Mass Spectra of Hydrocarbons

It will also be noted that in Table 10.1 for benzene and in Table 10.16 for naphthalene, peaks at noninteger ndz values are observed. For naphthalene, there are peaks at mIz values of 64.5,63.5, and 62.5. These are not fragments with fractional masses. Rather, they are doubly charged ions. It will be remembered (Chapter 9) that 

There is another phenomenon that gives rise to peaks with noninteger masses. Ions with lower internal energy than those that produce the normal product ions can fragment after they leave the ion source but before entering the mass analyzer. These metastable ions have the velocity of the precursor ion, m but the mass of the product ion, m2. In a magnetic sector instrument, a metastable peak will appear at m, where 

The value of m is generally not an integer. The spectral interpretation texts cited in the bibliography should be consulted for more details. 

The lack of significant fragmentation and the intense molecular ion illustrate how stable aromatic molecules are. As a general rule, increased unsaturation increases stability. It is harder to break double bonds than single bonds and harder to break apart the very stable, delocalized molecular orbitals in aromatic compounds than to break bonds in aliphatic compounds. Also, rings are more stable than straight chains. 

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Removal of one electron should make no difference to the relative stabilities of polyene molecule ions or even electron polyene fragments as compared to their neutral counterparts, e.g. butadiene and the allyl radical should have the same relative stabihties as the butadiene molecule ion, and the allyl cation. Removal of one electron will, however, alter the stabihties, and thus the reactivities of cychc polyenes. The molecule ions of aromatic hydrocarbons will be substantially less aromatic then their neutral counterparts. Correspondingly the molecule ions of antiaromatic hydrocarbons will not be as antiaromatic as their neutral analogs, e.g. cyclobutadiene + should be relatively more stable than cyclobutadiene. The largest charge effects in hydrocarbons will be observed in nonaltemant ) monocychc hydrocarbons. The cyclopropenium ion 7 and the tropillium ion 2 are both strongly aromatic as compared to their neutral analogs. Consequently CsHs is a very common ion in the mass spectra of hydrocarbons while cyclopropene is not a common product of hydrocarbon pyrolysis or photo-... 

A further collection of mass spectra of hydrocarbons of the carane and menthane series has been published. The identification of monoterpenoid alcohols in complex mixtures is sometimes tedious it has been suggested that the problem would be simplified by preparing the trifluoroacetates, and using the n.m.r. spectra. ... 

We will begin with the mass spectrum of -pentane. Although it is a simple molecule, it reveals some important information about the mass spectra of hydrocarbons that can be extended to more complex molecules (Figure 14.24). 

The mass spectra of TMS ethers are characterized by weak or absent molecular ions the [M-15] ion formed by cleavage of a methyl to silicon bond is generally more abundant. This ion can be used to determine the molecular weight provided that it is not mistaken for the molecular ion itself. Dissociation of the molecular ion often results in prominent secondary fragment ions containing the ionized dimethylsiloxy group attached to a hydrocarbon portion of the molecule. In common with alkyl ethers,... 

Mass spectra of the cis- and /ram-isomers of the pyrimido[2,TA [l,3]thiazin-6-ones 294 and 295 were studied. Retro-Diels-Alder fragmentation of the hydrocarbon ring was of medium to low stereospecificity. A number of highly selective processes were discovered allowing differentiation between stereoisomers <1996RCM721>. The mass spectral fragmentation pattern of 296 was studied in detail <1996PS(113)67>. 

T.R. Sharp, H. Lee, A. Ferguson, K.N. Marsh and R.G. Harvey, Electron impact mass spectra of polycyclic aromatic hydrocarbons A reference collection. Presented at the 36th ASMS Conference on Mass Spectrometry and Allied Topics, San Francisco, CA, 5-10 June 1988. 

The excess energy deposited onto a molecular ion can obviously be decreased at low electron energy. The use of 12-15 eV electrons instead of the routinely employed 70 eV electrons still allows to ionize most analytes while reducing disadvantageous fragmentation, e.g., the El mass spectra of large hydrocarbons benefit from such measures. [12] Especially in conjunction with low ion source tempera-... 

The RDA reaction is often observed from steroid molecular ions, and it can be very indicative of steroidal stmcture. [107,110,113,114] The extent of the RDA reaction depends on whether the central ring junction is cis or trans. The mass spectra of A -steroidal olefins, for example, showed a marked dependence upon the stereochemistry of the A/B ring juncture, in accordance with orbital symmetry rules for a thermal concerted process. In the trans isomer the RDA is much reduced as compared to the cis isomer. The effect was shown to increase at 12 eV, and as typical for a rearrangement, the RDA reaction became more pronounced, whereas simple cleavages almost vanished. This represented the first example of such apparent symmetry control in olefinic hydrocarbons. [114]. 

ROH is a low molecular weight alcohol, the alcohol serves as both reactant and solvent for the reaction. When ROH is a solid alcohol or phenol, the reaction can be conducted in a hydrocarbon solvent, e.g., toluene, or by fusing a mixture of ROH and P4S10. Only moderate heating of the reaction mixtiues should be employed since temperatures much above 100 cause secondary reactions. Mass spectra of acids prepared by the above reaction usually indicate from about 1—5% impurities with molecular weights greater than the acids. 

Cleavage with proton transfer is also common in the mass spectra of drug molecules. In the first two examples thC initial step is homolytic a-cleavage as shown in Figure 9.11 this is followed by loss of a neutral hydrocarbon fragment. 

Mass spectral data have frequently been used in the structural determination of boron heterocycles. One paper has been devoted to the mass spectra of some six-membered boron-nitrogen systems. It was concluded that the spectra could be interpreted analogously to their hydrocarbon counterparts. In all cases the molecular peak was the base peak of the spectrum (68T6755). Doubly charged molecular ions, a feature typical of aromatic compounds, are often encountered. It should be noted, however, that some certainly non-aromatic aminoboranes give such doubly charged ions as well. 

During the last 30 years the literature has included many reports on the variation of the properties (such as m., solubility, vapor pressure, IR, and mass-spectra) of samples of different alkoxides in time. Such examples were provided by the samples of Al(OEt)3and Al(OPr% [1640, 1642], Ga(Ol )3 [1234, 1233], [Th(OPr )4]B [106], Er(OPri)j arROH [1734], and so on. The properties of the polymeric and tetrameric samples of Ti(OMe)4 ( A and B forms) [866] that differ in solubility in hydrocarbons, structure, and IR spectra are compared in Section 12.11. Dioxomolybdates and tungstates M02(0R)2 are also prone to polymerization in time their solubility decreases to practically... 

Fig. 3.79 Comparison of the mass spectra of straight chain and branched chain saturated hydrocarbons (a) decane (b) 2,6-dimethyloctane.

The mass spectra of diamonoid substrates are highly characteristic. The parent hydrocarbons exhibit a remarkable resistance to fragmentation. Large parent peaks which are also generally the base peaks are observed 4- 29> 33 169>. 

Pomonis, J. G., Nelson, D. R. and Fatland, C.L. (1980). Insect Hydrocarbons 2. Mass spectra of dimethylalkanes and the effect of the number of methylene units between methyl groups on fragmentation../. Chem Ecol., 6, 965-972. 

The rationale used in the interpretation of the mass spectra of methylalkanes has been presented in several reports 2- vs. 4-methylalkanes (Baker et al., 1978 Scammells and Hickmott, 1976 McDaniel, 1990 Bonavita-Cougourdan et al., 1991) 2,X- and 3,X-dimethylalkanes (Nelson et al., 1980 Thompson et al., 1981) and internally branched mono-, di- and trimethylalkanes (Blomquist et al., 1987 Pomonis et al., 1980). In the majority of reports, identification is based on GC and MS data, but the conclusions are not confirmed with standards or synthesis of the proposed structures. However, there are reports of chemical ionization (Howard et al., 1980) and electron impact of synthetic methyl-branched hydrocarbons (Carlson et al., 1978, 1984 Pomonis et al., 1978, 1980) and these have been very useful in confirming mass spectral fragmentation patterns with chemical structures. 

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