Tropylium ion structures

It is possible to derive structural information from the fragmentation pattern in a spectrum. The appearance of prominent peaks at certain mass numbers is empirically correlated with certain structural features. For example, the mass spectrum of an aromatic compound is usually dominated by a peak at m/z 91, corresponding to the tropylium ion. Structural information can also be obtained from the differences between the masses of two peaks in a spectrum. For instance, a fragment ion occurring 20 mass numbers below the molecular ion strongly suggests a loss of a HF moiety. Thus, a fluorine atom is likely to be present in the substance analyzed. 

Draw an MO diagram for tropylium ion (structure shown on the previous page), and fill in the 7t electrons showing the ground state of this ion. 

The 1,2-dithiolylium and 1,3-dithiolylium ions (115) and (116) are iso-rr-electronic with the tropylium ion, from which they may be formally derived by replacing double bonds by sulfur atoms. Various calculations and structural data demonstrate that the rings are stabilized by tt-electron delocalization. 

Following the first observation, eight substituted tropylium ions [94" ]-[98" ], [30 ], [34" ] and [35" ] (Scheme 1) were combined with Kuhn s carbanion [2 ] to make ionically dissociative hydrocarbons [94-2]-[98-2], [30-2], [34-2] and [35-2]. The structures of these hydrocarbons were determined as indicated in (23) on the basis of their and nmr spectra. Compounds [94-2], [95-2], [98-2] and [35-2] were mixtures of two positional isomers, in which [2 ] is connected to different positions of the seven-membered ring. Positions that formed a carbon-carbon bond with [2 ] are indicated by dots in Scheme 1. 

The best guess at present is that both of these q values represent rearrangement barriers on the way to the low-energy tropylium (benzotropylium) fragment ion structures. 

Structures 3a and 3b are examples of tropylium ions (see Section I,C) corresponding to tropones 1 and 2, respectively. 

For fused tropylium ions the formulation of resonance structures is not uniform in the literature. In compounds of type 3a a resonance structure... 

Using MS-MS, the product ion scan of m/z 422 from mosapride showed m/z 109 for the fluorotropylium ion, whereas the product ion scan of m/z 404 for the impurity showed a tropylium ion at m/z 91 (see Figure 6.15). On the basis of this information the des-fluoro structure was proposed for the impurity, as indicated in Figure 6.16. Its formation was rationalised by the presence of a des-fluoro impurity in the starting material. 

A special case of carbocation stability arises where the cation complies with the Hiiekel (4n+2) rule governing aromatic structures. Of these, the best known and most useful is the cycloheptatrienyl cation, more frequently referred to as the tropylium ion. For an informative and wide ranging account of structures, stabilities, properties and reactions of almost every type of carbocation, reference should be made to the series of monographs edited by Olah and Schleyer (18). 

See Table XX, Footnote (a). b Parent tropylium ion.c Corrected structure (69ZOR961). 

The high stability of certain tropylium ions (e.g., 237 and 238, in contrast to 351) has also been demonstrated by spectroscopic observations (Section III,B,2,a-d) and by reactivity estimation (73ACS2257 78JHC285). Thus, the facile formation and ready transformation of the corresponding tro-pones are explained by intermediates or mesomeric forms having a tropylium structure. 

Another aromatic molecule containing six n electrons is C7H7+, the tropylium ion, derived from cycloheptatriene. This positive ion forms fewer complexes than does benzene, and they are less thoroughly studied. A molecule that has 10 electrons and has an aromatic structure is the cyclooctatetraenyl ion, C8H82. Some sandwich compounds containing this ligand are known as well as complexes of the type... 

Unsubstituted benzofuran and dibenzofuran show very intense peaks of the molecular ions,22b reflecting their high stability, caused by the lack of bonds which can be cleaved easily. The spectra of benzofurans substituted with a methyl group in the benzene ring (15) are characterized by abundant ions at M-l. They are assumed to have the oxonium ion structure [17] (see Budzikiewicz,11 p. 230) rather than that of a tropylium ion [16],22b as might be supposed considering the decomposition of analogous benzene compounds.22b... 

The interesting explanation offered by Wexler for this discrepancy is based on a striking parallel with the behaviour of the isomeric deuteriated toluenes under electron impact. The mass spectrometric work by Meyerson and Rylander (1957), Rylander et al. (1957) and by Meyerson et al. (1968), provided convincing evidence that the isomeric tolyl and benzyl ions produced by electron impact rearrange, before fragmentation, to a common seven-membered ring structure, the tropylium ion, which... 

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