Interpreting Mass-Spectral Fragmentation Patterns

Mass spectrometry would be useful even if molecular weight and formula were the only information that could be obtained. In fact, though, we can get much more. For example, the mass spectrum of a compound serves as 

Mass spectrum of hexane (QH, MW = 86). The base peak is at m/z = S7, and numerous other ions are present. 

An example of how information from fragmentation patterns can be used to solve structural problems is given in Practice Problem 12.2. This example is a simple one, but the principles used are broadly applicable for organic structure determination by mass spectrometry. Well see 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. 

Mass spectra of unlabefed samples A and B for Practice Problem 12.2. (Sample A is ethylcydopentane sample B is methylcydohexane.) 

Strategy Look at the two possible structures and decide on how they differ. Then think about how any of these differences might give rise to differences inti mass spectra. Methylcydohexane, for instance, has a -CH3 group, and ethyl-1 cyclopentane has a -CH2CH3 group, which should affect the fragmentation patterns.  

List the possible formulas of molecules with M = 100. Assume that C, H, and O may be present. 

A good approach to this kind of problem is to begin by calculating the possible hydrocarbon formulas. First divide the molecular weight by 12 to find the maximum number of carbons possible. Each carbon is equal in mass to 12 hydrogens, so the next step is to replace 1 C by 12 H, giving another possible formula. 

Oxygen-containing formulas can be calculated by realizing that one oxygen is equal in mass to CH.  

Dividing M by 12 gives 100/12 = 8 (remainder 4), so a possible hydrocarbon formula is C8H4. Replacing 1 C by 12 H gives the second possible hydrocarbon formula C7H16. 

Problem 12.1 Write as many molecular formulas as you can for compounds that have the following molecular ions in their mass spectra. Assume that all the compounds contain C and H, and that O may or may not be present. 

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Interpreting Mass Spectra 444 Interpreting Mass-Spectral Fragmentation Patterns Mass-Spectral Behavior of Some Common Functional Groups 449... 

Mass spectrometric studies yield principally three types of information useful to the radiation chemist the major primary ions one should be concerned with, their reactions with neutral molecules, and thermodynamic information which allows one to eliminate certain reactions on the basis of endothermicity. In addition, attempts at theoretical interpretations of mass spectral fragmentation patterns permit estimates of unimolecular dissociation constants for excited parent ions. 

The characteristics of mass spectral fragmentation patterns for the interpretation of methyla-lkane mass spectra were first put forward by McCarthy et al. (1968). The effect of the position... 

The rationale used in the interpretation of the mass spectra of methylalkanes has been presented in several reports 2- vs. 4-methylalkanes (Baker et al., 1978 Scammells and Hickmott, 1976 McDaniel, 1990 Bonavita-Cougourdan et al., 1991) 2,X- and 3,X-dimethylalkanes (Nelson et al., 1980 Thompson et al., 1981) and internally branched mono-, di- and trimethylalkanes (Blomquist et al., 1987 Pomonis et al., 1980). In the majority of reports, identification is based on GC and MS data, but the conclusions are not confirmed with standards or synthesis of the proposed structures. However, there are reports of chemical ionization (Howard et al., 1980) and electron impact of synthetic methyl-branched hydrocarbons (Carlson et al., 1978, 1984 Pomonis et al., 1978, 1980) and these have been very useful in confirming mass spectral fragmentation patterns with chemical structures. 

Routine, but useful, interpretations of the mass spectral fragmentation patterns under electron impact have been reported for esters of the menthane and camphane series,39-40 for thioketones with the thujane, pinane, camphane, and fenchane skeleta,41 for [2H]limonene,42 for cannabinoids,43-46 and for the volatile components from Pinus seedlings.47... 

C. Mass and N.M.R. Spectra of Steroidal Amines.—A study has recently been made18 of the mass spectral fragmentation pattern of the dimer (28), obtained by reduction of the cyanamide of conanine with lithium aluminium hydride.19 As with certain benzylamines, a major fragment has been interpreted to arise from a heterolytic rupture of the C—N= bond. 

The solid compound has been characterized by elemental analysis, infrared and NMR ( H and spectroscopy, and X-ray diffraction. The analytical and spectral data confirm that the compound is 4-amino-3,5-dimethyl-1,2,4-triazole. The mass spectral fragmentation pattern has been successfully interpreted on the basis of its structure. 

The constitutional structural features just described were deduced by classical methods. Both and 3C NMR spectroscopy have proven valuable in unraveling these complex structures, and interpretation of mass spectrometric fragmentation patterns is often used to pinpoint the location of substituents along the carbon chains. In recent years, two-dimensional NMR techniques have provided even more structural detail, and spectral trends among the many known compounds has made determination of connectivity a relatively straightforward task. ... 

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