Raman frequencies of triple bonds

Table XLIX, Raman Frequencies of Triple Bond, C C...

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. 

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 triple bond, CsG — The Raman frequency of the triple bond in acetylene is 1,960 cm i, on monosubstitution this value is increased to about 12,120 cm and with the di-substituted derivatives is raised to approximately 2,235 cm (see Table XLIX). 

Micromechanisms of reinforcement. It was pointed out earlier that the vibrational frequencies of certain main-chain Raman active modes are found to change with the level of applied strain (26). The biggest change is found for the C=C triple bond stretchTng frequency which is of the order of 20 cm-i/%. This property can be used to determine the strain in any polydiacetylene fibre subjected to any... 

The NMR resonances of carbyne complexes fall between 200 and 350 ppm down-field of tetramethylsilane. Detailed IR and Raman studies show the metal-carbon triple bond vibrates between 1250 and 1400 cm, but that the M-C vibration is strongly coupled with vibrational modes of fhe phenyl ring in aryl-substituted carbyne complexes. The carbyne complex in Figure 2.18 illustrates these spectroscopic features. Neutral car-b5me complexes exhibit C-0 bands at low frequencies, and this low frequency shows that the carbyne ligands withdraw little electron density from the metal. 

Problem 6.12. What are the products obtained on the mercury (Il)-catalyzed hydration of (a) ethyne (acetylene, HC=CH), (b) propyne (methylacetylene, CHjC CH), and (c) 2-butyne (dimethylacetylene, CHsC CCHs) Because ethyne (acetylene, HC=CH) and 2-butyne (dimethyl-acetylene, CHsC CCHs) are symmetrical, the carbon-carbon triple bond stretching frequency is IR inactive (Raman active) and... 

One of the effects of the application of pressure with a DAC to the resonance Raman spectrum in PDA-TS is shown in Fig. 29. Figure 29a shows the 952-cm Raman band of PDA-TS at 1 atm and at 50 kbar. This Raman band corresponds to a bond-bending mode with large amplitude of vibration about the triple bond on the backbone of the PDA-TS. It can be seen from Fig. 29a that the Raman band both shifts in frequency and splits into two peaks at 50 kbar. Figure 29b plots the center wave number of each of these two components as a function of pressure as determined by least-squares fitting of two Lor-... 

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. 

The extent of metal-P coordination is reflected in the Raman C=C stretch frequency [72]. Uncoordinated dppa has a lower v(C=C) of 2,097 cm than most disubstituted acetylenes (v = 2,190-2,260 cm ) as the conjugated phosphorus lone pair causes 7i-electron depletion from the triple bond (Fig. 2.18). Transition metal complexation suppresses this effect. The magnitude of Av(C=C), i.e., v(complexed dppa) — v(free dppa), then reflects the degree of P M a-bonding. Electron depletion of the triple bond has been inadvertently attributed to the contribution of P(d,7i) orbitals, as well as M P 7r-bonding to the increase in Av(C=C) [72]. 

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