Unimolecular Ion Decompositions

McLafferty, F.W. Wachs, T. Lifshitz, C. Irmorta, G. Irving, P. Substituent Effects in Unimolecular Ion Decompositions. XV. Mecharristic Interpretations and the Quasi-Equilibrium Theory. J. Am. Chem. Soc. 1970,92, 6867-6880. 

Such unimolecular ion-decomposition reactions can be viewed as another field of chemistry, but fortunately for most chemists studying this book, there are many close similarities to pyrolytic, photolytic, radiolytic, and other energetic reactions, and there are even many general similarities to condensed-phase (solution) organic reactions. The largest points of difference are that ionic and often radical species are involved in each reaction in the mass spectrometer, and their combined effects sometimes appear unusual to the organic chemist. Chemists may also question the reliability of structural relationships based on rearrangement reactions. However, many of these are based on well-established chemistry and can provide key molecular information... 

The following general guidelines for predicting unimolecular ion decompositions follow basic chemical principles. 

To understand thoroughly the capabilities, and especially the limitations, of mass spectrometry for structure elucidation, one must be familiar with the basic theoretical aspects of unimolecular ion decompositions. Sections 7.1 and 7.2 are appropriate for introductory courses designed to teach fundamental interpretive skills. The nomenclature and abbreviations adopted here follow those of the GIANT tables (Lias et al 1988), whose introduction is recommended reading, as well as the references at the end of this chapter. 

Figure 7.1. The Wahrhaftig diagram relationship of P E) and k E) for unimolecular ion decompositions of ABCD. See text for definitions. (Wahrhaftig 1962,1986.)...

Figure 7.9. Thermochemical energy relationships for unimolecular ion decompositions in the mass spectrometer.

The wide utility of aryl diazonium ions as synthetic intermediates results from the excellence of N2 as a leaving group. There are several general mechanisms by which substitution can occur. One involves unimolecular thermal decomposition of the diazonium ion, followed by capture of the resulting aryl cation by a nucleophile. The phenyl cation is very unstable (see Part A, Section 3.4.1.1) and therefore highly unselective.86 Either the solvent or an anion can act as the nucleophile. 

Theories of translational energy release in unimolecular decompositions are discussed in Sect. 8.1. Qualitative lines of explanation are discussed, in conjunction with the experimental results to which they relate, in Sects. 8.2—8.4. The extensive data on translational energy releases in source reactions, including PIPECO, and in metastable ion decompositions are collected together in tables (Sect. 8.5). The emphasis is on decompositions of polyatomic ions, although many triatomics are included in the tables. The coverage includes both fundamental and mechanistic studies. 

An overall view shows the CID process as a sequence of two steps. The first step is very fast (10 14 to 10 16 s) and corresponds to the collision between the ion and the target when a fraction of the ion translational energy is converted into internal energy, bringing the ion into an excited state. The second step is the unimolecular decomposition of the activated ion. The collision yield then depends on the activated precursor ion decomposition probability according to the theory of quasi-equilibrium or RRKM. This theory is explained elsewhere. Let us recall that it is based on four suppositions ... 

The unimolecular decomposition of C6H5Cr has been studied by analysing the distorted C5H5 peak shape observed in a TOP mass spectrometer following MPI of jet-cooled chlorobenzene. The analysis provided values for the specific reaction rate constants, k(E), which agreed well with the results of previous studies, validating the MPI/TOF m.s. technique as a means for investigating ion decomposition rates. 

There is some difficulty with the energetics of unimolecular hydroperoxide decomposition. The endothermicity for the reaction ROOH RO + OH is of the order of 50 kcal., whereas the observed activation energy is as low as 30 kcal. The question is, therefore, bound to arise To what extent is decomposition trace metal--catalyzed It can be demonstrated that ferrous phthalocyanine, even at concentrations below lO M, is a most powerful activator of hydroperoxides—e.g., in the oxidation of quercetin, rhamnetin, or 8-carotene. The action of ferrous phthalocyanine is in principle similar to that of ferrous ion with hydrogen peroxide, already discussed. It may be described as reduction activation. 

Benzene has been investigated rather thoroughly by PIPECO [269, 277, 278, 758, see also 715] however, its unimolecular ion chemistry remains somewhat enigmatic. It was suggested, on the basis of charge exchange results, that the decompositions of the molecular ion to form (CaHa)" and (04114) did not compete with those forming and... 

Tajima, Okada and coworkers reported the unimolecular metastable decompositions of aUcoxysilanes in a number of recent studies - . These authors found significant differences in the ion fragmentation characteristics between the silanes and their carbon analogues. Thus, for example, the principal fragmentation process of ionized diethoxy-dimethylsilane was found to correspond to a consecutive loss of ethylene and aldehyde molecules from the siUcenium ion formed by loss of an ethoxy radical. In contrast, the carbon analogue acetone diethyl acetal does not exhibit a significant loss of aldehyde molecules in its metastable ion mass spectrum. 

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