Mass spectral fragmentations

Thiazole disulfides absorb at 235 and 258 nm (320-322) and characteristic infrared bands are reported in Ref. 320. The activities of 2-cyclo-hexyldithiomethylthiazoles as vulcanization accelerators have been correlated with their mass-spectral fragmentation patterns (322). 

Mass spectral fragmentation patterns of alkyl and phenyl hydantoins have been investigated by means of labeling techniques (28—30), and similar studies have also been carried out for thiohydantoins (31,32). In all cases, breakdown of the hydantoin ring occurs by a-ftssion at C-4 with concomitant loss of carbon monoxide and an isocyanate molecule. In the case of aryl derivatives, the ease of formation of Ar—NCO is related to the electronic properties of the aryl ring substituents (33). Mass spectrometry has been used for identification of the phenylthiohydantoin derivatives formed from amino acids during peptide sequence determination by the Edman method (34). 

The mass spectral fragmentation of the fully saturated parent heterocycles has been studied on a comparative basis (65JA2920) and the principal processes are indicated in Figure 10. All of the compounds exhibit appreciable molecular ions. The M-1 ions (v)... 

There are correlations between mass spectral fragmentations and thermal and photochemical fragmentations and rearrangements see Sections 4.02.1.2.1 and 4.02.1.2.2. 

The structures of the isoxazoles (393) were all consistent with their mass spectral fragmentation patterns. The reaction of hydroxylamine with 3-phenylchromone (394) gave exclusively 5-(o-hydroxyphenyl)isoxazole (395) (78ACH(97)69). 

The mass spectral fragmentations of 9,10-dimethoxy-2,3,4,6,7,ll/)-hexa-hydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140 and -2,4-diones 141, under electron ionization (at 70 eV) were examined by metastable ion analysis, a collosion-induced dissociation technique and exact mass measurement (97RCM1879). Methyl substituent on N(3) in 140 (R = Me) had a larger effect on both the fragmentation and on the peak intensities, than a methyl substituent on C(6) (R = Me). The ionized molecules of 140 (R = H) were rather stable, whereas 4-phenyl substitution on C(4) of 140 (R = Ph) promoted the fragmentations of the molecular ions. The hexahydro-1//-pyrimido[6,l-n]isoquinoline-2,4-diones 141 were more stable, than the hexahydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140, and the molecular ions formed base peaks. 

Mass spectral fragmentation patterns are usually complex, and the molecular ion is often not the base peak. The mass spectrum of propane in Figure 12.2, for instance, shows a molecular ion at m/z = 44 that is only about 30% as high as the base peak at m/z = 29. In addition, many other fragment ions are present. 

Because mass-spectral fragmentation patterns are usually complex, it s often difficult to assign structures to fragment ions. Most hydrocarbons fragment in many ways, as the mass spectrum of hexane shown in Figure 12.4 demonstrates. The hexane spectrum shows a moderately abundant molecular ion at m/z = 86... 

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. 

As each functional grotip is discussed in future chapters, mass-spectral fragmentations characteristic of that group will be described. As a preview, though, we Jl point out some distinguishing features of several common functionaj groups. 

McLafferty rearrangement (Section 12.3) A mass-spectral fragmentation pathway for carbonyl compounds. 

Coupling of the diazotetrazole with ethyl cyanoacetate gave 1034. Its cyclization in boiling acetic acid or pyridine afforded 1035 as the major product in addition to 1036. Mass spectral fragmentation of 1035 confirmed that the azole ring is more stable than the 1,2,4-triazine ring on electron impact [76JCS(P1) 1496] (Scheme 194). 

Mass spectral fragmentation pattern of dime thy l-nitramine, CH3N(N02)CH3, mw 90 Instrument, CEC 21-104 Ionization Voltage 70 eV... 

The purity of the product is greater than 99% as determined by gas chromatographic analysis using a 6-m. column of 30% Carbowax 20M on 60-80 Chromosorb W. The major impurity (<1%) was shown to be 3-heptanol by comparison of gas chromatographic retention times and mass spectral fragmentation patterns with those of an authentic sample. 

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 total yield of hydrogen under the conditions of these measurements was about 1.6 molecules/100 e.v. If one-half resulted from the primary dissociation also leading to acetylene ion, a yield of 0.8 acetylene ions/100 e.v. may be estimated. This value is a minimum since acetylene ion production can also be accompanied by hydrogen atom formation and is highly uncertain but consistent with the mass spectral fragmentation pattern of acetylene and W which lead to an estimate of ca. 0.94 acetylene ions/100 e.v. 

Table 5.5 Nomenclature of the ions formed in the mass spectral fragmentation of polypeptides. From Chapman, J. R. (Ed.), Protein and Peptide Analysis by Mass Spectrometry, Methods in Molecular Biology, Vol. 61, 1996. Reproduced by permission of Humana Press, Inc. 

The reaction products from 2,4-dichlorophenol were tetrachloro-phenoxyphenols and tetrachlorodihydroxybiphenyls (Figure 5), as determined from their mass spectra and those of their methyl ethers. 4,6-Dichloro-2-(2, 4 -dichlorophenoxy)phenol (V) was the major phenoxy-phenol the mass spectral fragmentation pattern of o-hydroxyphenol ethers is quite characteristic since a hydrogen transfer occurs during the fragmentation (Figure 6). A trace of a trichlorophenoxyphenol also was detected and was formed presumably by the unsensitized reductive loss of chlorine, discussed previously. 

Figure k. Low and high resolution mass spectral fragmentation patterns of Component II. 

Molecular stmcture (from mass-spectral fragmentation... 

Molecular structures from mass-spectral fragmentation patterns... 

Fig. 8. The most characteristic ions in the mass spectral fragmentation of atropine (26).

The mass spectral fragmentation of the adducts (72), (73), and (78) occurs by the loss of ketene from the molecular ion, and in accord with this and the results of other research groups 119-122>, both (72) and (73) form ketene and (79) both thermally and photochemically. 

Mass spectra of the cis- and /ram-isomers of the pyrimido[2,TA [l,3]thiazin-6-ones 294 and 295 were studied. Retro-Diels-Alder fragmentation of the hydrocarbon ring was of medium to low stereospecificity. A number of highly selective processes were discovered allowing differentiation between stereoisomers <1996RCM721>. The mass spectral fragmentation pattern of 296 was studied in detail <1996PS(113)67>. 

Table 5. Mass spectral fragmentation pattern of miconazole...

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