Distonic ions formation

Localization of an unpaired electron in the framework of a definite molecular fragment can sometimes lead to the formation of ion-radicals with spatially separated charge and radical sites. They can be considered free radicals with an appended, remote charge. These species form a particular class of distonic ion-radicals. Distonic is from the Greek word diestos and the Latin word distans, both meaning separate. Yates et al. (1984) introduced this term for ions that formally arise by the removal of an electron from a zwitterion or a biradical. 

Gas-phase properties of a molecule have, by definition, an intrinsic character and they could be modified by the environment. Although the formation and reactions of gaseous ionized phenol 21 (cf. Chart 5) and its cyclohexa-2,4-dienone isomer 22 have been studied in numerous ionization and mass spectrometric studies , thermochemical parameters of these isomers as well as information on other non-conventional isomers, such as the distonic ion 23, were rather scarce. Conventional cations of analogous aromatic systems (X—CeH5) + and their distonic isomers generated by simple 1,2-hydrogen shifts within the ring were demonstrated to be observable gas-phase species . In addition, the mechanism of the CO-loss upon phenol ionization has only recently been unraveled . ... 

The proton transfer reaction shown in equation 14b represents a general class of reactions (equation 17) known as self CF reactions in which a compound acts as its own Cl reagent gas Such reactions can be observed in EI/CI sources of conventional mass spectrometers, and are important in ICR mass spectrometers. The rates of formation of MH ions from M (equation 17) ions of organophosphorus species are shown in Table 5 It should be pointed out that although the structures of the M ions in these experiments were assumed to have the same connectivity as in the neutral precursors M, distonic ions may also be responsible for the proton-transfer reactions observed, based on the work of Kenttamaa and coworkers ... 

Cycloreversion Reactions A cycloreversion reaction is the reverse of a cycloaddition reaction and leads to the formation of the starting reactants through the cleavage of two bonds in the ring [18], A typical example is the formation of C2H4+ and neutral C2H4 from the cyclobutane radical cation. As shown in reaction (6.37), this reaction proceeds through the intermediacy of a distonic ion. The radical cations of a variety of other four-membered cyclic compounds, such as cyclobutanones (3), diketene (4), oxetane (5), cyclobutylamine (6), and thiocyclobutane (7), are known to participate in cycloreversion reactions [27]. 

The H-transfer to the radical site in the distonic ion, preferably via a six-membered transition state, followed by the loss of alkyl radicals and eventual formation of an allyl cation, also competes with the ethene loss [reaction (6.40)]. In alkyl-substituted cycloalkanes, loss of the alkyl side chain overwhelms other fragmentation processes due to branching at that site. For example, m/z 83 is the base peak in the spectra of methyl- and n-bntylcylohexanes. 

It is of interest to note that distonic ions also isomerize by shifts of the protonated group that contains the heteroatom. A typical example is the 1,2-NH3 shift in the a-distonic ion of an n-alkyl amine. This shift converts the molecular ion of the n-alkyl amine into the molecular ion of an s-alkylamine and is responsible for the small but distinct signals at mlz 44 that are found in addition to the expected base peak at m/z 30 in the mass spectra of these amines (Scheme 15). Originally it was supposed that the ions were generated by P-cleavage and formation of protonated ethylene imine. 

Fig. 6.13. Activation energies for isomerization of primary amine molecular ions to distonic isomers with the heats of formation of the precursor M ions normahzed to zero. [46]...

Nitroarenes are recognized from their characteristic neutral losses due to the NO2 substituent. Normally, all theoretically possible fragment ions, the plausible [M-N02] and [M-O] ions as well as the unexpected [M-NO] ion, are observed. It is worth noting that molecular ions are 1,2-distonic by definition, because nitroarene molecules are best represented as zwitterion (Chap. 6.3). The molecular ion may either dissociate directly by loss of an oxygen atom or a NO2 molecule or it may rearrange prior to loss of NO. For the latter process, two reaction pathways have been uncovered, one of them involving intermediate formation of a nitrite, and the other proceeding via a three-membered cyclic intermediate. [208]... 

Section 3.2 includes an extensive discussion on the formation of odd-electron bonds, ion pairing, and the distonic stabilization of ion-radicals at the expense of separation between their spins and charges. Section 3.3 deals with ion-radicals from the class of even spin-charge distribution. This class occnrred more frequently in scientific works of past decades. However, the reader will find newly developed manifestations of the principle of the released electron, concerning spread conjugation and the fates of ion-radical precursors with increased dimensionality. 

Single-electron transfers to or from electronically neutral molecules result in the formation of anion radicals and cation radicals, respectively. The unpaired spin and the charge can delocalize within the molecular carcass, can be located on the same fragment or even atom, or can be spatially separated (distonic species). Each type has its own synthetic opportunities, which were discussed in the previous chapters. All of that material shows that these ion radicals cannot be treated either as conventional radical species or even as their ionic counterparts. They are characterized by unique behavior. 

Oxidation leads to formation of the new a -bond and aryl-stabilized car-bocationic centers in 62. Interestingly, there is no evidence of proton loss from the dihydro[5]helicene dication 62 and the dication is stable in the presence of the reasonably nucleophilic counter ion, I3-. This again suggests that these stabilized systems are considered only as weakly distonic superelectrophiles. 

The conversion is thought to involve formation of the carboxonium ion (77) by protonation of the carbonyl oxygen, and subsequent protonation then occurs at the C-H bond. The resulting carboxonium-carbonium dication (78) possesses the maximum possible charge-charge separation for this bicyclic framework. Subsequently, an intermediate carboxonium-carbenium dication (79) is produced, which isomerizes to the tertiary -carbenium ion, and deprotonation provides the product enone (80). Similar distonic superelectrophiles are proposed in other rearrangements of terpenes in superacid.28... 

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