Subject unpaired spins

With the radical 29, even though loss of an equatorial hydrogen should be sterically less hindered and is favored thermodynamically (by relief of 1,3 interactions of the axial methyl), there is an 8-fold preference for loss of the axial hydrogen (at 100 ( i. The selectivity observed in the disproportionation of this and other substituted cyclohexyl radicals led Beckwith18 to propose that disproportionation is subject to stereoelectronic control which results in preferential breaking of the C-H bond which has best overlap with the orbital bearing the unpaired spin. 

Systems with more than one unpaired electron are not only subject to the electronic Zeeman interaction but also to the magnetic-field independent interelectronic zero-field interaction, and the spin Hamiltonian then becomes... 

Radical ions - charged species with unpaired electrons - are easily generated by a number of methods that are discussed in more detail below. Their properties have been characterized by several spectroscopic techniques, and their structures and spin density contributions have been the subject of molecular orbital calculations at different levels of sophistication. The behaviour of radical ions in rearrangement and isomerization reactions as well as in bond-cleavage reactions has been extensively studied [for recent reviews see Refs. 11-13 and references cited therein]. Useful synthetic applications, such as the radical-cation-catalyzed cycloaddition [14-20] or the anfi-Markovnikov addition of nucleophiles to alkenyl radical cations [21-25], have been well documented. In... 

Observation of ESR from a particular sample is contingent upon the presence of a macroscopic spin magnetic moment jZ i.e., die sample under investigation must contain some minimum number of unpaired electron spins. Upon insertion into the cavity, the sample is subjected to the dc magnetic field H, and the unpaired electrons align themselves both parallel... 

Paramagnetic compounds can also be difficult NMR subjects because the unpaired electron(s) couple(s) with the nuclear spin to provide rapid relaxation. Relaxation times can, however, yield useful information about a system certain parameters which contribute to the magnitude of 7) are directly related to the oxidation number and spin state of the metal ion, the nature of the metal ion and its coordination geometry. For example, high-spin Com in octahedral complexes has a relaxation time more than an order of magnitude faster than the same ion in a tetrahedral arrangement of ligands (Banci et at., 1992). 

Ruthenium(III), d5 Ru111 is often associated with classical-type ligands, e.g. ammine, water, halides. They are octahedral low spin t2/ species with one unpaired electron and generally sub-stitutionally inert. The electronic structure of polynuclear carboxylates and mixed valence Ru" 1" complexes of the type [Ru(NH3)5]2L5+ has been the subject of much interest particularly with respect to the degree of unpaired electron delocalization within these molecules. 

Consequently, although the conclusion that the spin density is located essentially in the 7r-framework of the ring is in accordance with the established experimental data, it is necessary to reserve final judgement upon the subject until values of the anisotropic susceptibilities are known experimentally. Similarly, the conclusion that the extent of covalent interaction in U(Me4Cot)2 is somewhat greater than in Np(Me4Cot)2 is not unequivocably established since it depends upon the approximate identity of the hyperfine coupling constants in the two species (as deduced from the calculated Fermi contact terms), which contain respectively two and three unpaired electrons. 

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