Triple bonds infrared frequencies

Much earlier information on the structure of diazonium ions than that derived from X-ray analyses (but still useful today) was obtained by infrared spectroscopy. The pioneers in the application of this technique to diazonium and diazo compounds were Le Fevre and his school, who provided the first IR evidence for the triple bonds by identifying the characteristic stretching vibration band at 2260 cm-1 (Aroney et al., 1955 see also Whetsel et al., 1956). Its frequency lies between the Raman frequency of dinitrogen (2330 cm-1, Schrotter, 1970) and the stretching vibration frequency of the C = N group in benzonitrile (2255 cm-1, Aroney et al., 1955). In substituted benzenediazonium salts the frequency of the NN stretching vibration follows Hammett op relationships. Electron donor substituents reduce the frequency, whereas acceptor substituents increase it. The 4-dimethylamino group, for example, shifts it by 103 cm-1 to 2177 cm-1 (Nuttall et al., 1961). This result supports the hypothesis that... 

The carbonyl groups in nickel carbonyl may be replaced by isonitriles as well as by phosphines etc. 162) in general all the carbonyl groups are replaced in the direct reaction, but a compound (MeNC)3Ni(CO) has been obtained. Cotton and Zingales have measured the infrared spectra of a few of these compounds in the triple-bond region (46), and some interesting facts emerge. The frequencies are shown in Table III and it is clear that... 

Nitriles Nitriles show a characteristic C = N stretching absorption around 2200 cm-1 in the infrared spectrum. This absorption can be distinguished from the alkyne C=C absorption by two characteristics Nitriles usually absorb at frequencies slightly higher than 2200 cm-1 (to the left of 2200 cm-1), while alkynes usually absorb at frequencies slightly lower than 2200 cm-1 and nitrile absorptions are usually stronger because the C = N triple bond is more polar than the alkyne C=C triple bond. 

Table V shows that the position of the ck-W(CO)2 infrared stretch is highly sensitive to the Jt-acidity of the alkylidyne group. Conjugation of the M C triple bond with Jt-donating alkylidyne substituents (Scheme 32) leads to a decrease in the 7t-acidity of the M=C bond in line with the heteroallenic (heterovinylidene) canonical form and a subsequent increase in available electron density for the n-acidic carbonyls, which leads to a decrease in the frequency of vco-...

These spectra were plotted from runs on a Jarrell-Ash 25-300 Raman spectrophotometer with a 4880 A argon ion laser. In some spectra the region from 4000 to 2000 cm" has been plotted so that the intensity is 0.5 times its true value compared to the rest of the spectrum. These are marked xO.5. Like the infrared spectra, these Raman spectra illustrate a group frequencies which are labeled directly on the spectra. Groups illustrated include alkanes in spectra 1-6, cyclohexanes 7-8, aromatics 9-12,15,17,18,20,21,25, 32-34, double bonds 13,14,24, isocyanate 15, triple bond 16, nitrile 17,18, carbonyls 19-26, alcohols 27-29, ether 30, amines 31, 32, nitro 33, C—Cl 34, C Br 35, and mercaptan 36. A molecular formula index of the Raman spectra follows. 

For example, a C=N double bond is about twice as strong as a C-N single bond, and the CsN triple bond is similarly stronger than the double bond. The infrared stretching frequencies of these groups vary in the same order, ranging from 1100 cm for C-N, to 1660 cm for C=N, to 2220 cm for C=N. 

Infrared spectroscopy is useful for determining the presence and identity of functional groups. These spectra measure the frequency of bending and stretching of bonds where the bond dipole changes with the movement. The stretching vibrations of double and triple bonds in alkenes and alkynes... 

Chapter 4 examines the group frequencies of triple bonds and cumulated double bonds. The alkyne hydrocarbons (acetylenes, —C=C—) are treated in I and the important nitrile group, —C=N, in detail in II. In the case of the alkynes the amount and type of substitution can often be identified from vibrational data (I, D, E). Systems containing cumulative double bonds (so-called back-to-back double bonds, >C=C=C<) such as the allenes in the hydrocarbon series are considered in III. A number of them contain hetero-atoms as for example, the ketenes (>C=C=0, III, B). Section F presents a problem that allows the reader to determine which of two possible structures is the correct one based on infrared and Raman data. 

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