Fundamental molecular vibrations

Unlike the relatively straightforward infrared spectra, which arise mainly from fundamental molecular vibrations and rotations, and where molecular components can be readily ascribed to the fingerprint of peaks, the visual interpretation of NIR spectra is virtually impossible and speculative at best. 

However, what unite all applications of NIRS for PAC are the unique features of the NIR spectrum. The NIR is in effect the chemical spectroscopy of the hydrogen atom in its various molecular manifestations. The frequency range of the NIR from about 4000 cm-1 up to 12 500 cm-1 (800-2500 nm) covers mainly overtones and combinations of the lower-energy fundamental molecular vibrations that include at least one X—H bond vibration. These are characteristically significantly weaker in absorption cross-section, compared with the fundamental vibrational bands from which they originate. They are faint echoes of these mid-IR absorptions. Thus, for example, NIR absorption bands formed as combinations of mid-IR fundamental frequencies (for example v + u2), typically have intensities ten times weaker than the weaker of the two original mid-IR bands. For NIR overtone absorptions (for example 2v, 2v2) the decrease in intensity can be 20-100 times that of the original band. 

Light that is scattered from a molecule is primarily elastically scattered that is, the incident and the scattered photons have the same energy. A small probability exists, however, that a photon is scattered inelastically, resulting in either a net gain or loss of energy of the scattered photon. This inelastic scattering, discovered by Raman and Krishna,1 allows fundamental molecular vibrational transitions to be measured at any excitation wavelength. 

Figure 4.5 Viscosity versus inverse temperature for glass-forming liquids, showing behavior classified as strong, typified by open tetrahedral networks, to fragile, typical of ionic and molecular liquids. Here Tg is defined by the criterion that nlT ) = 10 P. For most of the liquids, the viscosities seem to extrapolate to a common value of around 10" P at high temperatures, corresponding to a fundamental molecular vibrational frequency of around 10 sec-i. (Reprinted from J. Non-Cryst. -Solids, 73 1, Angell (1985), with kind permission from Elsevier Science—NL, Sara Burgerhartstraat 25, 1055 KV Amsterdam, The Netherlands.)...

The mid-IR spectral range contains much denser and more selective information compared to the near-IR, where overtone and combination bands of the fundamental molecular vibrations occur. As the intensity of bands in the mid-IR is higher, optical path lengths are on the micrometer scale. Such path lengths can be achieved with a special technique that uses attenuated total reflection, and which renders optical materials compatible with aqueous biofluids. Recently, dry films of biosamples of nanoliter volumes have been successfully applied for reagent-free... 

Various aspects differentiate a mid-infrared spectrum from the corresponding near-infrared spectrum (Table 6). In the mid-infrared region fundamental molecular vibrations always occur between 4000 and 200 cm , while in the near-infrared region overtones and combination bands of these fundamentals are observed between 12 8(X) and 40(X)cm". Near-infrared bands are due primarily to hydrogenic stretches of C-H. N-H, and... 

The term infrared spectroscopy can be generally applied to measurements in at least three spectral regions the near infrared, the mid-infrared, and the far infrared. Commonly, when the term infrared is applied, this implies the mid-infrared spectral region, today defined by the frequency range 4000-400 cm (2.5-25 / m 2500-25,000 nm in units of wavelength). This range tends to be arbitrarily defined, but the upper spectral limit does define the extent of all the fundamental molecular vibrations, with the exception of that of hydrogen fluoride. 

Prior to World War II, the near-infrared (NIR) region was not considered to be particularly useful for organic compositional spectroscopy. The explanation for this line of reasoning seemed obvious. The NIR region consisted only of overtone and combination bands of fundamental molecular vibrations. It was also obvious that all these combination and overtone bands occur in a relatively narrow region (750-3,000 nm) as compared to the fundamental bands occurring at 2,800-50,000 nm. Thus it was observed that NIR bands are severely overlapped, difficult to resolve, and, once resolved, difficult to interpret. If the same information is obtained with better resolution in the infrared (IR) region, then why would any chemist be interested in the difficulties inherent with NIR spectroscopy ... 

Molecular vibration absorptions are typically observed at infrared wavelengths, which correspond to the resonance frequencies for fundamental molecular vibrations. Because of the molecular potential anharmonicity, however, overtone and combination absorption bands also appear at visible and near-infrared wavelengths. The predominant factor for attenuation in POFs has been the stretching overtone absorptions of C-H bonds. 

Fundamental Molecular Vibrations Mid-infrared and Raman Bands... 

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