Crystal radical molecules

Figure 6.25. Spatial arrangement of reactants for long-range hydrogen transfer in a y-irradiated dimethylglyoxime crystal. Radical pair J transforms into radical pair J as a result of hydrogen transfer from molecule C to radical B. (From Toriyama et al. [1977].)...

The paramagnetic absorption of alkane radical cations is critically dependent on their conformation. In neat n-alkane crystals, alkane molecules are in the extended all-trans conformation (see Fig. 5.1) and FDMR spectroscopy unequivocally shows that alkane radical cations retain that conformation in such systems. The extended all-trans conformation is also the preferred conformation of many n-alkane radical cations in chlorofluorocarbon and perfluorocarbon matrices. In this conformation, the unpaired electron occupies the planar cr molecular orbital and delocalizes over the entire extended chain. Only two C-H bonds (both chain-end, one on each side) are in the planar ct molecular frame in the extended structure and high unpaired-electron and positive-hole density appears only on these in-plane protons. Alkane radical cations in the extended conformation (as well as in other conformations) are thus fj-delocalized paramagnetic species. The associated hyperfme interaction with the two (equivalent) in-plane chain-end protons results in a 1 2 1 three-line (triplet) EPR spectrum. The fact that the hyperfme in-... 

Crystals of Molecules with Charge Transfer, Radical-ion Salts... 

Figure 12.25 Schematic drawing of the crystal structure (a Y c < ft) of the p-phase of p-NPNN. Each ellipsoid represents the radical molecule. Note that the structure is similar to that of diamond. The expected dominant exchange paths are given by the solid and dotted lines.

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Fig. 63. Molecular arrangement in (a, c) plane of a mixed ethylene-chlorine binary crystal illustrating (a) radical pair formation, (b) single chain growth and (c) chain growth in the vicinity of product line. Molecules labelled 1-4 are ethylene (C2H4), chlorine, chloroethyl radical (C2H4CI) and anti 1,2-dichloroethane (C2H4CI2), respectively.

A free radical (often simply called a radical) may be defined as a species that contains one or more unpaired electrons. Note that this definition includes certain stable inorganic molecules such as NO and NO2, as well as many individual atoms, such as Na and Cl. As with carbocations and carbanions, simple alkyl radicals are very reactive. Their lifetimes are extremely short in solution, but they can be kept for relatively long periods frozen within the crystal lattices of other molecules. Many spectral measurements have been made on radicals trapped in this manner. Even under these conditions, the methyl radical decomposes with a half-life of 10-15 min in a methanol lattice at 77 K. Since the lifetime of a radical depends not only on its inherent stabihty, but also on the conditions under which it is generated, the terms persistent and stable are usually used for the different senses. A stable radical is inherently stable a persistent radical has a relatively long lifetime under the conditions at which it is generated, though it may not be very stable. 

Concerning molecule-based magnets, the first spin-ladder was synthesized (/ -EPYNN)[Ni (dmit)2] (/ -EPYNN = / -7V-ethylpyridinium o-nitronyl nitroxide). Within the crystal lattice, the radical cation /7-EPYNN units are arranged in ID chains with ferromagnetic interactions. The chains of [Ni(dmit)2] moieties in the ladder formation exhibit coexistent antiferromagnetic interactions.1031,1032... 

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