M + 2 peak

Interpreting the mass spectra of sulfur compounds is aided by the observation of an M+2 peak because of the presence of the mass 34 isotope of sulfur The major cleav age pathway of thiols is analogous to that of alcohols... 

Rule 3). The size of the M + 2 peak indicates the absence of sulphur and halogens and the empirical formula C9H10O2 given in Beynon s tables best fits the isotope peak ratios. The number of saturated sites is 9 + 1 - 5 = 5, i.e. one ring and four double bonds. 

Determination of the elemental composition should be started with the M + 2 peak. Chlorine, bromine, sulfur, and silicone are easily detected due to characteristic signal multiplicity for each of these elements. There is a simple mle to check the presence of the main A + 2 elements. If the intensity of the M + 2 peak constitutes less than 3% of the intensity of the M peak, the compound does not contain chlorine, bromine, sulfur, or silicon atoms. This rule is valid for the fragment ions as well, while its applicability is confirmed by the data summarized in Table 5.4. 

Formally oxygen is also an A + 2 element. However, the natural abundance of the lsO isotope is only 0.2% of that of the main isotope ieO. That is why reliable calculation of the number of oxygen atoms based on the intensity of the isotopic peaks is hardly possible. Nevertheless, it is quite possible to estimate this number. Thus, if the intensity of the M + 2 peak in the mass spectrum of a sample with a low number of carbon atoms is higher than 0.5% relative to M+, this compound may contain one or more oxygen atoms. 

When the number of carbon atoms is calculated on the basis of the M + 1 peak one should return to the M + 2 peak, to exclude contributions of 13C isotopes and estimate the number of oxygen atoms with greater precision. Let us consider as an example the mass spectmm represented in Fig. 5.21. 

It is evident that in cases where the amination has taken place 100% according to the ANRORC process, no enrichment in the M + 2 peak could be measured and that only enrichment of the M+1 peak in the chloro compound could be observed. Based on these mass spectrometric determinations it was established that the 2-halogenopyrimidines react for the greater part according the Sn(ANRORC) mechanism see Table II.3. 

The presence of a chlorine or bromine atom results in an intense (M + 2) peak due to the presence of either two chlorine isotopes (chlorine-35 and chlorine-37) or two bromine isotopes (bromine-79 and bromine-81). In the case of chlorine, the (M + 2) peak is about one-third the intensity of the M ... 

Sulfur has a less obvious (M + 2) peak because the abundance of sulfur-34 is only 4.4 percent of sulfur-32. (A little sulfur-33 is also present, which contributes to the [M + 1] peak.)... 

The existence of the alcohol shows that the butyl lsopropenyl ketone formed was again attacked by BuLi. The field desorption mass spectra of the trimer, tetramer and pentamer showed only their own ", (M+1)+ and (M+2)+ peaks, and the mass numbers of M " peaks... 

If sulfur or silicon, is present, the M + 2 will be more intense. In the case of a single sulfur atom, 34S contributes approximately 4.40% to the M + 2 peak for a single silicon in the molecule, 30Si contributes about 3.35% to the M + 2 peak (see Section 2.10.15). The effect of several bromine and chlorine atoms is described in Section 2.10.16. Note the appearance of additional isotope peaks in the case of multiple bromine and chlorine atoms. Obviously the mass spectrum should be routinely scanned for the relative intensities of the M + 2, M + 4, and higher isotope peaks, and the relative intensities should be carefully measured. Note that F and I are monoisotopic. 

The presence of an M - 15 peak (loss of CH3), or an M - 18 peak (loss of H20), or an M — 31 peak (loss of OCH3 from methyl esters), and so on, is taken as confirmation of a molecular ion peak. An M — 1 peak is common, and occasionally an M - 2 peak (loss of H2 by either fragmentation or thermolysis), or even a rare M - 3 peak (from alcohols) is reasonable. Peaks in the range of M - 3 to M — 14, however, indicate that contaminants may be present or that the presumed molecular ion peak is actually a fragment ion peak. Losses of fragments of masses 19-25 are also unlikely (except for loss of F = 19 or HF = 20 from fluorinated compounds). Loss of 16 (O), 17 (OH), or 18 (H20) are likely only if an oxygen atom is in the molecule. 

Aromatic and Aralkyl Hydrocarbons An aromatic ring in a molecule stabilizes the molecular ion peak (rule 4, Section 2.7), which is usually sufficiently large that accurate intensity measurements can be made on the M + 1 and M + 2 peaks. 

As required by Table 2.3, the M + 2 peak in the spectrum of p-chlorobenzophenone (Fig. 2.12) is about one-third the intensity of the molecular ion peak (m/z 218). As mentioned earlier, the chlorine-containing fragments (m/z 141 and 113) show (fragment + 2) peaks of the proper intensity. 

The relative intensity of the M + 1 and M + 2 peaks can be used to deduce the empirical formula of the compound. Software packages can be used to perform this operation, based on the relative abundance analysis. This is a commonly used method for low-resolution mass spectra, but it has limitations. 

Sulfur gives a significant M+2 peak (4.52% of M per sulfur). The observed M+2 is 12/122 = 9.8%, which could represent two sulfur atoms. The composition C4H10O2S2 has two sulfur atoms and has a molecular mass of 154. The known structure is shown here, but you could not deduce the structure from the composition. 

The relative intensities of diese peaks can be calculated by taking into account die number of combinations diat can give die required isotopic substitution and die probability of an isotope being present. An M + 2 peak in die above example will result if either of die chlorine atoms is 37C1 tiius die intensity of an M + 2 peak will be 2 x (1/3.058). An M + 4 peak will occur only if both chlorine atoms are 37C1 thus die intensity of die M + 4 peak will be 2 x (1/3.058)2. The squared term follows from die necessity tiiat both chlorine atoms must be 37C1. Thus the intensities of die peaks in die isotopic cluster of the molecular ion are (approximately) given as... 

Measurement of the isotopic cluster of the molecular ion showed an M + 1 peak of 5.30% and an M + 2 peak of 0.15% of the molecular ion. From tables of isotopic abundance ratios it was found that the expected product C4H8N2 should give M + 1 and M + 2 peaks of 5.21 and 0.11%, respectively, while cyclohexane C6H12 should give M + 1 and M + 2 peaks of 6.68 and 0.19%, respectively. It is clear that the isolated product is most likely the expected cyclic azo compound and not cyclohexane. 

Of course nowadays exact mass measurement could also distinguish these two molecules, as could a variety of other instrumental techniques. The analysis of isotopic clusters is most useful for detecting the presence of halogens, sulfur, and silicon, all of which have abundant isotopes of two atomic weight units higher, thus leading to relatively large M + 2 peaks. 

Relative intensity of M+ 2 peak 100 s Relative intensity of M+ peak 4.4... 

The mass spectrum of an unsymmetrical diaryl ketone, p-chlorobenzophenone, is displayed in Figure 1.23. The molecular ion peak (mlz 216) is prominent and the intensity of the M + 2 peak (33.99%, relative to the molecular ion peak)... 

A compound that contains one chlorine atom will have an M + 2 peak approximately one-third the intensity of the molecular ion peak because of the presence of a molecular ion containing the 37C1 isotope (see Table 1.4). A compound that contains one bromine atom will have an M + 2 peak almost equal in intensity to the molecular ion because of the presence of a molecular ion containing the 81Br isotope. A compound that contains two chlorines, or two bromines, or one chlorine and one bromine will show a distinct M + 4 peak, in addition to the M + 2 peak, because of the presence of a molecular ion containing two atoms of the heavy isotope. In general, the number of chlorine and/or bromine atoms in a molecule can be ascertained by the number of alternate peaks beyond the molecular ion peak. Thus, three chlorine atoms in a molecule will give peaks at... 

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