Vibrational Circular Dichroism Measurements

Polavarapu PL (1989) New developments in Fourier-transform infrared vibrational circular-dichroism measurements. Appl Spectrosc 43 1295-1297... 

Malon P, Keiderling TA (1996) Spinning quarter-wave plate polarization modulator test of feasibility for vibrational circular dichroism measurements. Appl Spectrosc 50 669-674... 

Cao XL, Dukor RK, Nafie LA (2008) Reduction of linear birefringence in vibrational circular dichroism measurement use of a rotating half-wave plate. Theor Chem Acc 119 69-79... 

P. L. Polavarapu, Appl. Spectrosc., 43, 1295 (1989). New Developments in Fourier Transform Infrared Vibrational Circular Dichroism Measurements. 

Long F, Freedman TB, Tague TJ and Nafie LA (1997) Step-scan Fourier transform vibrational circular dichroism measurements in the vibrational region above 2000 cm"L Applied Spectroscopy 51 508-511. 

Molecular chirality is most often observed experimentally through its optical activity, which is the elfect on polarized light. The spectroscopic techniques for measuring optical activity are optical rotary dispersion (ORD), circular di-chroism (CD), and vibrational circular dichroism (VCD). 

Over the past decade two forms of vibrational optical activity have become established. One is called vibrational circular dichroism (VCD), the extension of electronic circular dichroism into the infrared vibrational region of the spec-tram. The first measurements of VCD were reported by George Holzwarth and co-workers at the University of Chicago in 1973 for crystals (3) and 1974 for neat liquids (4). In VCD one measures the small difference in the absorption of a sample for left versus right circularly polarized incident infrared radiation. The early stages of the development of VCD have been reviewed from several perspectives (5-8). 

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. 

To confirm the importance of ester linkage for chiral formation, we conducted vibrational circular dichroism (VCD) measurements [64]. FT-IR spectrum of this molecule shows many C=0,0-C-O related peaks in addition to peaks of N=N and phenyl and CH2 vibrational modes, as shown in Fig. 22a. It is noted that strong VCD signals are observed in C=0 and O-C-O vibrations, as shown in Fig. 22b. This implies that ester linkage is strongly related to the chiral structure. 

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. 

Vibrational optical activity (VOA) is a relatively new area of natural optical activity. It consists of the measurement of optical activity in the spectral regions associated with vibrational transitions in chiral molecules. There are two basic manifestations of VOA. The first is simply the extension of electronic circular dichroism (CD) into the infrared region where fundamental one-photon vibrational transitions are located. This form of VOA is referred to as vibrational circular dichroism (VCD). It was first measured as a property of individual molecules in 1974 [1], and was independently confirmed in 1975 [2]. Within the past twelve years, VCD has been reviewed on a number of occasions from a variety of perspectives [3-15], and two more reviews are currently in press [16,17], The second form of VOA has no direct analog in classical forms of optical activity. Optical activity in Raman scattering, known simply as Raman optical activity (ROA), was measured successfully for the first time in 1973 [18], and confirmed independently in 1975 [19], ROA has been described in detail and reviewed several times in the past decade from several points of view [20-24], and two additional reviews [25,26], one with a view toward biological applications [25] and the other from a theoretical perspective [26], are currently in press. In addition, two articles of a pedagogical nature are in press that have been written for a general audience, one on infrared CD [27] and the other on ROA [28],... 

Keywords Absolute configuration and conformation determinations Density functional theory Induced solvent chirality Transient VCD measurements Vibrational circular dichroism... 

Buffeteau T, Lagugne-Labarthet F, Sourisseau C (2005) Vibrational circular dichroism in general anisotropic thin solid films measurement and theoretical approach. Appl Spectrosc 59 732-745... 

Yang GC, Xu Y (2008) The effects of self-aggregation on the vibrational circular dichroism and optical rotation measurements of glycidol. Phys Chem Chem Phys 10 6787-6795... 

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. 

The measurement of vibrational circular dichroism was confined to the region below 15 pm resulting from the transmittance characteristics of the photoelastic modulator made from zink selenide (ZnSe). This was changed by the employment of a photoelastic modulator made from cadmium telluride (CdTe) with sufficient transparency down to about 300 cm. ... 

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. 

Vibrational Circular Dichroism Vibrational circular dichroism (VCD) could be measured at good signal-to-noise levels. Vibrational optical activity is observed in the classic method of Grosjean and Legrand. 

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. " ... 

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