Vibrational spectroscopy chemical functional groups

Further details of the theory and application of Raman spectroscopy in polymer studies can be found elsewhere (1. 9). However, vibrational frequencies of functional groups in polymers can be characterized from the spacing of the Raman lines and thus information complementary to IR absorption spectroscopy can be obtained. In addition, since visible radiation is used the technique can be applied to aqueous media in contrast to IR spectroscopy, allowing studies of synthetic polyelectrolytes and biopolymers to be undertaken. Conformation and crystallinity of polymers have also been shown to influence the Raman spectra Q.) while the possibility of studying scattering from small sample volumes in the focussed laser beam (-100 pm diameter) can provide information on localized changes in chemical structure. 

Practical problems associated with infrared dichroism measurements include the requirement of a band absorbance lower than 0.7 in the general case, in order to use the Beer-Lambert law in addition infrared bands should be sufficently well assigned and free of overlap with other bands. The specificity of infrared absorption bands to particular chemical functional groups makes infrared dichroism especially attractive for a detailed study of submolecular orientations of materials such as polymers. For instance, information on the orientation of both crystalline and amorphous phases in semicrystalline polymers may be obtained if absorption bands specific of each phase can be found. Polarized infrared spectroscopy can also yield detailed information on the orientational behavior of each component of a pol3mier blend or of the different chemical sequences of a copoljnner. Infrar dichroism studies do not require any chain labelling but owing to the mass dependence of the vibrational frequency, pronounced shifts result upon isotopic substitution. It is therefore possible to study binary mixtures of deuterated and normal polymers as well as isotopically-labelled block copolymers and thus obtain information simultaneously on the two t3q>es of units. 

Mbrational spectroscopy is well suited to elucidating the details on the surface of ionic liquids. This is due to the high degree of chemical information that is provided in the vibrational spectrum The various techniques mentioned earlier each have advantages and limitations, but the level of information is clearly unique and highly informative. From the vibrational spectrum, information as to the identity of the molecules is available. However, more specifically, the chemical functional groups are identified as they each have unique... 

If infrared absorption or Raman scattering is used as the contrast mechanism, vibrational spectra of samples can be obtained. The combination of the nanoscale spatial resolution of a scanned probe with the chemical specificity of vibrational spectroscopy allows in situ mapping of chemical functional groups with subwavelength spatial resolution. Figure 12 is a shear force image of a thin polystyrene film along with a representative near-field spectrum of the... 

THE INFRARED (IR) spectrum results from the interaction of radiation with molecular vibrations and, in gases, with molecular rotations. The spectrum itself is a plot of sample transmission of IR radiation as a function of wavelength or related units. Infrared spectroscopy is the physics that deals with the theory and interpretation of this spectrum and is one of the most popular techniques for identifying molecules. The IR spectrum can be used as a type of fingerprint unique to a molecule. In addition, the presence or absence of many chemical functional groups such as phenyls and carbonyls usually can be established from the spectrum. Quantitative analyses of mixtures can be obtained. Infrared spectra can be run for liquids, solids, or gases without special difficulties. Different types of spectrometers can be used, and a wide variety of sample handling techniques are available, many of which are described in this article. 

The interactions of photons with molecules are described by molecular cross-sections. For IR spectroscopy the cross-section is some two orders of magnitude smaller with respect to UV or fluorescence spectroscopy but about 10 orders of magnitude bigger than for Raman scattering. The peaks in IR spectra 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. The frequencies of the characteristic absorption bands lie within a relatively narrow range, almost independent of the composition of the rest of the molecule. The relative constancy of these group frequencies allows determination of the characteristic... 

IR spectra have many shaip absorption bands corresponding to fundamental vibrational transitions of different functional groups, the positions of which are influenced by the chemical environment of the group. Therefore, IR spectroscopy is well suited for the structural elucidation and identification of organic compounds. 

Vibrational spectroscopy is a very versatile and, chemically, well-resolved technique for the characterization of carbon-oxygen functional groups. The immense absorption problems of earlier experiments seems to be overcome in present times with modem FT-IR, DRIFTS or photoacoustic detection instruments. 

Near-infrared absorption is therefore essentially due to combination and overtone modes of higher energy fundamentals, such as C-H, N-H, and O-H stretches, which appear as lower overtones and lower order combination modes. Since the NIR absorption of polyatomic molecules thus mainly reflects vibrational contributions from very few functional groups, NIR spectroscopy is less suitable for detailed qualitative analysis than IR, which shows all (active) fundamentals and the overtones and combination modes of low-energy vibrations. On the other hand, since the vibrational intensities of near-infrared bands are considerably lower than those of corresponding infrared bands, optical layers of reasonable size (millimeters, centimeters) may be transmitted in the NIR, even in the case of liquid samples, compared to the layers of pm size which are detected in the infrared. This has important consequences for the direct quantitative study of chemical reactions, chemical equilibria, and phase equilibria via NIR spectroscopy. 

Since bonds between specific atoms have particular frequencies of vibration, IR spectroscopy provides a means of identifying the type of bonds in a molecule, e.g. all alcohols will have an O-H stretching frequency and all compounds containing a carbonyl group will have a C=0 stretching frequency. This property, which does not rely on chemical tests, is extremely useful in diagnosing the functional groups within a covalent molecule. 

Vibrational spectroscopy has been used to make significant contributions in many areas of chemistry and physics as well as in other areas of science. However, the main applications can be characterized as the study of intramolecular forces acting between the atoms of a molecule the intermolecular forces or degree of association in condensed phases the determination of molecular symmetries molecular dynamics die identification of functional groups, or compound identification the nature of the chemical bond and the calculation of thermodynamic properties. Ciuient plans are for the reviews to vary, from the application of vibrational spectroscopy to a specific set of compounds, to more general topics, such as force-constant calculations. It is hoped tiiat many of the articles will be sufficiently general to be of interest to other scientists as well as to the vibrational spectroscopist. 

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