Aromatic compounds heterocyclic spectra

Mass spectral data have frequently been used in the structural determination of boron heterocycles. One paper has been devoted to the mass spectra of some six-membered boron-nitrogen systems. It was concluded that the spectra could be interpreted analogously to their hydrocarbon counterparts. In all cases the molecular peak was the base peak of the spectrum (68T6755). Doubly charged molecular ions, a feature typical of aromatic compounds, are often encountered. It should be noted, however, that some certainly non-aromatic aminoboranes give such doubly charged ions as well. 

Through his studies on the aromatic character of the heterocyclic compounds, Bonino inevitably confronted the classical problem of the structure of benzene. [50] He completed his work on the Raman spectrum of aromatic compounds which included benzene, presenting his results in April 1934 at the 9th International Congress of Pure and Applied Chemistry in Madrid. At the Madrid Congress, Bonino recommended a new formula for benzene this formula, however, lacked a rigorous quantum mechanical grounding. Rather, it represented an attempt to summarize qualitatively some fundamental ideas in the wave-mechanical interpretation of benzene (Figure 4.4). 

Six-Membered Heterocyclic Aromatic Compounds When a CH group in an aromatic hydrocarbon is replaced by =N, there is very little change in the spectrum, except for a slight intensification of the... 

Ultraviolet spectra have frequently been used as evidence for the aromaticity or lack of aromaticity of heterocycles, usually as supporting evidence. Most frequently, the spectrum is compared with some structurally analogous compound of known aromaticity, and the degree of similarity is taken as a qualitative, and often at least by implication as a semi-quantitative, indication of the aromaticity of the compound investigated. 

In Table IV some physical data and spectral characteristics of 6,7-secoberbines are listed. Only methyl corydalate (55) is optically active. Formula 55 presents the spatial structure of this compound, deduced by Nonaka et al. (65) and confirmed by Cushman et al. by both correlation with (+)-mesotetrahydrocorysamine (72) (<5S) and total synthesis (69). It is difficult to find common characteristic features in both the mass and H-NMR spectra of these alkaloids because they differ significantly from each other in their structures. On one hand, corydalic acid methyl ester (55) incorporates a saturated nitrogen heterocycle, while the three aromatic bases (56-58) differ in the character of the side chain nitrogen. For example, in mass fragmentation, ions of the following structures may be ascribed to the most intensive bands in the spectrum of 55 ... 

Check which ionization method was used and examine the general appearance of the mass spectrum. Is the molecular ion peak intensive (as with aromatic, heterocyclic, polycyclic compounds) or weak (as with aliphatic and multifunctional compounds) Are there typical impurities (solvent, grease, plasticizers) or background signals (residual air, column bleed in GC-MS) ... 

While luminescence in vapor-deposited matrices accordingly should be a powerful technique for detection and quantitation of subnanogram quantities of PAH in complex samples, it suffers from two major limitations. First, it is obviously limited to the detection of molecules which fluoresce or phosphoresce, and a number of important constituents of liquid fuels (especially nitrogen heterocyclics) luminesce weakly, if at all. Second, the identification of a specific sample constituent by fluorescence (or phosphorescence) spectrometry is strictly an exercise in empirical peak matching of the unknown spectrum against standard fluorescence spectra of pure compounds in a hbrary. It is virtually impossible to assign a structure to an unknown species a priori from its fluorescence spectrum qualitative analysis by fluorometry depends upon the availabihty of a standard spectrum of every possible sample constituent of interest. Inasmuch as this latter condition cannot be satisfied (particularly in view of the paucity of standard samples of many important PAH), it is apparent that fluorescence spectrometry can seldom, if ever, provide a complete characterization of the polycyclic aromatic content of a complex sample. 

The ultraviolet spectrum of the parent heterocycle in water shows three main peaks, at 221.5, 282, and 302 nm. The similarity of this spectrum to that of indolizine is apparently due to the fortuitous cancellation of the effects of the introduction of successive nitrogen atoms into indolizine. UV data are available for several quaternary derivatives of this heterocycle and their V-oxides." 2,3-Dihydroimidazo[l,2-a]pyr izine (5) has only two peaks, at 258 and 394 nm, in its ultraviolet spectrum in water. Both this compound and the unsubstituted aromatic heterocycle undergo large hypsochromic shifts on protonation. This contrasts with the bathochromic shift observed on protonation of aminopyrazines and is consistent with the hypsochromic effect obtained on protonation of the analogous imino compounds. These observations confirm the conclusion that protonation occurs at the 1-position in this heterocycle to give the cation 6. 

As the reaction proceeds a band appears in the 300-310 nm region. However, the use of a heterocyclic amine with epoxy compounds does not always result in such changes in the UV spectrum. For instance, with 2,6-dimethylpyridine the consumption of aromatic molecules on interaction with PhGE is not observed. The reaction of 2,6-dimethylpyridine with PhGE is characterized by the appearance and accumulation of substances producing two bands in the visible spectrum (at 408 and 505 nm), and by a decrease in the nmnber of epoxide groups. 

---