Vibrational CD

The distribution of conformers in the unordered conformation depends on the amino acid sequence and on the solvent and temperature. However, there is a substantial body of evidence 151 that poly(Pro)II-like conformers are present at significant levels in unordered peptides at room temperature, and become increasingly important as the temperature decreases. This was first pointed out by Tiffany and Krimm/152 who noted the striking similarity of the CD spectra of ionized poly(Lys) and poly(Glu) to that of poly(Pro)II. The spectra are indeed very similar, if one allows for a red shift of -10 nm in the poly(Pro)II spectrum, attributable to the difference between tertiary and secondary amides. The case for poly(Pro)-Il-like conformers in unordered peptides is greatly strengthened by observations that the vibrational CD spectrum of unordered peptides is qualitatively like that of poly(Pro)Il 153 154 ... 

This chapter is not an update of a previous chapter and will therefore try to review the reported chiroptical data on the carbon-carbon double bond, starting from 1968. It will refer only to the available literature on the C=C chromophore itself. It will analyze the available data of molecules which contain only one chromophore, the carbon-carbon double bond. It will not dwell on molecules which have the C=C bond as one of the chromophores which are responsible for its optical activity. It will cover the literature in the field of electronic excitations and will not provide information on vibrational CD (VCD) or Raman optical activity. The chiroptical properties provide information regarding the spectroscopy of the chromophore, as well as its absolute configuration. The latter is usually done with the help of sector rules around the chromophore of interest. In this review both aspects will be discussed. 

Fig. 22 (a) IR absorption spectra of a bent-shaped twin dimer, (b) Vibrational CD spectra in two chiral domains stabilized by right- and left-CPL [64]... 

For multilayer films (five monolayers thick), the depletion of the CH2 band was found to depend linearly on temperature in the range between —30°C and 120°C, with a change of slope occurring at 60°C. The observed linear dependence was attributed to the reactivity being controlled by the penetration of 0(3P) atoms between the vibrating Cd arachidate chains, while the chain vibrations are described by two-dimensional harmonic potential... 

A general discussion regarding the instrumentation which can be used for the measurement of CD spectroscopy has been provided by Donald Bobbitt, while the more specialized requirements associated with vibrational optical activity have been discussed by Laurence Nafie. This latter chapter deals with both the methods of vibrational CD spectroscopy as well as raman vibrational optical activity. The use of CD as a detector in liquid chromatography represents an important area of heightened activity, and this has been covered by Andras Gergely. 

In VCD, the differential absorption of left and right circularly polarized infrared radiation by a vibrational transition of a chiral molecule is observed. Equations (1-5) hold equally well for electronic and vibrational transitions, but in all transition moments, the electronic wave functions need to be replaced by vibrational (or vibro-nic) wave functions. In vibrational CD, the ratio of differential absorption to the infrared absorption, defined as... 

Polavarapu PL, He J (2004) Chiral analysis using mid-IR vibrational CD spectroscopy. Anal Chem 76 61A-67A... 

Rhee H, JuneYG KZH et al (2009) Phase sensitive detection of vibrational optical activity free-induction-decay vibrational CD and ORD. J Opt Soc Am B 26 1008-1017... 

An expression which is structurally similar to the last term of equation 26 appears in the (conceptionally simple) fixed partial charge (FPC) model of vibrational CD In... 

Over 15 years ago a new technique [15] using new instrumentation and known as FT-IR vibrational CD was developed and has been continuously used since then. 

In this chapter, the characterization of natural products by circular dichroism (CD) spectroscopy in the UV/Vis region, called electronic CD (ECD), as well as a brief explanation of optical rotation (OR) [ct] values, which are fundamental constants for chiral compounds, is reviewed. ECD is a very sensitive diagnostic tool for determining not only AC and conformation, but also for monitoring intermolecular interactions where chiral systems are involved.2 3 There will be no discussion of vibrational CD (VCD) even though this method has made significant advances in the last decade in combination with theoretical ab initio calculations of VCD spectra.4-6... 

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).

Theoretical predictions of vibrational CD spectra of D-glyceraldehyde, d-erythrose and D-threose have been made through development of a method to scale calculated force constants to agree with experimental values. 

See also Near-IR Spectrometers Vibrational CD, Applications Vibrational CD Spectrometers Vibrational CD, Theory. 

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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