Mass spectral formula weight

The first piece of information we try to get is the molecular formula from the mass spectral formula weight and the percent composition from a combustion analysis if available. If we don t have a molecular formula, we can still get a minimum count on the number of carbons and hydrogens from the NMR spectra, which will be discussed later. 

Analysis and mass spectral molecular weight determination established the empirical formula, C21H26N2O3, for stemmadenine (8, 116, 117). Its UV-spectrum was characteristic of an indole (cf. ref. 55), while the IR-spectrum indicated the presence of a normal ester grouping (1718 cm-1) and the absence of any substituent in the indole aromatic ring (116). These findings were fully borne out by NMR-spectroscopy which showed the presence of an indole NH (9.3 S), four aromatic protons, and a carbo-methoxyl methyl singlet (3.79 S). A single vinyl proton quartet (5.4 S)... 

Compilations of Reference Spectra There are several compilations of reference mass spectra available of which the oldest is the American Petroleum Institute (Ref 82) collection of spectra obtained mostiy on the older type instruments. Recent collections index spectra variously, eg, under reference number (Ref 19). molecular weight (Refs 12 19), molecular formula (Ref 19), fragment ion values (Ref 19), and base peak (Refs 12 19). A quarterly journal, Archives of Mass Spectral Data ... 

The Eight-Peak Index of Mass Spectra published by the Mass Spectrometry Data Centre of the Royal Society of Chemistry is a popular printed index of mass spectral data that now contains some 81000 spectra of over 65000 different compounds [4], These spectra are published in the shape of lists of the eight main peaks. The complete data are sorted in three different ways to allow easy identification of unknown compounds by (i) molecular weight subindexed on molecular formula, (ii) molecular weight subindexed on m/z value and (iii) m/z value of the two most intense ions. 

The combination of IR and mass spectral data provides key information on the structure of an unknown compound. The mass spectrum reveals the molecular weight of the unknown (and the molecular formula if an exact mass is available), and the IR spectrum helps to identify the important functional groups. 

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. 

The kind of information available from mass spectrometry falls into two categories. First, the m/z value for the molecular ion provides information useful in calculating the molecular formula of the molecule. Second, the lower molecular weight fragments that appear in the mass spectrum contain clues concerning structural features of the molecule in question. Be sure that you understand how to extract these kinds of information from mass spectral data. 

Mass spectrometry has long been an important tool in natural products research (ref. 27), in part because of its sensitivity and also because mass spectral fragmentations often serve to class a compound with related structural groups. The technique also provides molecular weights (and in the high resolution mode, molecular formulas) together with fragmentation data... 

Mass spectrum interpretation is essential to solve one or more of the following problems establishment of molecular weight and of empirical formula detection of functional groups and other substituents determination of molecular skeleton (atom connectivity) elucidation of precise structure and, even in favorable cases, certain stereochemical features. As discussed in the previous chapters, electrospray (ESI) and atmospheric pressure chemical ionization (APCI) are two of the most effective and successful interfaces for the liquid chromatography—mass spectrometry (LC—MS) that have been developed. Thus, we will focus on how to interpret the mass spectral data generated by either ESI or APCI in this section. 

These are very simple examples, but provide a plan of attack for the problems at the end of the chapter. It is highly unlikely that an analyst can identify a complete unknown by its IR spectrum alone (especially without the help of a spectral library database and computerized search). For most molecules, not only the molecular weight, but also the elemental composition (empirical formula) from combustion analysis and other classical analysis methods, the mass spectrum, proton and C NMR spectra, possibly heteroatom NMR spectra (P, Si, and F), the UV spectrum, and other pieces of information may be required for identification. From this data and calculations such as the unsaturation index, likely possible structures can be worked out. 

One of the first steps in determining the molecular structure of a compound is to establish the compound s molecular formula. In the past, this was most commonly done by elemental analysis, combustion to determine the percent composition, and so forth. More commonly today, we determine molecular weight and molecular formula by a technique known as mass spectrometry (an explanation of the technique is beyond the scope of this book). In the examples that follow, we assume that the molecular formula of any unknown compound has already been determined, and we proceed from there, using spectral analysis to determine a structural formula. 

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