17-electron radicals

Another aspect of wave function instability concerns symmetry breaking, i.e. the wave function has a lower symmetry than the nuclear framework. It occurs for example for the allyl radical with an ROHF type wave function. The nuclear geometry has C21, symmetry, but the Cay symmetric wave function corresponds to a (first-order) saddle point. The lowest energy ROHF solution has only Cj symmetry, and corresponds to a localized double bond and a localized electron (radical). Relaxing the double occupancy constraint, and allowing the wave function to become UHF, re-establish the correct Cay symmetry. Such symmetry breaking phenomena usually indicate that the type of wave function used is not flexible enough for even a qualitatively correct description. 

Similar arguments can be used to predict the relative stabilities of the cyclo-heptatrienyl cation, radical, and anion. Removal of a hydrogen from cyclohepta-triene can generate the six-77-electron cation, the seven-77-electron radical, 01 the eight-77-elec iron anion (Figure 15.6). All three species again have numerous resonance forms, but HiickeTs rule predicts that only the six-7r-electron cyclohep-tatrienyl cation should be aromatic. The seven-77-electron cycloheptatrienyl radical and the eight-77-electron anion are antiaromatic. 

Reactions that proceed photochemically do not necessarily involve observations of an excited state. Long before observations are made, the excited state may have dissociated to other fragments, such as free radicals. That is, the lifetime of many excited states is shorter than the laser excitation pulse. This statement was implied, for example, by reactions (11-46) and (11-47). In these systems one can explore the kinetics of the subsequent reactions of iodine atoms and of Mn(CO)s, a 17-electron radical. For instance, one can study... 

Molecular modeling helps students understand physical and chemical properties by providing a way to visualize the three-dimensional arrangement of atoms. This model set uses polyhedra to represent atoms, and plastic connectors to represent bonds (scaled to correct bond length). Plastic plates representing orbital lobes are included for indicating lone pairs of electrons, radicals, and multiple bonds—a feature unique to this set. 

Arsenic(IV) is a 25 electron radical, the molecular parameters of which can be found in the monograph by Atkins and Symons ... 

At electrodes of the mercury type, electron-radical reactions can also be carried ont when there is a dehcit of proton donors in the solution. Then the radicals Q H formed initially, instead of adding a second electron and second proton, will couple to a dimeric product ... 

As in reduction reactions, two possible mechanisms exist for substitution reactions (1) electron-radical, involving the intermediate formation of radicals and their reaction with nucleophiles X ... 

The hexakis(methyl isocyanide) dimers, [Pt2(CNMe)6], undergo photolytic cleavage of the Pt—Pt bond to give 15-electron radicals, Pt(CNMe)3.94 Mixtures of platinum and palladium dimers give rise to heteronuclear complexes under photolytic conditions. Mixtures of normal and deuterium-labeled methyl isocyanide complexes reveal that the metal-ligand bonds undergo thermal redistribution.94... 

But homolytic fission can also take place, thus generating species possessing an unpaired electron—radicals, e.g. (1) and (2) ... 

McConnell, H.M. and Chesnut, D.B. 1958. Theory of isotropic hyperfine interactions in ir-electron radicals. The Journal of Chemical Physics 28 107-117. 

The six -electron [RCN2S2]+ ring is readily reduced to the corresponding seven re-electron radical [RCN2S2] .8 The structures and properties (conductivity, magnetic behaviour) of the radicals are discussed in Section 10.1. 

The species -CH3 and -CH3CO are radicals species containing unpaired electrons. Radicals are formed by homolytic fission of a covalent bond, where the electron pair constituting the bond is redistributed such that one electron is transferred to each of the two atoms originally joined by the bond. 

The presence of O2 clues you in that this is a free-radical mechanism, specifically a free-radical substitution. Because it is an intermolecular substitution reaction, it probably proceeds by a chain mechanism. As such it has three parts initiation, propagation, and termination. (We do not draw termination parts in this book.) The initiation part turns one of the stoichiometric starting materials into an odd-electron radical. This can be done here by abstraction of H- from C by 02. 

The anion BH formed in Eq. (5) is thermodynamically a stronger base than B , and BH will react with another molecule of substrate, Eq. (6). Each PB will therefore consume a total of two protons (and two electrons). Radical anion EGBs are normally produced in situ. 

Fig. 8. (a) Homoaromatic lit-electron conjugation in silyl cation 3. (b) Allyl-type resonance in Sit-electron radical 26. ... 

ERR is a powerful tool for unpaired electron/radical studies [55]. Therefore, EPR has the potential to fill a unique niche in the solution of Eq. (2). Only the subset of species that are radicals are accessible (dim Sq s << dim S). EPR is also promising because it produces localized signals rather than broad signals. 

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