Presence of the molecular ion

In El the molecular ion is only weakly observed in the case of linear saturated hydrocarbons. The presence of branching usually entails the disappearance of this peak. However, an unsaturation, and especially an aromatic ring, makes the molecular ion peak more intense. The presence of electronegative saturated heteroatoms (oxygen, fluorine) normally prevents the observation of the molecular ion. In fact, its intensity depends on the groups that are present. Thus an aliphatic ester yields a weak or absent molecular ion, whereas an aromatic one usually yields an intense molecular ion peak. 

Using soft ionization techniques in the positive ion mode, (M-H)+ is most often observed in the case of saturated compounds and (M + H)+ in the case of unsaturated compounds or heteroatom-containing compounds. However, halogen compounds or compounds containing an sp3 oxygen often prevent the observation of the molecular ion. Thus, for example, it is very difficult to form the molecular ion or the pseudomolecular ion of acetals or orthoesters. 

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Sidwell and Zondervan [10] used LC-MS with APCI detection for the identification and quantification of extractable antioxidants from food-contact plastic materials. Identification is based on the presence of the molecular ion (M + FI)+, (M—H) , other key ions or on further ion breakdown (MSn) transitions. The following antioxidant/stabiliser types were examined hindered phenols,... 

The presence of the molecular ion as an intense peak in the mass spectra of the pyrazolo[3,4-c][l,2,5]thiadiazole (24a) and pyrazolo[3,4-c][l,2,5]selenadiazole (24b) was central to their structural identification <81JOC4065>. 

Digermoxane (vp4 about 66 torr at 0°) is a clear colorless liquid that is best stored at liquid nitrogen temperature in break-seal glass ampules. The 1H nmr spectrum,17 measured in cyclohexane, consists of a singlet [5(GeH)J at 5.28 ppm, while the infrared spectrum4 18 shows prominent absorptions at 2112 (s), 928, 882, 798 (vs), 784 (vs), 674, and 452 cm-1. The mass spectrum confirms the presence of the molecular ion at m/e 156-172 [H Ge20]+. 

Flow injection analysis mass spectrometry (FIA-MS) has been reported to be a fast method for the characterization of combinatorial libraries (55,56). The method verifies the presence of the molecular ions of the expected product and side products or impurities but does not provide information on the quality of the analyzed samples. Significant improvements related to the increased analytical throughput, obtained by reducing the time between each injection without increasing the intersample carry-over from each analysis, were recently reported (57, 58). When coupled with RP-HPLC, FIA-MS allows the separation and the determination of the molecular weight of the components of each sample. This is normally enough to unequivocally attribute the structure of the expected library component and of any side products from a library synthesis. 

El mass spectroscopy gives valuable information about the fragmentation of barbiturates, but the molecular ion is not usually observed. The presence of the molecular ion is very important for the identification of particular derivatives. Therefore, other methods of ionization of a molecule are also employed. 

The H NMR spectrum and the presence of the molecular ion peak in its FAB mass spectrum are in agreement with the structure proposed. Note that attempts to use Ni as template led to the formation of an almost black precipitate of unidentifiable composition. A number of other derivatives, [Ni(L1777)]-[Ni(Ll780)], differing by the number of oxyethylene units, are available [127],... 

The solid probe EI-MS spectrum of PV 23 (Figure 21-13) shows the presence of the molecular ions near 588 to 592 as well as the molecular ions of the mono-chlorinated analog (molecular ion at 554) and the non-chlorinated product (molecular ion... 

The mass spectra of phenylthiazoles are characterized by the presence of intense molecular ion peaks, due to the aromatic nature of the molecules, which represent 35, 41, and 44% of the total ionization for 2-, 4-, and 5-phenylthiazoles, respectively. 

Look for characteristic isotopic abundances that show the presence of bromine, chlorine, sulfur, silicon, and so on. If the deduced molecular ion is of sufficient intensity, the probable molecular formula may be determined using the observed isotopic abundances of the molecular ion region. Set the deduced molecular ion, M, at 100% abundance, and then calculate the relative abundances of M + 1 and M + 2 either manually or using the data system. 

Figure 18.2 is the mass spectrum of a branched hydrocarbon. Note the intensity of the molecular ion peak and the presence of an M - 15 peak. The M - 15 peak is typical, particularly if the side chain is a methyl group. The position of the side chain is indicated by the m/z 112 and 113 ions (CH3(CH2)5CHCH3). Usually the site of branching is more difficult to establish. 

The M - 1 peak due to the loss of the aldehyde hydrogen by a-cleavage is usually abundant. The loss of 29 Daltons is characteristic of aromatic aldehydes. Peaks at m/z 39, 50, 51, 63, and 65 and the abundance of the molecular ion show that the compound is aromatic. Accurate mass measurement data indicate the presence of an oxygen atom. 

The presence of chlorine and/or bromine is easily detected by their characteristic isotopic patterns (see Appendix 11). As in many aliphatic compounds, the abundance of the molecular ion decreases as the size of the R group increases. For example, in the El mass spectra of methyl chloride and ethyl chloride, the molecular ion intensities are high, whereas in compounds with larger R groups such as butyl chloride, the molecular ion peak is relatively small or nonexistent. 

Mass spectrometry is a useful tool to detect the existence of reactive iron-imido intermediates. In intramolecular aromatic aminations, Que and coworkers used electrospray ionization mass spectrometry to show the presence of a molecular ion at m/z 590.3 and 621.2, which could be attributed to the formation of [(6-(o-TsN-C6H4)-TPA)Fe ]+ and [(6-(o-TsN-C6H4)-TPA)Fe° OMe)]+. With the isoto-... 

Hence, the presence in an El spectrum of the molecular ion at mJz 109 suggests that the molecule must contain an odd number of nitrogen atoms, while if it is at m/z 232 the molecule may contain no nitrogen atoms or an even number of them. 

The presence of one carbon atom in a molecule of carbon dioxide results in registration of the molecular ion peak of m/z 44 and of the A + 1 isotopic peak of m/z 45. The intensity of the latter is 1.1 % of that of M+. It appears due to the presence of 13C02 molecules. An increase in the number of carbon atoms in a molecule leads to an increase of the intensity of the M + 1 ion peak to 1.1 n% of M+, where n is the number of carbon atoms in the molecule. To calculate the number of carbon atoms in a molecule using a mass spectrum one should divide the intensity of the M + 1 peak as a percentage of M by 1.1. The result defines the maximum possible number of carbon atoms. One should remember that calculations may be more complicated if an [M — H]+ ion peak is present. 

All analytical techniques are designed to provide the answer to one or both of the two important questions what is it and how much is there Mass spectrometry possesses attributes that allow it to contribute answers to both of these questions. The nominal (integral) m/z ratio of the molecular ion can sometimes be sufficient to identify a chemical compound, particularly if there is additional information available (either from the mass spectrum itself or another analytical technique). The presence of other signals in a mass spectrum attributable to... 

In general, the stability of the molecular ion increases if n-bonding electrons for the delocalization of the charge are available and it decreases in the presence of preferred sites for bond cleavage, e.g., by a-cleavage. 

The molecular ion is relatively abundant (30-50%), as is the (M — ion. The latter may be represented as either (a) or (b) (Scheme 32). The clue which points out unequivocally to an isopavine nucleus is the presence of the (M — 43) + ion in intensities varying from 30 to about 60%. It is formed by the retro-Diels-Alder condensation of the molecular ion, resulting in loss of a CH2=NCH3 unit. This particular ion is practically absent in the spectrum of a pavine (77). An additional peak of moderate intensity is associated with the (M — 86) ion which is formed by a subsequent loss of a methyl radical and carbon... 

The appearance of this type of peak arrangement at the high end of the m/z scale on the mass spectrum is characteristic of the molecular ion. The two higher mass peaks are due to the presence of isotopes of the compound s atoms. The ratio found in the peak pattern is the result of natural isotope abundance of the atoms in the molecule. In a... 

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