The Absence of Absorption Bands

The absence of an absorption band can be as useful as the presence of one in identifying a compound by IR spectroscopy. 

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It has been shown that polypyridyl Rh(III) complexes induce photo-cleavages of the sugar phosphate backbone of double-stranded DNA with a higher relative quantum yield than Ru(II) complexes of phen or DIP. Thus replacement of Ru(II) ions by Rh(III) in Tris(phen) complexes, increases the efficiency of DNA photo-cleavages. However, in contrast to the Ru(II) complexes, Rh(III) samples have to be illuminated in the UV because of the absence of absorption bands in the visible region. 

Titanium modified beta zeolite (Ox-Ti-p) was prepared by treating aluminum containing beta zeolite (Na-K-, NH4- or H-form) with ammonium titanyl oxalate solutions of different concentrations. The obtained catalysts were characterized by atomic absorption, XRD, IR, UV-Vis and XPS. The incorporation of Ti into the zeolite framework was evidenced by (i) the increase in the [302] interplanar d-spacing, (ii) the increase in the intensity of the 960 cm IR band, (iii) the presence of an absorption band at 220 nm and the absence of absorption band due to TiOa in UV-Vis, (iv) the high binding energy of Ti 2pa/2 (459.9 eV) and (v) the catalytic activity in epoxidation reactions. The amount of Ti in the samples was found to depend on the concentration of ammonium titanyl oxalate solutions and also on the nature of the counter-cations of the parent beta zeolite. 

The adsorption of the rutheniiim complex on the surface of Pd is confirmed by the fact that the saturating concentration of the supported Ru3(CO)] 2 comprises one ruthenium atom per 3-5 palladium surface atoms (Table II). In addition, the absence of absorption bands from CO adsorbed on palladium is due to the blockage of its surface by the adsorbed ruthenium complex. 

The spectrum of R has absorption bands in the 1000 cm-1 region that can be attributed to coordinated THF.(7, 8) The broad absorption in the spectrum of P in the 1000-1150 cm"" region is probably due to skeletal and deformation vibrations.(5) The absence of absorption at 1200-1350 cm-1 indicates that there are probably no hydride hydrogens.(8)... 

In the ultraviolet SnCl2 showed a continuous absorption with a maximum intensity at about 21,044 cm"1 (3220 A). The absence of discrete bands is probably due to overlapping of closely spaced diffuse bands. For PbCl2 three regions of continuous absorption were observed. These had maximum intensities at 3600, 3200 and below 2916 A. The SnCl2 and PbCl2 spectra were interpreted as being due to 1A2 - 1B2 transitions 119 ... 

Strictly speaking, the values of e, Ac, A, and AA need to be obtained by integration over the spectral band however, since, for a fundamental transition, the VCD and its parent absorption band have the same shape, the anisotropy ratio can be obtained, in the absence of interfering bands due to other transitions, by taking the ratios of intensities at corresponding spectral positions, such as peak locations. The anisotropy ratio is also of interest for theoretical reasons since it is a dimensionless quantity that can be compared to the results of calculations vide infra). 

R-bands—In addition to the high intensity K-bands, all double bond groups such as C C, C N, C 0, C S N N, N 0, N0 0 are responsible for the appearance of absorption bands of very low intensity, in simple compounds at considerably longer wave-lengths than the K-bands (e, <20 in simple and up to a few thousand in more complicated molecules). In the absence of any evidence to the contrary, these R-bands may be considered to correspond to transitions of the same type, since, as far as they are accessible to experimental test, they have been shown to obey the same very characteristic optical rules (effects of substituents, of solvents and of proton addition) [15, 17]. 

Recycled HDPE items (blow-moulding grade) produced from HDPE made by the Phillips process were analysed by IR spectroscopy. The absorption bands at 888 and 965 cm-1, corresponding to unsaturations of vinylidene and vinylene type, respectively, are common in PE produced by either the Ziegler process or metallocenes [104]. The absence of these bands in the IR spectrum of the different samples confirmed that the resins had been produced using a Cr-type catalyst [118]. 

Table 13.2 summarizes the positions of the various absorption bands that have been discussed so far. On the basis of these absorptions, it is usually possible to determine the nature of the functional group that is present in the compound whose spectrum is being considered. Many functional groups require the presence of several characteristic absorptions, whereas the absence of a band in a particular region of the spectrum can often be used to eliminate the presence of a particular group. 

Let s try a problem. The IR spectrum of an unknown compound is shown in Figure 13.23. First, let s examine the hydrogen region. The absence of absorptions in the 3600 to 3100 cm-1 region indicates that the compound does not have any O—H or N—H groups. The bands in the 3100 to 3000 cm-1 region indicate the presence of... 

Independently and simultaneously, the anhydronium base structure XXVII was proposed for sempervirine (90, 91). This formulation clearly provides a more satisfactory explanation of the color of the alkaloid, its high basicity, and the absence of absorption corresponding to an imino group in the IR-spectrum (90, 91). In contrast, the synthetic base XXV exhibits a sharp NH band at 3480 cm-1 (88). 

A strong carbonyl absorption is observed at 1690cm and the absence of a band between 3000cm and 4000cm indicates the absence of a hydroxy group. The H NMR spectrum run in DMSO-r4 also implies the exclusive existence of the keto form, but it is gradually converted into the enol form 213. The rate of conversion depends on the amount of water present in the solvent. 

The data manipulation capabilities of the FTIR spectrometer can be used to quantitatively resolve the structural features which are superimposed upon the intrinsic absorption. The spectral features which exceed a baseline drawn between 1880 and 880 cm 1 in five independent preparation and measurement experiments are shown in Figure 4. The superposition of these five spectra illustrate both the reproducibility and the quantitative nature of this technique. The "resolved" spectra consist of three broad absorptions centered around 1725,1595 and 1245 cm1 and two superimposed sharp bands at 1135 and 1340 cm1. These latter peaks are characteristic of the particular type of carbon black and are presumably caused by impurities introduced in the manufacturing process. They may reflect the presence of residual sulfur compounds present in the form of sulfones or sulfonic esters in which the symmetric and antisymmetric stretching modes of the S02 vibrational modes occur in the range of 1140-1160 cm1 and 1300-1350 cm 1 [24], Note the absence of discemable bands above 1730 cm 1. This implies that these carbon blacks do not contain the lactone and cyclic anhydride functionalities observed on other carbon surfaces (see Figure 1) [17]. 

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