Determination of the Molecular Formula by Mass Spectrometry

Although mass spectra usually show the particle masses rounded to the nearest whole number, the masses are not really integral. The nucleus is defined to have a mass of exactly 12 atomic mass units (amu), and all other nuclei have masses based on this standard. For example, a proton has a mass of about 1, but not exactly Its mass is 1.007825 amu. Table 12-3 shows the atomic masses for the most common isotopes found in organic compounds. 

Consider a molecular ion with a mass of 44. This approximate molecular weight might correspond to C3Hg (propane), C2H4O (acetaldehyde), CO2, or CN2H4. Each of these molecular formulas corresponds to a different exact mass  

If the HRMS measured the exact mass of this ion as 44.029 mass units, we would conclude that the compound has a molecular formula of C2H4O, because the mass corresponding to this formula is closest to the observed value. Published tables of exact masses are available for comparison with values obtained from the HRMS. Depending on the completeness of the tables, they may include sulfur, halogens, or other elements. 

Whether or not a high-resolution mass spectrometer is available, molecular ion peaks often provide information about the molecular formula. Most elements do not consist of a single isotope, but contain heavier isotopes in varying amounts. These heavier isotopes give rise to small peaks at higher mass numbers than the major M molecular ion peak. A peak that is one mass unit heavier than the M peak is called the M+1 peak two units heavier, the M+2 peak and so on. Table 12-4 gives the isotopic composition of some common elements, showing how they contribute to M+1 and M+2 peaks. 

Isotope ratios can help to identify banned substances in Olympic athletes. For example, mass spectrometry can distinguish between synthetic testosterone and the naturally occurring hormone by detecting differences ri the isotope ratios of and 

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The Infrared Region 515 12-4 Molecular Vibrations 516 12-5 IR-Active and IR-lnactive Vibrations 518 12-6 Measurement of the IR Spectrum 519 12-7 Infrared Spectroscopy of Hydrocarbons 522 12-8 Characteristic Absorptions of Alcohols and Amines 527 12-9 Characteristic Absorptions of Carbonyl Compounds 528 12-10 Characteristic Absorptions of C—N Bonds 533 12-11 Simplified Summary of IR Stretching Frequencies 535 12-12 Reading and Interpreting IR Spectra (Solved Problems) 537 12-13 Introduction to Mass Spectrometry 541 12-14 Determination of the Molecular Formula by Mass Spectrometry 545... 

Calculation of the Molecular Formula How do we know the correct molecular formula We can choose the right multiple of the empirical formula if we know the molecular weight. Molecular weights can be determined by methods that relate the freezing-point depression or boiling-point elevation of a solvent to the molal concentration of the unknown. If the compound is volatile, we can convert it to a gas and use its volume to determine the number of moles according to the gas law. Newer methods include mass spectrometry, which we will cover in Chapter 11. 

One of the first pattern recognition applications in mass spectrometry was the attempt to determine the molecular formula by a decision tree C120, 128, 1293. The decision tree contained several binary classifiers. Each of the classifiers decided whether a compound contains more atoms than a given number- A run through the decision tree yields the molecular formula of an unknown whose low resolution mass spectrum is known. A tree with 26 classifiers was necessary for a set of 346 compounds of formulas --i 6 0-3 0-2 spectra with an artifi-... 

A compound with the molecular formula C7///JNO3, determined by mass spectrometry, was isolated from the plant Petiveria alliacea (Phytolaccaceae). What is its structure given the set of NMR results 52 ... 

Amongst products isolated from Heliotropium spathulatum (Boraginaceae) were 9 mg of a new alkaloid which gave a positive Ehrlich reaction with p-dimethylaminobenzaldehyde The molecular formula determined by mass spectrometry is CisH2sNOs. What is the structure of the alkaloid given the set of NMR results 54 Reference is useful in providing the solution to this problem. Conditions CDCI3, 9 mg per 0.3 ml, 25 °C, 400 MHz ( //), 100 MHz ( C). (a) HH COSY plot ... 

To find the molecular formula of a compound, we need one more piece of information—its molar mass. Then all we have to do is to calculate how many empirical formula units are needed to account for the molar mass. One of the best ways of determining the molar mass of an organic compound is by mass spectrometry. We saw this technique applied to atoms in Section B. It can be applied to molecules, too and, although there are important changes of detail, the technique is essentially the same. 

The molecular formula of a compound shows the actual number of each kind of atom present in a molecule of the compound. To work out the molecular formula of a compound, we need to know both the empirical formula and the relative molecular mass of the compound. The latter can be determined by mass spectrometry (see p. 73). 

The exact mass of a compound determined by high-resolution mass spectrometry is 212.0833. What is the molecular formula of the compound ... 

The chemical makeup of a substance is described by its percent composition—the percentage of the substance s mass due to each of its constituent elements. Elemental analysis is used to calculate a substance s empirical formula, which gives the smallest whole-number ratio of atoms of the elements in the compound. To determine the molecular formula, which may be a simple multiple of the empirical formula, it s also necessary to know the substance s molecular mass. Molecular masses are usually determined by mass spectrometry. 

An organic compound was prepared containing at least one and no more than two sulfur atoms per molecule. The compound had C and H, no N, but O could have been present. The molecular mass of the predominant nuclidic species was 110.020 u, as determined by mass spectrometry. What is the molecular formula of the compound ... 

Rationalization of known compounds provides a level of usefulness that justifies the rule. But the rule also permits observed molecular stoichiometries of newly synthesized compounds to be translated into acluster shape. For example, [Al Bu ]2-has eve = 50 or sep =13 consistent with n = 12 and a deltahedral structure. The compound has been synthesized and an X-ray diffraction study reveals an icosahe-dral shape. The ability to suggest reasonable structures based on knowledge of a molecular formula generated by a technique like mass spectrometry accelerated the development of cluster chemistry simply because rapid spectroscopic methods can be more productively applied. Although efficient X-ray crystallographic structure determination reduces its importance for compounds that can be isolated in pure crystalline forms, transient intermediates detected in a reaction mixture can now be given reasonable structures. 

To find the molecular formula, we must determine the molecular weight today, almost certainly by mass spectrometry, which gives an exact value (Sec. 13.2). Ethane, for example, has an empirical formula of CH3. A molecular weight of 30 is found, indicating that, of the possible molecular formulas, C2H6 must be the correct one. 

If it turns out to be a new alkane, the proof of structure can be a difficult job. Combustion and molecular weight determination give its molecular formula. Clues about the arrangement of atoms are given by its infrared and nmr spectra. (For compounds like alkanes, it may be necessary to lean heavily on x-ray diffraction and mass spectrometry.)... 

A substance with the molecular formula CnH2o04, as determined by mass spectrometry, was isolated from the light petroleum extract of the Chilean medicinal plant Centaurea chilensis (Compositae) 8 mg were available for the set of NMR experiments 45. Beyond the shift range shown in (c), the C NMR spectrum shows the signals of quaternary C atoms at 170.1,169.2,149.8,142.9 and 137.5 ppm. A CH COLOC plot was not recorded owing to shortage of material and time. It was nonetheless possible to identify the natural product which was already known. What is its structure ... 

Methods of characterization aim to determine the products of a reaction. The level of detail expected depends on the circumstances, and determines the range of methods required. If the aim has been to make a known compound, one needs to check its identity and purity. Fingerprinting techniques measure a spectrum or some other property and compare it with results published for known compounds and available in literature databases. Such techniques may also show whether impurities are present, but it is often desirable to check the purity of the compound independently, for example by elemental analysis. However, if the compound prepared is a new one, more thorough investigation is appropriate. The stoichiometric formula may be found by elemental analysis, and the full molecular formula in principle by mass spectrometry (MS). MS combined with other spectroscopic techniques, especially infrared (IR) and nuclear magnetic... 

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