VCD spectrometer

The most recent advance in VCD instrumentation has been its adaptation to Fourier transform infrared (FTIR) measurement (23-25). The details of this technique involve a new method of FTIR measurement termed double-modulation FTIR spectroscopy. Thus spectra of very high quality and resolution have been obtained using a standard VCD modulator and detector, a glower source, and a commercially available FTIR spectrometer system. In fact an entire FTIR-VCD spectrometer can be assembled from a few commercially available components. It is found that the major advantages of resolution, throughput, and... 

Figure 4. Optical and electronic diagram of the double modulation FT-VCD spectrometer at Syracuse University.

Figure 10. VCD and absorbance spectrum for (+)-limonene measured with a PMI FT-VCD spectrometer.

Figure 1. Schematic of a dispersive VCD spectrometer. Heavy lines denote the light path, medium lines the signal path, and thin lines the reference signals.

Fig. 1 Block diagram of the optical-electronic layout of an FTIR-VCD spectrometer...

Fig. 2 The VCD spectra of camphor with the anisotropy ratio on the order of magnitude of 10 5 are used as examples to illustrate the performance of several generations of FTIR-VCD spectrometers. Raw (bottom traces) and subtracted (top traces) VCD spectra of 0.6 M R-camphor in CC14 in a 0.10 mm CaF2 cell from 4,000 to 2,000 cm-1 are shown (a) SPM, single PEM only, for a pair of enantiomers. The subtracted spectrum is the true spectrum (b) SPM, single PEM, with only solvent baseline correction (c) SPM/RHWP, single PEM (SPM) with RHWP, with only solvent baseline correction (d) DPM, DPM only, with only solvent baseline correction (e) DPM/RHWP, with both DPM and RHWP, with only solvent baseline correction. Reproduced with permission from [41]. Copyright (2008) Springer...

A few scanning dispersive VCD instruments are still in use for biological applications in the mid-IR region [46,47]. In 2009, a newly designed and optimized dispersive VCD instrument was reported [47]. A collection of spectra for peptides and proteins having different dominant secondary structures (alpha-helix, beta-sheet, and random coil) measured with this new instrument showed substantially improved signal-to-noise (S/N) ratios as compared with the earlier version. The instrument provides protein VCD spectra for the amide I region that are of comparable or better quality than those obtained with a standard commercial FTIR-VCD spectrometer [47]. 

Besides the continuous improvements of FTIR-VCD instruments described above, some exciting new developments related to VCD measurements have been reported in recent years. These include the developments of matrix isolation FTIR-VCD instruments and of laser based real time VCD spectrometers. These new developments are associated with brand new applications and research directions, such as combining the matrix isolation technique with VCD spectroscopy to probe conformationally flexible chiral molecules and H-bonded chiral molecular complexes, and using femtosecond laser VCD instruments to record time resolved VCD spectra for monitoring fast chemical reactions or folding and unfolding events of peptides and proteins in solution. These will be discussed in more detail in Sects. 4.5 and 4.6. 

The spectrum of (-)-a-pinene as a 50 pm film of neat liquid using a Bomem spectrometer with a polarizing interferometer attachment is a composite of 60 000 VCD scans and 12 000 transmission scans and took 19.5 hours to be measured. A spectrum with comparable signal to noise ratio is shown in Fig. 6.3-9. This was measured in our laboratory with a PEM-based VCD spectrometer and is a composite of only 5000 scans (five blocks of 1024 scans each). The noise estimate shown as the upper trace is the difference between two 5000 scan VCD spectra. 

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. " ... 

Recently, due to both availability of the Fourier transform VCD spectrometers (FT-VCD) and DFT software for predicting the VCD spectra, the VCD technique has become widely recognized and used [108-110]. DFT has been accepted by the ab initio quantum chemistry community as a cost-effective approach to computations of molecular structures and spectra (vibrational and NMR) of molecules of chemical interest. Many studies have shown that vibrational frequencies and VCD intensities calculated by means of DFT methods are more reUable than those obtained at the MP2 level [87]. 

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).

A block diagram of a VCD spectrometer is shown in Figure 22.5. The units to generate circularly polarized radiation (an optical filter, a linear polarizer, and a PEM) are inserted between a Fourier transform-infrared (FT-IR) spectrometer and the detector. Left- and right-circularly polarized radiation are generated with a fixed frequency by the PEM, and the signal synchronized with the PEM is detected. 

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. 

The precise way in which the detector signal is processed electronically depends on the kind of VCD spectrometer used. In the following sections, three different VCD spectrometer designs are discussed. The simplest of these is the dispersive VCD spectrometer, and we will use this design to illustrate the basic concepts associated with the electronic processing of VCD spectra. The two subsequent cases involving Fourier-transform VCD spectrometers are more complex, but share the same underlying conceptual basis as the dispersive VCD spectrometer. 

In a dispersive VCD spectrometer, the IR source in Figure 2 consists of a thermal or arc source of infrared radiation, a light chopper, and a grating monochromator. The infrared source of radiation is first... 

Figure 2 Diagram illustrating the basic optical layout and electronic pathways for the measurement of VCD. The diagram is applicable to both dispersive VCD spectrometers and FT-IR spectrometers that use a photoelastic modulator (PEM) as the source of the polarization modulation of the light beam between left (L) and right (R) circular states.

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