M/e values

Similarly, methane Cl spectrum of 18a was found to be dominated by the (C6H5C= CC6H5 + H)+ ion. A distinct molecular ion species at m/e value corresponding to (M + H)+ was observed in the methane mass spectra of this thiirene oxide (26% 2 40). Furthermore, the relative intensity of the (M +H)+ peak of 18a was shown to increase substantially in the isobutane and dimethyl amine Cl mass spectra91. 

Thus we think of the chemical ionization of paraffins as involving a randomly located electrophilic attack of the reactant ion on the paraffin molecule, which is then followed by an essentially localized reaction. The reactions can involve either the C-H electrons or the C-C electrons. In the former case an H- ion is abstracted (Reactions 6 and 7, for example), and in the latter a kind of alkyl ion displacement (Reactions 8 and 9) occurs. However, the H abstraction reaction produces an ion oi m/e = MW — 1 regardless of the carbon atom from which the abstraction occurs, but the alkyl ion displacement reaction will give fragment alkyl ions of different m /e values. Thus the much larger intensity of the MW — 1 alkyl ion is explained. From the relative intensities of the MW — 1 ion (about 32%) and the sum of the intensities of the smaller fragment ions (about 68%), we must conclude that the attacking ion effects C-C bond fission about twice as often as C-H fission. 

FIGURE 5.1 Major fragmentation pattern of the parent ion in propane. Numbers on the curves denote m/e values. The relative abundance of these, as well as of the minor ions, stabilizes beyond -40 eV. Adapted from Eyring and Wahrhaftig (1961), with permission from Am. Inst. Phys. ... 

Figure 15. Major fragmentation pathways of CTC involving molecular ion and other significant high mass ions. Underlined m/e values indicate confirmation by accurate mass measurements.

In addition to this there are some additional peaks which is unusual in the typical organic compounds, The mass spectrum of methane shows m/e values of 14, 13, 12, 2 and 1. This is explained as due to the formation of cationic fragments as follows ... 

Searches through the MSSS data base can be carried out in a number of ways. With the mass spectrum of an unknown in hand, the search can be conducted interactively, as is shown in Figure 5. In this search the user finds that 24 data base spectra have a base peak (minimum intensity 100% maximum intensity 100%) and an m/e value of 344. When this subset is examined for spectra containing a peak at m/e 326 with intensity of less than 10%, only 2 spectra are found. If necessary, the search can be continued in this way until a manageable number of spectra are retrieved as fulfilling all the criteria that the user cited. These answers can then be listed as is shown. Alternatively, the data base can be examined for all occurrences of a specific molecular weight or a partial or complete molecular formula. Combinations of these properties can also be used in searches. Thus all compounds containing for example, five chlorines and whose mass spectra have a base peak at a particular m/e value can be identified. 

Although isotopic patterns and deconvolutions may be calculated manually (6), to achieve full potential a computer is virtually a necessity. Manual calculations often omit the 1.1% contribution for for molecules with large ligands, thirty carbons are not unusual, and these would give a 33% contribution to the m/e value, greater than that from the nominal mass by one mass unit. 

Unknown compound D, which has the formula CjqHjq, readily decolorizes a bromine in Ccirbon tetrachloride solution. When D is allowed to react with hydrogen and a catalyst, E is obtained. The M for E has an m/e value of 134. When E is heated with a mixture of nitric and sulfuric acid, only one product is produced. That compound has an M of 179. Heating either D or in hot chromic acid produces a solid F, CgHgO, whose melting point is in excess of 300 degrees Celsius. Give structures for D, E, and F. 

The reaction with NaOEt/EtOH is a dehydrohalogenation, which indicates that I is an alkene. Since both G and H give / they must have similar regiochemistry. The m/e value (160) corresponds to the loss of HBr. 

Synchronous thermal analysis was carried out with STA 449C Jupiter thermal analyzer (Netzsch) in argon at flow rate 15 cm min". Polymer samples were heated from 20 to 700°C at a rate of 10°C min and then allowed to cool down. Gaseous products were analyzed with a QMS 403C Aeolos quadrupole mass spectrometer (MS) connected to the main analyzer. MS data were collected for lines with given m/e values. 

The parent peak, usually having the highest m/e value, can easily be identified in all cases. Triphenyl-X -phosphorin also has an intense peak at m/e = 120, which Markl attributes to the C Hs-C = P fragment. Fig. 11 shows the mass spectra... 

Problem 18.67 Amines A, B, D, and E each have their parent-ion peaks at mle = 59. The most prominent peaks for each are at m/e values of 44 for A and B, 30 for D, and 58 for E. Give the structure for each amine and for the ion giving rise to the most prominent peak for each. ... 

A new development, the selected-ion-drift tube or SIFT technique (Smith and Adams, 1979), enables the isolation of reactions due to a given set of ions, of a selected mfe value, with a neutral. Ions are produced outside the flow cell under high vacuum, and injected into the flow tube through a mass filter. This filter eliminates all ions except the ones selected (according to a specific m/e value), and removes the neutral precursor of the ion in question by differential pumping. 

Species Inside Deactivated Catalysts. The total extract of the adsorbate in deactivated parent H-mordenite was heated using the solids probe of the mass spectrometer. Intense fragment ions at the higher m/e values, previously not observed to be of such intensity in the dynamic de-... 

It was demonstrated that the highest intensity of the peak Bi (m/e 176) is observed with (13) and (16), having three methoxyl groups cis to each other. All the other isomers produce a less intense peak at this m/e value. The highest intensity of peaks Fi and J (m/e 101 and 75) has been observed with (13) and (14). It is somewhat lower with (17) and lowest with (15) and (16). Hence, the formation of these ions is facilitated when the methoxyl groups at C-l and C-3 are trans to each other. 

As expected, the spectrum of the 6-deoxyhexose (23) contains no peaks of the E series however, the B series appears (m/e 176, 131), the peaks of the F, G, H, and J series have the same intensities and positions as with permethylated methyl hexosides, and the peaks of fragments involving C-5 and C-6 are shifted 30 mass units to lower m/e values compared with the peaks of permethylated methyl hexosides (Di, m/e 119 C2, m/e 129, and so on). 

The fragments of the C series expected have the same structures and m/e values as in the spectra of pentoses, and are shifted 72 mass units (the difference between H and CH OCOCH ) to lower m/e values compared with those of isomeric aldohexoses. This feature may also serve to dis-... 

Mass Spectra of Methyl Tetra-O-methyl-a-D-glucopyranoside (MTMG)and its Analogs m /e Values and Relative Intensities... 

Before abandoning the idea, however, the work of Dejongh et al. was repeated. A lower ionization voltage was used to limit secondary fragmentation processes and hopefully to accentuate stereochemical differences. The 17 ev mass spectra of the pertrimethylsilylated derivatives of < - and / -D-glucopyranose are shown in Figure 6. The two spectra are very similar however, intensity differences do occur. In particular, attention has been focused on the ratio of the two peaks with m/e values of 435 and 393. The m/e 435 peak corresponds to the parent molecular ion (PMI) with loss of CH3 and TMSiOH, the 393 peak to the shard A minus CH3 (57). 

Finally, the ions (lowing down ihc quadrupole strike the Faraday plate defector. In some cases. Ihc signal is amplified further by an electron multiplier. Thus, there is obtained a spectrum of signal intensity versus m/e value. Each molecule has a unique fragmentation pattern so that a spectrum can he used as a fingerprint lor compound idenlilication. In addition, it is possible lo quantitate the amount of a particular compound by comparing sample signal intensity with the intensity produced hy a known amount of ihe compound. 

With the molecular weight available from the M+ peak with reasonable certainty, the next step is to determine the molecular formula. If the resolution of the instrument is sufficiently high, quite exact masses can be measured, which means that ions with mje values differing by one part in 50,000 can be distinguished. At this resolution it is possible to determine the elemental composition of each ion from its exact m/e value (see Exercise 9-44). 

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