Functional groups, determination infrared spectroscopy

Infrared and Raman spectroscopy are complementary methods used, for instance, to study molecular structure, identify compounds and functional groups, determine interatomic forces and bond-stretching distances, perform quantitative and qualitative analyses, and determine thermodynamic properties. The three states of matter may be studied by these methods over wide ranges of temperature and pressure. Selection rules for different molecular structures determine which spectral lines are allowed, and these rules differ for the infrared and Raman methods. This difference is used to advantage in studies of molecular structure, because two types of information are brought to bear on the same problem. 

Infrared spectroscopy, NMR spectroscopy, X-ray methods such as X-ray absorption near-edge structure spectroscopy and ESR spectroscopy have been used primarily to probe the detailed chemistry of heteroatom speciation, polar functional group determination, and hydrogen and carbon types in asphaltenes. The consensus seems to indicate that most asphaltene molecules have one to three heteroatoms (S, N, and O) per molecule. Sulfur exists predominantly as thio-phenic heterocycles (typically 65—85%) with the remainder as sulfidic groups (46,47). Thiophenic moieties are not... 

We saw in Chapter 10 that mass spectrometry gives a molecule s formula, infrared spectroscopy identifies a molecule s functional groups, and ultraviolet spectroscopy identifies a molecule s conjugated tt electron system. Nuclear magnetic resonance spectroscopy complements these other techniques by mapping a molecule s carbon-hydrogen framework. Taken together, mass spectrometry, IR, UV, and NMR make it possible to determine the structures of even very complex molecules. 

Before the advent of NMR spectroscopy infrared (IR) spectroscopy was the mstrumen tal method most often applied to determine the structure of organic compounds Although NMR spectroscopy m general tells us more about the structure of an unknown com pound IR still retains an important place m the chemist s inventory of spectroscopic methods because of its usefulness m identifying the presence of certain functional groups within a molecule... 

Infrared (IR) spectroscopy (Section 12.6) A kind of optical spectroscopy that uses infrared energy. IR spectroscopy is particularly useful in organic chemistry for determining the kinds of functional groups present in molecules. 

There is supporting evidence in the literature for the validity of this method two cases in particular substantiate it. In one, tests were made on plastics heated in the pressure of air. Differential infrared spectroscopy was used to determine the chemical changes at three temperatures, in the functional groups of a TP acrylonitrile, and a variety of TS phenolic plastics. The technique uses a film of un-aged plastic in the reference beam and the aged sample in the sample beam. Thus, the difference between the reference and the aged sample is a measure of the chemical changes. 

Previous authors have taught the principles of solving organic structures from spectra by using a combination of methods NMR, infrared spectroscopy (IR), ultraviolet spectroscopy (UV) and mass spectrometry (MS). However, the information available from UV and MS is limited in its predictive capability, and IR is useful mainly for determining the presence of functional groups, many of which are also visible in carbon-13 NMR spectra. Additional information such as elemental analysis values or molecular weights is also often presented. 

Several assumptions were made in order to analyze kinetic data in terms of this expression (2). First it was assumed that k 2 m kj, k2 k 3, and kj/k j k /k ( - If). Second it was assumed that the rate constants were independent of the extent of reaction i.e., that all six functional groups were equally reactive and that the reaction was not diffusion controlled. The concentration of polymer hydroxyl functionality was determined experimentally using infrared spectroscopy as described elsewhere (7). A major unknown is the instantaneous concentration of methanol. Fits to the kinetic data were made with a variety of assumptions concerning the methanol concentration. The best fit was achieved by assuming that the concentration of methanol was initally constant but decreased at a rate proportional to the concentration of residual polymer hydroxy groups towards the end of the reaction. As... 

A technique called infrared (IR) spectroscopy is valuable in the study of organic compounds. This technique allows researchers to determine the kinds of bonds and functional groups that are present in a molecule. Using a more advanced analysis, researchers are even able to determine other groups and bonds that are nearby. This information, paired with the molecular formula of a compound, helps researchers puzzle out the precise structure of an unknown molecule. 

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. 

The degradation of the cellulose fraction of the copolymer and subsequent recovery of the polyvinyl polymer have often been used to characterize the polymer. For example, cellulose may be acetylated and acid hydrolyzed to remove it from the copolymer. Then the recovered polymer can be dissolved, in solvent normally used for the polymer, and i the molecular weight of the polymer determined viscometrically (12, 42). As reported previously for polymers, such as polyacrylonitrile, a functional group on the polymer may be altered during the fractionating. These changes have been determined by infrared spectroscopy. For free-... 

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