Mid-infrared VCD

The mid-infrared VCD of some members of this series has also been investigated (59, 60) with an aim, in part, to identify features diagnostic of conformation. In the methyl deformation region (1450 cm ), all members of the... 

Polavarapu and Michalska (63) have reported mid-infrared VCD in (S)-(-)-epoxypropane both in the vapor phase and as a neat liquid. This paper reports the first comparison of gas phase and liquid phase VCD as well as the first report of VCD from degenerate methyl rocking modes. The stereochemical significance lies in the identification of VCD marker bands for the molecular geometry since the molecule is completely rigid from the standpoint of vibrational spectroscopy. Due to its extreme simplicity as a chiral molecule, epoxypropane (propylene oxide) is very important from a theoretical perspective. 

In an effort to establish an intensity standard for VCD measurements, a number of laboratories have undertaken the measurement of the mid-infrared VCD spectrum of (-)-a-pinene. The results from several laboratories have been obtained to date and the results have been plotted in molar absorptivity units. In Figure 7, we present the absolute VCD measurements from three locations. The results are still preliminary, and though they show a variation of the order of 10%, these intensities and others appear to... 

The VCD spectra of molecules of the form CH3C HR,R2 have been investigated in the CH stretching region (20, 57-59) and the 16(X)-900-cm mid-infrared region (59, 60). The ROA of this type of molecule has also been recorded (II-14). In particular, a-deuterated ethanol and a-phenylethane derivatives have been studied. 

The mid-infrared region is considerably more difficult to interpret than the CH stretching region. This fact again points to the fact that chromophores responsible for VCD are delocalized over the chiral frame. 

More recently, Su et al. (81) have measured the VCD in the mid-infrared region for a series of four chiral cyclophosphamides, 26, where X is either chlorine or hydrogen. They observe an absorption band that occurs between 1205 and 1225 cm in the four molecules having strong negative VCD in all cases. They refer to this band as a VCD marker band because it appears to correlate better with the absolute configuration of these molecules than does the optical rotation at the sodium D line in the visible region of the spectrum. 

The Co(en)3 VCD spectra also exhibit interesting anion dependency in the CH stretching and mid-infrared 1600-1100-cm regions. Corresponding features are also observed for the chromium complex. 

Dispersive VCD instrumentation has been further refined for the mid-infrared region from 800 to 1800 cm l [51] and this design was further optimized for the 6(0. spectral region between 1600 and 2000 cm l [52], Additional discussion of dispersive VCD instrumentation can be found elsewhere in this book [53]. 

The vibrations discussed so far occur in the 6 pm spectral region in the mid-infrared however, most vibrational modes in a molecule will exhibit VCD, and other vibrations in the 12 pm, as well as the 3.5 to 2.5 pm range, have shown interesting VCD results. In this article, only the 6 pm region (the C=0 stretching vibration) will be discussed. To observe VCD in this spectral region, a chiroptical instrument with infrared optics (with materials such as CaF,) needs to be constructed. 

Although VCD was first reported nearly two decades ago in 1974 [1,2], progress in the field has been slow, and observation of VCD in the mid-infrared region (5 to 10 pm, or 2000 to 1000 cm1) in aqueous solutions was not possible until the mid 1980 s. VCD is now at a stage where it can reveal solution structural information on biological molecule which is not available from any other technique. 

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. 

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