Raman frequencies of aromatic compounds

TABLE 7.35 Raman Frequencies of Aromatic Compounds (Continued)... 

The N=N frequency of aromatic azo compounds is also difficult to identify in the infra-red. The frequencies are lower in these cases and are not only weak but are overlaid by other aromatic bands. LeFevre et al. [34, 57] studied many aromatic azo compounds and made a very tentative identification of the N= N band in the infra-red at 1406+ 14cm , but Dolinsky and Jones [58] were unable to find any specific absorptions from this group. The problem has been resolved by Kubler et al. [77] and by Hacher [79] using Raman spectra. This shows that trans-azobenzene absorbs at 1442 cm and the cis form at 1511 cm . Derivatives of azobenzene and of azonaphthalene absorb in the 1450—1380 cm range, but are difficult to observe in the infra-red [78, 79]. 

One important catalytic reaction cycle which starts from a primary gas mixture of carbon monoxide and hydrogen is the Eischer—Tropsch synthesis. Depending on the reaction parameters (temperature of the catalytic surface, gas pressure and composition of the gas mixture) a great variety of aliphatic, aromatic and even oxygen-containing compounds can be obtained. The understanding of reaction mechanisms in terms of the appearance of intermediates on the surface, their structure and symmetry, is of fundamental interest for the development of well-defined reaction pathways. The frequency of the C—H stretching Raman band is a measure of the state of hybridization of the adsorbed molecule. 

A second problem of this revision is the vast literature which has grown up in the twenty years since this book was first written. At that time it was possible, at least to aspire, to include all the relevant references. To try to do so now would require a bibliography as large as the book itself. Moreover a number of specialist texts have appeared in which the whole book is devoted to the material which has to be presented here in one chapter, and these already offer the complete documentation that a specialist may need. Thus, two reviews by Katritzky and his coworkers deal exclusively with the group frequencies of heterocyclics and between them cite over 1500 references. Varsanyi has provided a comparable review of the aromatics, Adams of coordination compounds and Bentley et al. of the far infrared. There is also a wide coverage on inorganics by Nakamoto and by Lawson, whilst the supplementary Raman group frequency data has recently been very excellently covered by Dollish, Fateley and Bentley. I have therefore confined myself to those references which are most relevant and contented myself with reference to these more detailed sources where appropriate. 

There are many reasons why scientists want to measure the Raman spectra of compounds. First, many bands that are weak in the infrared spectrum are among the strongest bands in the Raman spectrum. For example, the S—S and C=C stretching bands are often so weak as to be essentially unrecognizable in the IR spectrum but stick out like the proverbial sore thumb in a Raman spectmm. Second, some Raman bands are found at very characteristic frequencies. For instance, monosubstituted aromatic compounds, together with 1,3-disubstituted and 1,3,5-trisubstituted aromatics, have a very intense band at 1000 cm. This band, along with the presence or absence... 

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