Characteristic absorption bands

Appreciable interaction between chromophores does not occur unless they are linked directly to each other, or forced into close proximity as a result of molecular stereochemical configuration. Interposition of a single methylene group, or meta orientation about an aromatic ring, is sufficient to insulate chromophores almost completely from each other. Certain combinations of functional groups afford chromophoric systems which give rise to characteristic absorption bands. 

The most powerful method for stmcture elucidation of steroid compounds during the classical period of steroid chemistry (- 1940 1950s) was ir-spectroscopy. As with the ultraviolet spectra, data collected on the infrared spectra of steroids are available in several books, spectmm atiases, and review articles (265,266). Unlike ultraviolet spectroscopy, even the least substituted steroid derivatives are relatively rich in characteristic absorption bands in infrared spectroscopy (264). 

Esters are usually readily identified by their spectroscopic properties (70). Among these, infrared spectroscopy (ir) is especially useful for identifying the carbonyl of the ester group that has characteristic absorption bands. The C=0 absorption is very strong in the ir at 1750-1735 cm in addition,... 

Since IR spectra are essentially due to vibrational transitions, many substituents with single bonds or isolated double bonds give rise to characteristic absorption bands within a limited frequency range in contrast, the absorption due to conjugated multiple bonds is usually not characteristic and cannot be ascribed to any particular grouping. Thus IR spectra afford reference data for identification of pyrimidines, for the identification of certain attached groups and as an aid in studying qualitatively the tautomerism (if any) of pyrimidinones, pyrimidinethiones and pyrimidinamines in the solid state or in non-protic solvents (see Section 2.13.1.8). 

IR spectra of PAN obtained from the CAN-1,3-diketone systems revealed that, besides the characteristic absorption band of CN group at 2244 cm , the characteristic absorption bands at 1727 and 1700 cm (keto form of 1,3-diketone) and 1625 cm (enol form of 1,3-diketone) were observed simultaneously. Therefore, the 1,3-diketone moiety is present as an end group in PAN. 

To use KBr discs for quantitative measurements it is best to employ an internal standard procedure in which a substance possessing a prominent isolated infrared absorption band is mixed with the potassium bromide. The substance most commonly used is potassium thiocyanate, KSCN, which is intimately mixed and ground to give a uniform concentration, usually 0.1-0.2 per cent, in the potassium bromide. A KBr/KSCN disc will give a characteristic absorption band at 2125 cm 1. Before quantitative measurements can be carried out it is necessary to prepare a calibration curve from a series of standards made using different amounts of the pure organic compound with the KBr/KSCN. A practical application of this is given in Section 19.9. 

After the laser flash, one then monitors the progress of events by some rapidly responding method. Conductivity, absorption spectroscopy, and fluorescence spectroscopy are the methods most commonly used. If a reaction product has a characteristic absorption band of sufficient intensity, one can monitor its buildup with time. This might be a UV, visible, or IR band. The need for a band with a high molar absorptivity arises because the reactive transient is usually present at a relatively low concentration, KT6-lCr5 M being typical. If the species of interest is phosphorescent, then the timed decay of its phosphorescence intensity can be recorded. 

Except in simple cases, it is very difficult to predict the infrared absorption spectrum of a polyatomic molecule, because each of the modes has its characteristic absorption frequency rather than just the single frequency of a diatomic molecule. However, certain groups, such as a benzene ring or a carbonyl group, have characteristic frequencies, and their presence can often be detected in a spectrum. Thus, an infrared spectrum can be used to identify the species present in a sample by looking for the characteristic absorption bands associated with various groups. An example and its analysis is shown in Fig. 3. 

This section is completed with a brief review of the synthesis and properties of this epimer (20) of the precursor of thiazole in bacteria. This pentulose is conveniently accessible by an unconventional route (Scheme 19). Methyl 2,3 4,6-di-O-isopropylidene-a-D-mannopyranoside, readily available from methyl ot-D-mannopyranoside, is converted to the ketonic glycoside by butyllithium in 91% yield, following a method first published by Klemer and Rodemeyer43 and scaled up by Horton and Weckerle.44 This was converted by means of lithium hydroxide in a water-ether mixture into 3,5-0-benzylidene-l-deoxy-D-eryf/iro-2-pen-tulose in 55% yield. Hydrolysis to the free pentulose (20) proceeded in 73% yield in aqueous acetic acid. This product was obtained as a syrup with a characteristic absorption band at 1705 cm 1 as a film. Thus, there is a fair proportion of the open-chain ketone under these conditions, as with the D-threo epimer.45... 

Figure lb shows the transient absorption spectra of RF (i.e. the difference between the ground singlet and excited triplet states) obtained by laser-flash photolysis using a Nd Yag pulsed laser operating at 355 nm (10 ns pulse width) as excitation source. At short times after the laser pulse, the transient spectrum shows the characteristic absorption of the lowest vibrational triplet state transitions (0 <— 0) and (1 <— 0) at approximately 715 and 660 nm, respectively. In the absence of GA, the initial triplet state decays with a lifetime around 27 ps in deoxygenated solutions by dismutation reaction to form semi oxidized and semi reduced forms with characteristic absorption bands at 360 nm and 500-600 nm and (Melo et al., 1999). However, in the presence of GA, the SRF is efficiently quenched by the gum with a bimolecular rate constant = 1.6x10 M-is-i calculated... 

The formation of monomer and dimer of (salen)Co AIX3 complex can be confirmed by Al NMR. Monomer complex la show Al NMR chemical shift on 5=43.1 ppm line width =30.2 Hz and dimer complex lb 5=37.7 ppm line width =12.7 Hz. Further instrumental evidence may be viewed by UV-Vis spectrophotometer. The new synthesized complex showed absorption band at 370 nm. The characteristic absorption band of the precatalyst Co(salen) at 420 nm disappeared (Figure 1). It has long been known that oxygen atoms of the metal complexes of the SchifT bases are able to coordinate to the transition and group 13 metals to form bi- and trinuclear complex [9]. On these proofs the possible structure is shown in Scheme 1. 

The infrared spectra, in the range 1200-400 cm of the acids and of all their salts exhibited the same characteristic absorption bands due to the vibrations of the PWi2O40 or SiWi204o anions as already well described by Rocchiccioli-DeltchefF et al [14,15]. 

The IR spectra of carbohydrazide 9 showed absorption bands at 3317 cm (OH,Hydrazide NH2), 3269 cm (aromatic CH), 1711 cm (CO stretching), and 1621-1640 cm (CO-NH-NH2 groups). The H NMR spectra exhibited a singlet due to the CONHNH2, NH proton at 9.32 ppm. Methylene protons resonated as a singlet at 4.23 ppm. The structures of the products lOa-1 were inferred from their analytical and spectral data. Thus, their IR spectra showed characteristic absorption bands at 3400-3240 cm (NH,OH), 1710-1700 cm (lactone CO), and NHCO at 1650-1600 cm . ... 

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... 

A method that is stated to be applicable to residues of benzene hexachloride (20) is based on the fact that benzene hexachloride yields essentially 1,2,4-trichlorobenzene on dehydrohalogenation with alkali. This product possesses a characteristic absorption band in the ultraviolet, which permits its quantitative determination. 

Complex formation constants could also be determined directly from UV spectrophotometric measurements. Addition of tert.-butyl hydroperoxide to a solution of nitroxide I in heptane at RT causes a shift of the characteristic absorption band of NO at 460 nm to lower wavelengths (Fig. 9). This displacement allows calculation of a complex equilibrium constant of 5 1 1/Mol. Addition of amine II to the same solution causes reverse shift of theC NO" absorption band. From this one can estimate a complex formation constant for amine II and +00H of 12 5 1/Mol (23 2 1/Mol was obtained for tert.-butyl hydroperoxide and 2,2,6,6-tetramethylpipe-ridine in ref. 64b). Further confirmation for an interaction between hindered amines and hydroperoxides is supplied by NMR measurements. Figure 10a shows part of the +00H spectrum in toluene-dg (concentration 0.2 Mol/1) with the signal for the hydroperoxy proton at 6.7 ppm. Addition of as little as 0.002 Mol/1 of tetra-methylpiperidine to the same solution results in a displacement and marked broadening of the band (Fig. 10b). 

Using characteristic absorption bands at higher wavenumber (shorter wavelength) — i.e., O-H and C-H absorptions (in many cases not really helpful unless they are specific). 

In the end, what matters obviously is whether the features of interest distinctly show up in the analytical data, e.g., in a spectrum or a map or image. Thus, even though IR is not ultimately a surface specific technique, when an outermost surface layer reveals characteristic absorption bands that are related to say a property such as adhesion, the technique can be a very valuable one and is extensively applied. 

In addition to the simple chemical methods for following these processes, infrared spectroscopy may also be used. In Fig. 9 is shown the spectrum of silica dried at 200°C before and after reaction with Zr(allyl)4- The characteristic absorption bands of the transition metal-allyl group are clearly displayed, also a significant reduction in the number of hydroxyl groups (3740 cm-1) is also clearly evident. 

Both ionic forms have the same set of characteristic bands in their IR spectra and thus cannot be identified in solution. The 3IP NMR spectrum contains only one averaged signal. The IR study revealed tautomeric transformations in solution. The characteristic absorption band of the P—H bond and carbonyl group appeared in solution spectra that is possible only on dissociation of the a-hydroxyalkyl fragment, present only in the second tautomeric form. An X-ray single-crystal study showed the compound (128) to be in a cyclic form. 

From the mechanism and values of the rate constants, the formation of B occurs very rapidly within a few hundred picoseconds and AB is formed on the microsecond time scale. These species exhibit characteristic absorption bands in the 550 to 600 nm region of the spectrum. At very long times, i.e. several seconds of steady state irradiation, the red shift in the absorption band is complete and presumably due to AnB as suggested by Krongauz (1 —2) Thus far, it has not been possible to clearly time resolve the formation of aggregates from AB dimers, although subtleties in the transient absorption indicate this is occurring. For instance, the time resolved buildup in absorbance at the red end of 600 nm band seems to be slower than it is 10 or 20 nm further to the blue. This may indicate a process such as ... 

Polymers with specific functionalities can be realized by incorporating various dopants, such as laser dyes, rare earth ions, quantum dots, and functional chromophores into the host polymer. Chromophores are molecules or chemical groups as part of a larger molecule, and they have characteristic absorption bands in the... 

Vibrational spectroscopy is the experimentalist s most powerful tool for studying the effects of changes in local environment on individual chemical bonds. Studies of simple adsorbates like CO which have strong characteristic absorption bands have contributed greatly to our understanding of adsorption processes at surfaces (1). As shown here and in other papers in this symposium, recent experimental developments have led to a renewed effort to use the vibrational spectroscopy of adsorbates as a probe for understanding the physical chemistry of metal/electrolyte interfaces. 

The values for the ionization and protonation constants (0.21 and 0.24 respectively) indicate that [Mo205(OH2)6]2+ is the major dimeric species in 1.0 M acid with about equal concentrations of the other two dimers. The dimeric cationic species show a characteristic absorption band in the UV at —245 nm. 

As shown in Figure 3, the change in Rg is non-linearly related to the cis content expressed by the increase in absorbance at 4.50 nm, a characteristic absorption band of cis-azobenzene. This nonlinearity is attributed to the intrinsic nature of phase transformation occurring at a critical condition. A sudden change in Rq commences when the cis content reaches the critical value which can trigger the transformation. 

The study of ZnCFO also was carried out by the method of IR-spectroscopy on spectrometer UR-20. For detection of possible chemical bonds between inorganic and organic components in ZnCFO the spectras of Zn(OH)2, CFO and ZnCFO were studied (fig. 3). As it is shown, IR-spectrum of ZnCFO repeats the characteristic absorption bands of CFO at 3350, 1640 and 1550 cm"1, caused by presence of secondary amide group [5], at the same time the... 

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