing rapidly in both experimental and theoretical aspects. Currently, the VCD has become one of the most effective and reliable spectroscopic technique to determine the absolute configuration of chiral molecules. Its success is related to the availability of instrumentation and software for quantum-chemical calculation of the spectra. Nowadays, large parts of the VCD spectra can be trustfully predicted by theory and critically verified by confiding experiment, and vice versa. In the last decade, several theoretical and experimental VCD studies reported on VCD chirality transfer phenomenon occurring when an achiral molecule becomes VCD active as a result of intermolecular interactions with a chiral one. There are still some theoretical and experimental uncertainties about the VCD chirality transfer, however, benefits from an comprehensive use of the phenomenon can push our ability to diversify the intermolecular complexes and deepen our understanding of intermolecular interactions. This chapter is a review of the computational studies on VCD chirality transfer phenomenon supported by the experimental references, and ended by perspectives. 

Electronic and vibrational circular dichroism spectroscopy With commercially available instruments the accessible spectral region of BCD and VCD spectroscopies lies nowadays between 800 (1.25 pm) and 62 500cm (160nm). Brom the spectroscopic point of view the chosen solvents should be free of absorption and in order to have only a small solvent/ solute interaction they should be as nonpolar as possible. Acyclic or cyclic hydrocarbons are a good choice for the UV/vis absorption region if sufficient solubility is guaranteed. As a compromise, dioxane. 

The relatively new chiroptical technique, vibrational circular dichroism spectroscopy, is coming of age. Instrumentation and theoretical models are sufficiently good to establish VCD as the only spectroscopic method for the determination of absolute configurations of molecules in solution and potentially in the gas phase. 

The measurement of vibrational optical activity (VOA) lacks some of the severe disadvantages mentioned. Vibrational spectral bands are less likely to overlap and can be measured using two complementary techniques namely infrared and Raman spectroscopy. They can be measured as well in the crystalline as in the liquid or gaseous state, and the techniques are applicable to solutions while nearly reaching (complemented with the appropriate theoretical models) the accurateness of the X-ray method. VOA has drawbacks too the effects are quite small and tend to be obscured by artifacts. They are about 10 times weaker than the optical rotatory dispersion (ORD) and the circular dichroism (CD) in the UV-VIS range. However, this apparent disadvantage is more and more relieved by instrumental advances. 

The fundamental requirement for the existence of molecular dissymmetry is that the molecule cannot possess any improper axes of rofation, the minimal interpretation of which implies additional interaction with light whose electric vectors are circularly polarized. This property manifests itself in an apparent rotation of the plane of linearly polarized light (polarimetry and optical rotatory dispersion) [1-5], or in a preferential absorption of either left- or right-circularly polarized light (circular dichroism) that can be observed in spectroscopy associated with either transitions among electronic [3-7] or vibrational states [6-8]. Optical activity has also been studied in the excited state of chiral compounds [9,10]. An overview of the instrumentation associated with these various chiroptical techniques is available [11]. 

Circular dichroism employs standard dispersive or interferometric instrumentation, but uses a thermal source that is rapidly modulated between circular polarization states using a photoelastic or electro-optic modulator. Using phase-sensitive detection, a difference signal proportional to the absorption difference between left- and right-polarized light, A A = AL — AR, is recorded as a function of wavenumber. Relative differential absorptions (dissymmetry factors) AA/A at absorption maxima are typically 0.1—0.01 for uv—vis electronic transitions and 10-4 —10-5 for vibrational modes in the ir. 

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 Biochemical Applications of Raman Spectroscopy Biomacromolecular Applications of Circular Dichroism and ORD Chiroptical Spectroscopy, Oriented Molecules and Anisotropic Systems Chiroptical Spectroscopy, General Theory ORD and Po-larimetry Instruments Raman Optical Activity, Applications Raman Optical Activity, Spectrometers Raman Spectrometers Vibrational CD, Applications Vibrational CD, Theory. 

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