Absorption bands and spectra

Absorption bands and spectra indicate how light absorption varies with wavelength. The absorption at a particular wavelength by a certain species is quantitatively described using an absorption coefficient", Ex (e is also... 

A number of analytical methods have been developed for the determination of chlorotoluene mixtures by gas chromatography. These are used for determinations in environments such as air near industry (62) and soil (63). Liquid crystal stationary columns are more effective in separating m- and chlorotoluene than conventional columns (64). Prepacked columns are commercially available. ZeoHtes have been examined extensively as a means to separate chlorotoluene mixtures (see Molecularsieves). For example, a Y-type 2eohte containing sodium and copper has been used to separate y -chlorotoluene from its isomers by selective absorption (65). The presence of ben2ylic impurities in chlorotoluenes is determined by standard methods for hydroly2able chlorine. Proton (66) and carbon-13 chemical shifts, characteristic in absorption bands, and principal mass spectral peaks are available along with sources of reference spectra (67). 

The side-chain chlorine contents of benzyl chloride, benzal chloride, and benzotrichlorides are determined by hydrolysis with methanolic sodium hydroxide followed by titration with silver nitrate. Total chlorine determination, including ring chlorine, is made by standard combustion methods (55). Several procedures for the gas chromatographic analysis of chlorotoluene mixtures have been described (56,57). Proton and nuclear magnetic resonance shifts, characteristic iafrared absorption bands, and principal mass spectral peaks have been summarized including sources of reference spectra (58). Procedures for measuring trace benzyl chloride ia air (59) and ia water (60) have been described. 

The intensity ratio between typical absorption bands and the least changing band was used to illustrate the changes in the spectra. The change (m) in the absorption bands is mainly observed at the initial irradiation stage (30-40 min). The most marked decrease in the band intensity was found for the carbonyl group in the anhydride unit (1770 cm-1). 

The major applications of the OTTLE cell are (i) to obtain spectra of electrogenerated species and thence to obtain the extinction coefficients of its major absorption bands, and (ii) to determine the standard redox potential of a reversible couple. The latter experiment relies on the thin-layer electrochemical characteristics of the cell. Thus, for the couple ... 

IR spectra are characterized by rather sharp absorption bands and each such band is characteristic of a particular covalent bond in the sample molecule. An example is shown in Figure 8.14. Thus, while IR spectra are molecular fingerprints, as are UV-VIS spectra, they have a greater worth to a qualitative analysis scheme due to the specificity of the absorption bands. 

In acetonitrile-dichloromethane 1 1 v/v solution, their absorption spectra are dominated by naphthalene absorption bands and they exhibit three types of emission bands, assigned to naphthyl localized excited states (/Wx = 337 nm), naphthyl excimers (Amax ca. 390 nm), and naphthyl-amine exciplexes (/lmax = 480 nm) (solid lines in Fig. 3). The tetraamine cyclam core undergoes only two protonation reactions, which not only prevent exciplex formation for electronic reasons but also cause strong nuclear rearrangements in the cyclam structure which affect excimer formation between the peripheral naphthyl units of the dendrimers. 

A certain transition metal ion presents two optical absorption bands in a host crystal whose zero-phonon lines are at 600 nm and 700 nm, respectively. The former band has a Huang-Rhys parameter 5 = 4, while for the latter 5 = 0. Assuming coupling with a phonon of 300 cm for the two bands (a) display the 0 K absorption spectrum (absorption versus wavelength) for such a transition metal ion (b) display the emission spectra that you expect to obtain nnder excitation in both absorption bands and (c) explain how you expect these two bands to be affected by a temperature increase. 

Molecules generally have a large number of bonds and each bond may have several IR-active vibrational modes. IR spectra are complex and have many overlapping absorption bands. IR spectra are sufficiently complex that the spectmm for each compound is unique and this makes IR spectra very useful for identifying compounds by direct comparison with spectra from authentic samples "fingerprinting"). 

ZnTPPS/ZnTTAP] (17). It is found that dimerization is accompanied by (a) a blue shift of the Soret band, (b) red shifts of the Q bands, (c) red shift of the fluorescence maximum, and (d) strong quenching of fluorescence. Optical spectra of face-to-face covalently linked diporphyrins show the same trends (20,21). However, in one instance formation of a covalently-linked diporphyrin does not lead to a Soret band shift ( ). Of the systems considered here, [TPPS]2 and [ZnTPPSl2 also fail to show a Soret shift even though the visible absorption bands and fluorescence show clear evidence of dimer formation (10). A summary of observed dimerization induced... 

The UV spectra of 4-hydroxy-5-iminoalkyl-3//-l,3-thiazine-2,6-diones 49 display three strong absorption bands at Amax = 199-209, 227-292, and 298-315 nm. The 5-acyM-hydroxy-3//-l,3-thiazine-2,6-diones 61 also display three absorption bands and these appear at A ax = 199, 216-218, and 276-279 nm <2005RJC134>. The UV bands in 62-66 undergo a red shift when the methyl (R = Me) is replaced by an allyl, benzyl, or aryl group <2005RJC134>. 

The transient absorption spectra of duplexes with [2AP]A4GGAs are depicted in Fig. 5. At a delay time of 100 ns, the transient absorption spectrum is attributed to the superposition of the spectra of the 2AP(-H) and G /G (-H) radical products and the hydrated electrons. The structureless tail of the eh absorption in the 350-600 nm region decays completely within At<500 ns. The formation of G VG(-H) radicals monitored by the rise of the 310-nm absorption band and associated with the decay of the 2AP V 2AP(-H) transient absorption bands at 365 and 510 nm (Fig. 5) occurs in at least three well-separated time domains (Fig. 6). The prompt (<100 ns) rise of the transient absorption at 312 nm due to guanine oxidation by 2AP was not resolved in our experiments. However, the ampHtude, A((=ioo), related to the prompt formation of the G /G(-H) radicals (Fig. 6a) can be estimated using the extinction coefficients of the radical species at 312 and 330 nm (isosbestic point) [11]. The kinetics of the G VG(-H) formation in the yits and ms time intervals were time-resolved and characterized by two well-defined components shown in Fig. 6a (0.5 /zs) and Fig. 6b (60 /zs). 

Organic and inorganic molecular species (except homonuclear molecules) absorb in the IR region. IR spectroscopy has the potential to determine the identity of an unusually large number of substances. Moreover, the uniqueness of a MIR spectrum confers a degree of specificity which is matched or exceeded by relatively few other analytical methods. This specificity has found particular applications for the development of quantitative IR absorption methods. However, these differ from quantitative UV/Vis techniques in their greater spectral complexity, narrower absorption bands, and the technical limitations of IR instruments. Quantitative determinations obtained from IR spectra are usually inferior in quality and robustness to those obtained with UV/Vis and NIR spectroscopy. In addition, univariate or linear cali-... 

Formation of MH+ ions on adsorption of 2V,iV-dimethylaniline and stabilization in the adsorbed state should be accompanied by the appearance of the known 260-nm absorption band (18, 23). This band was observed in our work in the spectra of adsorbed aniline at low adsorption. In the spectra (Figure 1) this band is not visible because of the superposition of the MH+ absorption bands and the presence of the excess adsorbed N,N-... 

Difference spectra are usually recorded by placing the unperturbed spectrum in the reference light beam of a spectrophotometer and the perturbed solution in the sample beam in carefully matched cuvettes. However, the spectrum shown in Fig. 23-11B was obtained by recording the two spectra independently and subtracting them with the aid of a computer. The same data have been treated in another way by fitting two log normal curves (p. 1283) to the absorption bands and plotting the differences between the mathematically... 

The luminescence spectra of rigid molecules like benzene, naphthalene, etc. show clear vibrational structures, especially in the gas phase. The first absorption band and the fluorescence band then show a mirror-image relationship in which the 0-0 vibrational transitions are nearly coincident (Figure 3.28). At room temperature only the lowest vibrational levels (v = 0)... 

Reactive Dyes. These dyes form a covalent bond with the fiber, usually cotton, although they are used to a small extent on wool and nylon. This class of dyes, first introduced commercially in 1956 by ICI, made it possible to achieve extremely high washfastness properties by relatively simple dyeing methods. A marked advantage of reactive dyes over direct dyes is that their chemical structures are much simpler, their absorption spectra show narrower absorption bands, and the dyeings are brighter. The principal chemical classes of reactive dyes are azo (including metallized azo), triphendioxazine, phthalocyanine, formazan, and anthraquinone (see Section 3.1). 

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