Homologous ions series

The alkane series is present in mass spectra of any compound containing an alkyl group. In case of isobaric series (e.g., alkanes and ketones) one should pay attention to the intensities of the isotopic peaks. Thus, for the isobaric ions of m/z 43 (CH3CO and C3H7) the abundance of the isotopic peak (m/z 44 ion) will be 2.2% and 3.3%, respectively. The situation is very simple with A + 2 elements. In this case there are two homologous ion series due to A and A + 2 ions. It is worth emphasizing that... 

TABLE 5.8. Homologous Ion Series of the Main Classes of Organic Compounds... 

The correct analysis of the homologous ion series has certain limitations. Low abundances of peaks in some series require the attention and experience of a researcher. Usually alkane series are dominated in the mass spectra of the most various compounds. Fragmentation initiated by one functional group may completely suppress or notably camouflage other reactions of polyfunctional substances. In the latter case it is useful to consider IR-spectroscopy data in mass spectral interpretation. 

Low volatile bituminous Pocahontas 3 coal shows different early mobile phase components in terms of alkylsubstituted benzenes (m/z 92, 106, 120) and further shows similar components (several homologous ion series) compared to medium volatile bituminous Upper Freeport coal (e.g. at m/z 192,206,220,234 m/z 216, 230, 244, 258 and m/z 266, 280,294). From the above observations on seven coals, it is clear that there is a chemically and/or physically distinct mobile phase, at... 

Another characteristic homologous ion series of phenylalkanes can be seen in Figure 3.7 at masses corresponding to CgH5(CH2) (m/z 77, 91, 105, 119,...) the abundances of the individual ions are much more dependent on the structure of the molecule than those of the low-mass ion series. However, ions stabilized by the aromatic nucleus can be formed by a variety of fragmentation pathways, so that the presence of a large peak, such as m/z 91, signifles only that the molecule contains one or more of several possible structural features. 

Upon extrapolation to larger ketones one can expect to observe larger acylium and alkyl fragments. The occurrence of series of homologous ions is a feature that can be very helpful to deduce structural information from mass spectra. Ions such as the acylium ion series and the carbenium ion series are also known as characteristic ions. Learning the nominal masses of the first members of each series by heart is useful (Tables 6.2 and 6.3). 

The next stage of the program processes the information contained in characteristic ion series. Searches are conducted for homologous as well as for non-homologous important ion series. With that information, the system then attempts to deduce the most probable compound classes and then to postulate structures within the chosen classes. Using character-... 

The relative abundance of seven classes (1-VII) of aromatics in petroleum aromatic fractions is determined by mass spectrometry using a summation of peaks most characteristic of each class. Calculations are carried out by the use of a 7 by 7 inverted matrix derived from published spectra of pure aromatic compounds. Each summation of peaks includes the polyisotopic homologous series that contains molecular ions and the monoisotopic homologous series one mass unit less than the molecular ion series. Using characteristic summations found in the monoisotopic molecular ion—I series of peaks, each class is further resolved to provide relative abundances of three compound types nominal (Type 0), first overlap (Type 1), and second overlap (Type 2). The aromatic fraction is obtained by liquid elution chromatography (see Method D 2549). 

Calculation can also explain why in some thiazole dyes vinyiene shift of the first two homologs is larger than the shift between higher members of the series, and also why wavelengths of absorption of nonsymmetrical dyes as calculated by the mean value rule differ from experimental data (6671. This deviation is caused by an interannular no-bond SS-interaction in the monomethine ion. 

For targets that lack structural information, such as GPCRs or ion channels, a pharmacophore model or multiple pharmacophore models for different series of compounds can explain SAR and guide the synthesis of new analogs. Alternatively, homology models based on bacteriorhodopsin have been used to explain the interactions of small molecules with GPCRs. 

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