Fragmentation patterns principle

An example of how information from fragmentation patterns can be used to solve structural problems is given in Worked Example 12.1. This example is a simple one, but the principles used are broadly applicable for organic structure determination by mass spectrometry. We ll see in the next section and in later chapters that specific functional groups, such as alcohols, ketones, aldehydes, and amines, show specific kinds of mass spectral fragmentations that can be interpreted to provide structural information. 

A fourth technique used for the characterization of molecules is mass spectrometry. It is included in this chapter because the structural information it provides is similar to that obtained from the other techniques although the principle is entirely different. It is a destructive method in which the fragmentation pattern of sample molecules is used to determine empirical formulae and molecular weights, and to identify structural features. 

The mass spectra of indolizine alkaloids have been reviewed in detail (B-75MI30800). Some principles may be taken from the fragmentation pattern of slaframine, given in Scheme 3. 

Mass spectra of buclizine, carried out with electron impact method, were registered using Varian 320-GC/MS spectrometer. Figure 1.16 shows the detailed mass fragmentation pattern and Table 1.5 shows the mass fragmentation pattern of the drug substance. Clarke [10] reported the presence of the following principle peaks at m/z = 231, 147, 285, 232, 201,132,165, and 166. 

Similar principles have been used to explain the fragmentation patterns of monoisopropylidenealdo-furanoses and -pyranoses.24,67 71 O-Ethylidene acetals15 display cleavage patterns analogous to those of isopropylidene acetals, except that both [M — H ] and [M — CHj]+ are prevalent. 

The key principle underlying the electron impact ionisation (El) process is the fact that substances are ionised under relatively high vacuum conditions, the number of ion-molecule collisions is very small, hence fragmentation patterns are almost exclusively the result of unimolecular dissociation reactions. The chemistry of such reactions is relatively simple, as exemplified by equations (2) and (3). 

Negative ion fragmentations have been studied in various azole systems. Oxazole anions have a somewhat more complex fragmentation pattern than the cationic species, but the principles involved are similar. 

We have seen that we can identify alcohols by IR spectroscopy and by proton NMR spectroscopy. The mass spectra alcohols also have characteristic fragmentation patterns. Alcohols lose a hydrogen atom to give a radical cation whose miz is one unit less than the parent ion. Methanol is the simplest example, but it illustrates a basic principle (Figure 14.27). 

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