Vibrations bonds, near-infrared

In short, near-infrared spectra arise from the same source as mid-range (or normal ) infrared spectroscopy vibrations, stretches, and rotations of atoms about a chemical bond. In a classical model of the vibrations between two atoms, Hooke s Law was used to provide a basis for the math. This equation gave the lowest or base energies that arise from a harmonic (diatomic) oscillator, namely ... 

The second type of atomic polarization is attributable to intramolecular vibrations of the bonds between atoms and the change of bond length. It occurs in the near-infrared region with the frequency V of the order of lO -10 s . ... 

Near-Infrared Spectroscopy. Near-infrared (NIR) spectroscopy is a technique that has been around for some time but, like NMR spectroscopy, has only recently been improved and developed for on-line applications. Near-infrared analysis (NIRA) is a nondestructive technique that is versatile in the sense that it allows many constituents to be analyzed simultaneously 112, 113). The NIR spectrum of a sample depends upon the anharmonic bond vibrations of the constituent molecules. This condition means that the temperature, moisture content, bonding changes, and concentrations of various components in the sample can be determined simultaneously. In addition, scattering by particles such as sand and clay in the sample also allows (in principle) the determination of particle size distributions by NIRA. Such analyses can be used to determine the size of droplets in oil-water emulsions. 

This works well for the fundamental vibrational frequency of simple diatomic molecules, and is not too far from the average value of a two-atom stretch within a polyatomic molecule. However, this approximation only gives the average or center frequency of the diatomic bond. In addition, one might expect that since the reduced masses of, for example, CH, OH, and NH are 0.85, 0.89, and 0.87, respectively (these constitute the major absorption bands in the near-infrared spectrum), the ideal frequencies of all these pairs would be quite similar. 

We have developed a simple physical model suggesting that for most, if not all, chiral macromolecules, their terahertz absorption will be accompanied by terahertz circular dichroism. Ab initio methods have been used to successfully calculate the vibrational circular dichroism spectra of small molecules, however, these models have largely focused on spectra in the near-infrared, spectra dominated by bond stretching and bending, and the wagging of small... 

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