Spectroscopy, infrared spectra, experimental

Material response in THz frequency region, which corresponds to far- and mid-infrared electromagnetic spectrum, carries important information for the understanding of both electronic and phononic properties of condensed matter. Time-resolved THz spectroscopy has been applied extensively to investigate the sub-picosecond electron-hole dynamics and the coherent lattice dynamics simultaneously. In a typical experimental setup shown in Fig. 3.5, an... 

Now let us concentrate on the properties of the noise amplitude Af (s) under the assumptions made above. The aim of these considerations is to derive some realistic expressions for the signal-to-noise ratio in infrared spectroscopy and its dependence on experimental parameters like scanning time, resolution etc. Since N s) is a statistical function, its average N [s) will be zero. With the computation of the spectrum, the noise N (s) is also subjected to multiplication by the scanning function S s) and to the Fourier transform. The result is the noise amplitude in the spectrum (Fig. 43)... 

Mid-infrared (IR) spectroscopy is a well-established technique for the identification and structural analysis of chemical compounds. The peaks in the IR spectrum of a sample represent the excitation of vibrational modes of the molecules in the sample and thus are associated with the various chemical bonds and functional groups present in the molecules. Thus, the IR spectrum of a compound is one of its most characteristic physical properties and can be regarded as its "fingerprint." Infrared spectroscopy is also a powerful tool for quantitative analysis as the amount of infrared energy absorbed by a compound is proportional to its concentration. However, until recently, IR spectroscopy has seen fairly limited application in both the qualitative and the quantitative analysis of food systems, largely owing to experimental limitations. 

This technique is used to study thin (down to submonolayer) films adsorbed on reflective substrates such as metals. Experimentally it involves measuring the change in the reflectance spectrum of the substrate that accompanies thin film formation. Various acronyms for the technique are used infrared reflection—absorption spectroscopy (IRRAS, IRAS) and reflection—absorption infrared spectroscopy (RAIRS). The Basics of IRRAS spectra are described in Chapter 5.2. 

Raman spectroscopy uses an incident laser beam that is focused on the sample. The intensity of the scattered light is measured as a function of its frequency. Bands appear in the spectrum that are shifted in frequency from the frequency of the incident light. Each shift in frequency corresponds to one of the vibrational frequencies of the molecule. Infrared spectroscopy measures the absorption of infrared light as a function of its frequency. Absorption bands appear in the spectrum that correspond to vibrational frequencies in the DNA. Infrared spectroscopy is hindered by the strong almost continuous absorption band of water while Raman spectroscopy benefits from the fact that water is a weak Raman scatterer. Infrared spectroscopy is of greater usefulness in the studies of films and fibers while Raman is of use in obtaining the vibrational frequencies of DNA in crystals, films, fibers, and aqueous solutions. A large amount of evidence, both theoretical and experimental, now exists which shows that there is a close relation between the conformation of a DNA and the frequencies and the intensities of certain bands in the Raman spectra. The theory for the relation between the frequencies and intensities and the DNA conformation is outlined in the next two sections. 

The vibrational states of a molecule are observed experimentally via infrared and Raman spectroscopy. These techniques can help to determine molecular structure and environment. In order to gain such useful information, it is necessary to determine what vibrational motion corresponds to each peak in the spectrum. This assignment can be quite difficult due to the large number of closely spaced peaks possible even in fairly simple molecules. In order to aid in this assignment, many workers use computer simulations to calculate the vibrational frequencies of molecules. This chapter presents a brief description of the various computational techniques available. 

The fingerprint region lies between 1300-400 cm-1 which is considered to be the most valuable component of the spectra and mainly comprises of a specifically large number of unassigned vibrations. Therefore, IR-spectroscopy aids in the identification of unknown compound by comparing its spectrum with a standard spectra recorded under exactly similar experimental parameters. Thus, pharmaceutical substances that exhibit the same infrared spectra may be inferred as identical. 

Infrared (IR) spectroscopy was the first modern spectroscopic method which became available to chemists for use in the identification of the structure of organic compounds. Not only is IR spectroscopy useful in determining which functional groups are present in a molecule, but also with more careful analysis of the spectrum, additional structural details can be obtained. For example, it is possible to determine whether an alkene is cis or trans. With the advent of nuclear magnetic resonance (NMR) spectroscopy, IR spectroscopy became used to a lesser extent in structural identification. This is because NMR spectra typically are more easily interpreted than are IR spectra. However, there was a renewed interest in IR spectroscopy in the late 1970s for the identification of highly unstable molecules. Concurrent with this renewed interest were advances in computational chemistry which allowed, for the first time, the actual computation of IR spectra of a molecular system with reasonable accuracy. This chapter describes how the confluence of a new experimental technique with that of improved computational methods led to a major advance in the structural identification of highly unstable molecules and reactive intermediates. 

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