Molecular formula from mass spectrum

Mass spectrometry (MS) provides the molecular weight and valuable information about the molecular formula, using a very small sample. High-resolution mass spectrometry (HRMS) can provide an accurate molecular formula. The mass spectrum also provides structural information that can confirm a structure derived from NMR and IR spectroscopy. 

The mass spectra of compounds 1-14 have been discussed in CHEC-II(1996) <1996CHEC-II(8)733>. The mass spectra of all the new reported compounds displayed the corresponding molecular ions consistent with their respective molecular formulas. The mass spectra of the derivatives 17a (R1 = Ph, R2 = H) and 17b (R1=Ph, Rz = OMe) showed fragment peaks due to loss of (M-NO-CO), characteristic of sydnone-containing molecules, at m/z = 346 and 376, respectively <2002IJH287>. The other significant peaks were observed at m/z =187 and 217 due to the formation of 3-aryM-cyanosydnone. Similarly, in the mass spectrum of 25 <2002IJH287>, the peak at 324 was due to the loss of C02 from molecular ion and the peak at mlz = 171 was due to the formation of 3-cyanocoumarin. The molecular ion peak of the compound 24 (R1 =/-Bu, R2 = H) was observed at m/z = 368. 

Rather, look for evidence of the presence or absence of a few common functional groups with very characteristic absorptions. Start with OH, C=O, and NH groups in Figure 2.7 since a yes/no answer is usually available. A yes answer for any of these groups sharpens the focus considerably. Certainly the answer will contribute to development of a molecular formula from the mass spectrum (Chapter 1) and to an entry point for the NMR spectra (Chapters 3-6). These other spectra, in turn, will suggest further leads in the IR spectrum. 

If a high-resolution mass spectrum is not available, it is still possible to obtain information about the molecular formula from the low-resolution spectrum. In the mass spectrum of benzene, shown in Figure 15.5, the molecular ion appears at mlz 78. In addition, there is a smaller peak at mlz 79, called the M + 1 peak, that is 6.8% of the intensity of the Mt peak. The M + 2 peak, at mlz 80, is 0.2% of the Mt peak. The M + 1 and M + 2 peaks are caused by the presence of isotopic atoms of heavier mass in some of the molecules. Their intensities relative to the Mt peak can be used to deduce information about the formula. Let s look at how these peaks arise in more detail. 

The reaction below was expected to give product 6A and did indeed give a product with the correct molecular formula by mass spectrometry. The ]H NMR spectrum of the product was however 5 (p.p.m.) 1.27 (6H,s),4.70 (4H, m),2.88 (2H,m), 5.4-6.1 (2H, broad s, exchanges with D2O), 7.0-7.5 (3H, m). Though the detail is missing from this spectrum, how can you already tell that this is not the compound expected ... 

Once we know a compound s molecular formula from its mass spectral data and the identity of its functional group from its IR spectrum, we can then use its H NMR spectrum to determine its structure. A suggested procedure is illustrated for compound X, whose molecular formula (C4Hjj02) and functional group (C=0) were determined in Section 13.7. 

The example of ethane can illustrate the determination of a molecular formula from a comparison of the intensities of mass spectral peaks of the molecular ion and the ions bearing heavier isotopes. Ethane, C2H6, has a molecular weight of 30 when it contains the most common isotopes of carbon and hydrogen. Its molecular ion peak should appear at a position in the spectrum corresponding to a mass of 30. Occasionally, however, a sample of ethane yields a molecule in which one of the carbon atoms is a heavy isotope of carbon, This molecule would appear in the mass spectrum at a mass of 31. The relative abundance of in nature is 1.08% of the atoms. In the tremendous number of molecules in a sample of ethane gas, either of the carbon atoms of ethane will turn out to be a atom 1.08% of the time. Since there are two carbon atoms in ethane, a molecule of mass 31 will turn up (2 x 1.08) or 2.16% of the time. Thus, we would expect to observe a peak of mass 31 with an intensity of 2.16% of the molecular ion peak intensity. This mass 31 peak is called the M+ peak, since its mass is one unit higher than that of the molecular ion. 

Appendix C contains tables of molecular formula counts for various nominal masses between 1 and 1000. Appendix D lists all molecular formulas from of masses between 1 and 150 containing at least one C atom. As mentioned before, it makes sense to include formulas only whose nominal masses in fact appear in the mass spectrum when calculating match values. 

Figure 17.2 is an example of a mass spectrum of an aromatic dichloro compound. The intensity of the molecular ion indicates that an aromatic compound is present. The isotope pattern is that of two chlorines, and subtracting 70 mass units from the molecular ion gives the formula QHj. (See Example 2.3 in Chapter 2 for another example of isotope abundances in the molecular ion region.)... 

F.13 Osmium forms a number of molecular compounds with carbon monoxide. One light-vellow compound was analyzed to give the following elemental composition 15.89% C, 21.18% O, and 62.93% Os. (a) What is the empirical formula of this compound (b) From the mass spectrum of the compound, the molecule was determined to have a molar mass of 907 g-mol 1. What is its molecular formula ... 

Analysis and mass spectrometry showed it to have the molecular formula C HeBrFa. This could have been formed by the addition of magnesium bromide to tetrafluorobenzyne followed by the elimination of magnesium bromide-fluoride to give bromotrifluorobenzyne (30) and hence the compound (31). Analysis of the 19F n.m.r. spectrum and more particularly the preparation of (31) from o-bromotetrafluorophenyl-magnesium bromide and benzene confirmed the suggested mechanism. In this latter reaction the ratio of (31) to (24) was 99 1 54>. 

For example, to determine the empirical formula of di-n-octylphthalate, the daughter spectrum of the containing molecular ion (392) was obtained (Figure 7). The relative peak areas of adjacent peak pairs at m/z 149 and 150 is 2 1. This indicates that the M+1 ion is twice as likely to lose a atom as retain it. Thus the ratio of the number of carbon atoms lost to those retained is 2 1. Since the identified phthalate substructure contains 8 carbons, the unknown compound (di-n-octylphthalate) must contain 24 carbon atoms. These data, along with the molecular weight of 390 as determined from the conventional Cl mass spectrum of the unknown was fed into the empirical formula generator and the output was one empirical formula C24H38O4. 

Pseudovinblastinediol (21) (formerly named pseudovincaleukoblas-tinediol) has been isolated from C. rose us as a minor constituent (44). The high-resolution mass spectrum of 21 established a molecular formula of C44H5ftN40g. The characteristic vindoline fragments of miz 469 and 282 were shifted 58 mass units to miz 411 and 224, respectively, indicating the... 

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