Infrared spectroscopy absorption types

In the case of infrared spectroscopy, various types of infrared measurement methods (transmission method, attenuated total reflection (ATR) method, reflection-absorption (RA) method, surface-enhanced method, etc.) may be applied. 

In order to characterize the surface regions of a sample that has been modified in some way, as is usually the case in adhesion-related investigations, some sort of a reflection experiment is required. Two types of experiments, attenuated total reflection (ATR) and reflection-absorption infrared spectroscopy (RAIR),... 

Ultraviolet and infrared spectroscopy indicate that quinoxaline-2,3-dione type structures are preferred to tlie tautomeric 3-hydroxy-quinoxalin-2 One or 2,3-dihydroxyquinoxaline forms. The light absorption properties (UV) of quinoxaline-2,3-dione have been compared with those of its NN -, ON-, and OO -dimethyl derivatives (79, 80, and 81), and also its N- and 0-monomethyl derivatives (43 and 82). The parent dicarbonyl compound and its mono- and di-A -methyl derivatives show very strong carbonyl absorption near to 1690 cm split into two peaks. 

We have not yet addressed the important topic of absorption by the ligands in complexes. For many types of complexes, this type of spectral study (usually infrared spectroscopy) yields useful information regarding the structure and details of the bonding in the complexes. This topic will be discussed later in connection with several types of complexes containing specific ligands (e.g., CO, CN-, N02-, and olefins). 

Using infrared spectroscopy, Yates (299) proved the existence of hydroxyl groups on anatase as well as on rutile. Both forms still contained some adsorbed molecular water after evacuation at 150°, as evidenced by the bending vibration at 1605 cm b After outgassing at 350°, no free water was detected. There remained two OH stretching absorptions in the case of anatase (at 3715 and 3675 cm ) and one weak band at 3680 cm with rutile. This is indication of the existence of two different types of OH groups on anatase. These results were confirmed by Smith (300). 

InfraRed Spectroscopy (IR). Infrared speetroscopy is an efifeetive method for eharaeterization of polymers as to ehemieal structure. IR speetra of the sample examined are obtained by two basie types of IR spectrometers dispersive or Fourier transform (FTIR) ones. Infrared spectra are usually presented as a dependence of absorption (in pereent transmission) on wave length or wave... 

The wavelengths of IR absorption bands are characteristic of specific types of chemical bonds. In the past infrared had little application in protein analysis due to instrumentation and interpretation limitations. The development of Fourier transform infrared spectroscopy (FUR) makes it possible to characterize proteins using IR techniques (Surewicz et al. 1993). Several IR absorption regions are important for protein analysis. The amide I groups in proteins have a vibration absorption frequency of 1630-1670 cm. Secondary structures of proteins such as alpha(a)-helix and beta(P)-sheet have amide absorptions of 1645-1660 cm-1 and 1665-1680 cm, respectively. Random coil has absorptions in the range of 1660-1670 cm These characterization criteria come from studies of model polypeptides with known secondary structures. Thus, FTIR is useful in conformational analysis of peptides and proteins (Arrondo et al. 1993). 

Infrared spectroscopy can be used to classify metal-dioxygen complexes as either superoxo species (p(O-O) from 1200 to 1070 cm-1) or as peroxo species (p(O-O) from 930 to 740 cm-1) (49). However, this system fails to accurately define the type of dioxygen species present in 8 and 15, as these complexes exhibit v(O-O) absorptions at 961 and 941 cm-1, respectively. Preparation of the complexes with 180-enriched dioxygen confirmed that the dioxygen was bound side-on (if) in these complexes Complex 8 exhibited isotopically shifted vibrations indicative of a side-on bound dioxygen 0(160-160) absorption at 961 cm-1, p(160-180) at 937 cm-1, and /(180-180) at 908 cm-1), as did complex... 

Infrared spectroscopy is not as inherently informative with regard to metal interactions in highly symmetrical metal-metal bound dimers as is Raman spectroscopy, since the totally symmetric metal-metal stretch is a forbidden absorption in the infrared experiment. Oldham and Ketteringham have prepared mixed-halide dimers of the type Re2ClxBr 2xto lower the symmetry and hence introduce some infrared allowedness into the Re-Re stretching mode (206). Indeed, the appearance of a medium-intensity band at 274 cm 1 in the infrared spectrum of the mixed-halo species was considered to be the result of absorption by the metal—metal stretching vibration, which was also observed in the Raman spectrum at 274 cm ". ... 

Infrared Spectroscopy. Spectra were obtained on a Perkin-Elmer model 521 grating infrared spectrophotometer. The amount of branching in these materials is very small and appears to be of the methyl type, which is indicated by the small absorption near 1140 cm"1. Ethyl branching was also sought but not detected at 770 cm 1. The results are summarized in Table II. 

The primary techniques used in this study include X-ray photoelectron spectroscopy (XPS), reflection-absorption infrared spectroscopy (RAIR), and attenuated total reflectance infrared spectroscopy (ATR). XPS is the most surface-sensitive technique of the three. It provides quantitative information about the elemental composition of near-surface regions (< ca. 50 A sampling depth), but gives the least specific information about chemical structure. RAIR is restricted to the study of thin films on reflective substrates and is ideal for film thicknesses of the order of a few tens of angstroms. As a vibrational spectroscopy, it provides the type of structure-specific information that is difficult to obtain from XPS. The... 

Individual near-infrared absorption bands have absorptivities on the order of 10 1 Al /mm/mM for peak absorption bands in aqueous matrices.30 Such low absorptivities limit detection to the major components within skin tissue. As a general rule of thumb, substances must be present at concentrations above 1 mM to be quantified by near-infrared spectroscopy. Although such low absorptivities greatly restrict the number of possible analytes one can measure in clinical samples, the inability to measure chemicals present below millimolar concentrations enhances selectivity by rendering measurements insensitive to many different types of endogenous molecules. Only the major chemical components of these biological samples must be considered for selectivity purposes. 

O-H bond. Among such properties a prominent one is the ultraviolet absorption spectrum and the theory may therefore be used for the examination of some of the spectroscopic shifts which accompany the lactam-lactim tautomerization. Much caution must, however, be exercised in this respect. Thus, in a recent paper Kwiatkowski135,137 performed Pariser-Parr-Pople-type calculations on the electronic structure of hydroxypurines, essentially to interpret their ultraviolet spectra. In these calculations he assumed that these compounds exist predominantly in their lactim form, and the results of his calculations, at least for 6- and 8-hydroxypurine, did not seem to contradict this assumption. It is only in the case of the 2-hydroxy isomer that a particularly striking disagreement between theory and experiment led him to admit that this last compound may exist in the lactam form. Calculations carried out for this form gave, in fact, a more satisfactory agreement with experiment.138 As we have seen, unambiguous infrared spectroscopy evidence clearly show s that all three isomers exist essentially in the lactam form. This shows that ultraviolet absorption may provide only very uncertain evidence about the lactam-lactim tautomerism in hydroxypurines and related compounds. 

In order to assemble gold nanorods (aspect ratio 2.4) by the click reaction, one batch of nanorods capped with 4-azidobutane-l-thiol, A, and another with hex-5-yn-l-thiol, B, were reacted in the acetonitrile-water mixture under standard conditions [12], The occurrence of the click reaction was established by infrared spectroscopy [13], The product of the reaction was investigated by electronic absorption spectroscopy and transmission electron microscopy. Fig. 1 compares the electronic absorption spectra of the gold nanorods before and after the click reaction between the A and B type nanorods. Isolated gold nanorods show transverse and longitudinal plasmon bands around 520 nm and 630 nm, respectively (see Fig. la). 

Different types of carbonyl groups give characteristic strong absorptions at different positions in the infrared spectrum. As a result, infrared spectroscopy is often the best method to detect and differentiate these carboxylic acid derivatives. Table 21-3 summarizes the characteristic IR absorptions of carbonyl functional groups. As in Chapter 12, we are using about 1710 cm-1 for simple ketones and acids as a standard for comparison. Appendix 2 gives a more complete table of characteristic IR frequencies. 

The combination of infrared spectroscopy and XAS has been extremely useful in the understanding of site structure. Infrared spectra [13, 50, 52] of adsorbed probe molecules can help to differentiate between different types of site. They are discriminative in the sense that the probe molecules will adsorb with different thermodynamic parameters on the different sites. XAS on the other hand will average over all the different sites present in the zeolite. This can of course be an advantage, but is also a disadvantage in the sense that the active site can be lost in the signal of the other species. Some combined X-ray absorption infrared instrumentation is currently being developed and tested for metal catalysts [53,... 

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