Enolate ions mass spectra

Example The NR mass spectrum of acetone closely resembles its 70 eV El mass spectrum (Chap. 6.2.1), thereby demonstrating that the molecular ion basically retains the structure of the neutral (Fig. 2.29). [144] However, the isomeric CsHeO" ions formed by McLafferty rearrangement of 2-hexanone molecular ion are expected to have enol structure (Chap. 6.7.1), and thus the corresponding NR mass spectrum is easily distinguished from that of acetone. 

There are rarely more than eight alkaloids present in a species, and the possibility of the same GC retention time is not normally a problem. In cases where overlap has been observed it has proved possible to identify both components from the mass spectrum of the mixture. The crude alkaloid extracts are treated with trimethylsilyldiethylamine to form volatile TMS derivatives of the hydroxylated components. The presence of a free phenolic or hydroxyl group is then detected by an ion with m/e 73 [(CH3)3Si+]. Positional isomers [e.g., erysovine (5) and erysodine (7)] are resolved although a- and /1-erythroidine are not. The presence of fi-erythroidine (60) can be estimated since it shows some enol content under the silylation conditions and gives rise to a monotrimisyl derivative with m/e 345 (15). 

The enol of glycine (4) has been generated by neutralization with trimethy-lamine of the corresponding cation-radical (4+ ) prepared by dissociative ionization of isoleucine (Scheme 3) [71]. The +NR+ mass spectrum of 4+ showed a substantial survivor ion attesting to the stability of isolated 4. In contrast, the survivor ion from glycine (5) is much less stable and appears as a very minor peak in the +NR+ mass spectrum of 5, in spite of the fact that neutral 5 is thermodynamically more stable than 4. 

The enol of acetamide (6) is also quite stable as an isolated species, as documented by the dominant survivor ion in the +NR+ mass spectrum (Fig. 6). Compared to 6, the thermodynamically more stable neutral acetamide (7) shows less... 

Unequivocal identification of the structure of gas-phase ions is not a simple task, owing primarily to the possibility that the ion formed initially may not retain the structure of its neutral precursor. As a consequence, the mass spectrum may not be a true representation of the ions formed initially. In many cases, ions undergo isomerization to a more stable structure before fragmentation can occur. In several examples in the literature, the solution-phase stability is reversed for gas-phase ions. Ketones are more stable in solution than are their enol tautomers, but the reverse is true for gas-phase radical cations. 

The +NR+ spectrum of 8 showed a small survivor ion, but differed substantially from the spectra of other C2H5NO isomers, e.g., 6, 7, AT-methylamino(hy-droxy)carbene (9), and N-methylformamide (10). The low intensity of survivor ions in the NR mass spectra of enol imines is due to Franck-Condon effects in collisional reionization that result in vibrational excitation of the resulting cation radical followed by dissociation. Franck-Condon effects were studied for collisional ionization of acetimidic acid, CH3C(OH)=NH, which was one of the neutral dissociation products of 1 -hydroxy- 1-methylamino-l-ethyl radical, a hydrogen atom adduct to AT-methylacetamide [37]. The cation-radical dissociates extensively upon reionization, and the dissociation is driven by a 74 kj mol-1 Franck-Condon energy acquired by vertical ionization. 

One way to study the presence of a 3-keto group is to convert it to an enol trimethylsilyl ether which gives pronounced peaks at mje 142 and 143 (cf. 23). The latter ion is the base peak in the spectrum of an enol silyl ether of methyl 3-keto-12a-hydroxy-5/3-cholanoate. An intense peak is also seen at mje 316, i.e., A/-(90- -142). In view of the preferred loss of the side chain following loss of a 12-trimethylsiloxy function the fragment of mass 142 is unlikely to represent the side chain with C-16 and C-17. A prominent peak at mje 201 (Af-[90-f 1424-115]) indicates that it represents carbon atoms 1-4. 

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