C—N Bands

Compounds having C=N— exhibit a band at 1660-1610 cm that is useful in determining whether such compounds as IV-substituted glycosylamines are cyclic or acyclic (Ellis, 1966) (see Determination of Structure, in this chapter). Care must be taken to dry the compound completely, since moisture gives a band at 1650-16(X)cm . Also, if water of crystallization is present, a band appears at 1650-1640cm . For isothiocyanate —N=C=S and carbodiimides —N=C=N— there is a strong band at 21(X)cm .  

The carbonyl stretching frequency of aldehydes and ketones is found at 1730-1665 cm For example, the acyclic form of some aldoses and ketoses (in a lyophili-zate of the equilibrium mixture of mutarotation) produces a very weak band (Tipson and Isbell, 1962) at 1718 cm . Kuhn (1950) attributed the band at 1613 cm exhibited by periodate-oxidized methyl a-D-glucopyranoside to aldehydic carbonyl. Periodate-oxidized cellulose has only a very weak band (Rowen et ai, 1951), and exists mainly as the hemialdal —CH(OH)—O—CH(OH)— (Spedding, 1960), formed by hydration of two aldehyde groups per oxidized residue. 

The C=0 stretching frequency of the COOH of un-ionized carboxylic acids (Orr et al., 1952 Stevenson and Levine, 1952 Levine eta/., 1953 and, Orr, 1954) lies at 1736cm , including polysaccharides such as hyaluronic acid, the chondroitin sulfates, alginic acid, and pneumococcal capsular material. 

Barker et al. (1958) showed that 22 out of 24 aldono-1,4-lactones displayed a band at 1790-1765cm , and eleven aldono-1,5-lactones all had a band at 1760-1726 cm . Thus,ai is 1,4, and a spectrum with to distinguish gluce (and )8-l-1789-1764 cm  

The C=0 stretching frequency for —C—NHj (amide I) is found near 1650 cm for solids, and near 1690cm for dilute solutions. The amide I band is also shown by secondary and tertiary amides. (See Chapters 8 and 10.) Three glycopy-ranuronamide derivatives displayed this band (Tipson and Isbell, 1961a) at 1667-1661 cm .  

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Infrared spectral evidence indicates that indazol 3-one probably exists in the oxo form (cf. 77). 4-Monosubstituted-l,3-diphenylpyrazol-5-ones have been assigned an oxo structure (cf. 78) on the basis of infrared (presence of a v C=N band) and ultraviolet spectral data. The structure of certain iV-acylated pyrazolones has been discussed on the basis of their infrared spectra, but in these cases the possibility of acyl migration is a complicating factor. 

Similar isomerizations have been noted for a number of complexes. As with metal nitrosyls, IR spectra can be used to indicate the manner of bonding, but there is an overlap region around 2080-2100 cm-1 where i/(C-N) is found for both N- and S-bonded thiocyanates (additionally, S-bonded thiocyanates usually give a much sharper i (C-N) band). 14N NQR has been shown to be a reliable discriminator, but X-ray diffraction is ultimately the most reliable method. 

The material balance is consistent with the results obtained by OSA (S2+S4 in g/100 g). For oil A, the coke zone is very narrow and the coke content is very low (Table III). On the contrary, for all the other oils, the coke content reaches higher values such as 4.3 g/ 100 g (oil B), 2.3 g/ioo g (oil C), 2.5 g/ioo g (oil D), 2.4/100 g (oil E). These organic residues have been studied by infrared spectroscopy and elemental analysis to compare their compositions. The areas of the bands characteristic of C-H bands (3000-2720 cm-1), C=C bands (1820-1500 cm j have been measured. Examples of results are given in Fig. 4 and 5 for oils A and B. An increase of the temperature in the porous medium induces a decrease in the atomic H/C ratio, which is always lower than 1.1, whatever the oil (Table III). Similar values have been obtained in pyrolysis studies (4) Simultaneously to the H/C ratio decrease, the bands characteristics of CH and CH- groups progressively disappear. The absorbance of the aromatic C-n bands also decreases. This reflects the transformation by pyrolysis of the heavy residue into an aromatic product which becomes more and more condensed. Depending on the oxygen consumption at the combustion front, the atomic 0/C ratio may be comprised between 0.1 and 0.3 ... 

The high energy metal carbonyl band would be consistent with a Ti(IV) cationic complex, while the C=N bands correspond closely to those of the (TCNE)2 dianion at 2160 and 2095 cm-1 and also to those in (PPh3)2-(CO)IrN=C=C(CN)C(CN)=C=NIr(CO)(PPh3)2. 

The direction of the potential shift of the surface C-N band is the same as for the C-O band on Pt and the shift rates are ca. 30 cm 1/V for Ag and Au (22,23,25) and ca. 45 cm-1/V for Cu (33). The measurement for copper is less accurate due to formation of ion complexes with large absorption crossections and the relatively weak signal from the surface species itself. 

Figure 7. The dependence of the C—N bands of the adsorbed 12CN and 13CN- on the isotopic composition based on the data of Figure 4. (Reprinted from ref. 25. Copyright 1988 American Chemical Society.)...

PI 2A. IR studies of N labeled azide la. We are concerned with providing further evidence that nitrene 2a was formed upon the photolysis of azide la. To this end, we plan to synthesize azide la as an isotope labeled in the N1 position (see Figure 15), obtain IR spectra before and after irradiation in an argon matrix, and compare the calculated shift for the C-N band in nitrene 2a with the experimental value. Because isotope shifts in IR bands can be calculated very accurately, this will be an excellent proof of the formation of a nitrene intermediate. (From Gudmundsdottir, 2001)... 

Many azapentalenes have been characterized by their IR spectra, although these are frequently presented in an oversimplified manner (e.g., only C=C or C=N bands assigned). In this section, we shall discuss only specially significant results. 

A similar suggestion was advanced by Alyea and Ramaswamy (lib). Another interesting result of the above calculations (81) was that small splittings ( 15 cm 1) of the C—N vibration also should be observed for unsymmetrical bidentate, or monodentate, binding of the R2Dtc ligands. Unfortunately, this contention cannot be verified experimentally because of the inherently large width of the C-N band. 

Najer and co-workers130,131 reported a higher frequency (1660 cm-1) for the C=N band in 5-amino-3-phenyl-l,2,4-oxadiazole. 

The intensity increase between 1600 and 1400 cm-1 is in fact attributed (a) to a gain of oscillators strengths of the v(C6H4) ring stretches, which might testify (b) of an intensity transfer of the two v(C=0) and v(C-N) bands (red and blue shifts respectively). The C-O-C ether linkages at about 1250 cm-1 are not affected at all by this first stage of the metallization. 

The -C=N group is represented by a sharp, strong i/(C=N) band in the Raman as well as in the IR spectrum, as shown in Fig. 4.1-4A. Electronegative substitution makes the jz(C=N) IR band so weak that it may be no longer recognizable in the IR. spectrum itself (Fig. 4.1-7A). The corresponding Raman band, on the other hand, is always strong. 

In contrast to the 2,4-dioxoimidazoles (and such compounds as 5-phenyl-2-thiohydan-toin), there is convincing evidence (UV, IR, NMR data) that imidazolidine-2,4-dithiones exist mainly in the thione-thiol forms (84 Scheme 27). The two tautomers (85,86) have been separated in 5,5-diphenylimidazoIidine-2,4-dithione in KBr the C=N bands appear at 1495 and 1575 cm , respectively. In the 5,5-spiropentane analogues the structure related to (85) increases in importance as the solvent polarity increases. As mentioned above, the oxo-thione structures for imidazolin-4-one-thiones is supported by IR spectra in the solid state, and by UV spectra in ethanol. The partially fixed derivatives (87,88 Scheme 28) can be crystallized separately, but give identical spectra in solution. Both X-ray crystal structures and solid state IR spectra confirm the existence of two structures (87 y(C=0) 1690, r(C=N) 1510-1490 cm ) and (88 r(C=0) 1720, r(C=N) 1590-1575 cm ). As with the dithiones, the importance of the cross-conjugated form (88) increases with decrease in solvent polarity. 

Other adsorbents have been used in an effort to measure the acid strength of the sites or eliminate diffusion limitations. Kubelkova et al. used low temperature adsorption of CO on H-ZSM-5, H-Y, NaH-Y, and various AlPO sieves to measure the shift in the acidic OH stretching frequency upon CO adsorption. The authors argue that this shift is related to the proton affinity of the zeolites and thus to the Brpnsted acid strength. Tvaruzkova et al. used d3-acetonitrile to characterize both the Brpnsted and Lewis acidity of a number of zeolites. Using the band intensities and the frequency of the C-N band they obtained relative concentrations and strengths of the various acid sites. 

Most thermodynamic studies of the equilibria between hydrogen-bonded complexes of phenols and their free component molecules have been conducted in a diluting solvent. Binary solutions of phenols (phenol o-cresol ) in the pure base propionitrile have also been studied by means of Raman and IrW3,i44 spectrometry. Factor analysis of the v(C=N) band indicates the formation of a 1 1 complex over a large concentration range. However, this procedure is not recommended for the determination of equilibrium constants because these exhibit a strong concentration dependence. 

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