Methyl radicals polarity

A UHF wave function may also be a necessary description when the effects of spin polarization are required. As discussed in Differences Between INDO and UNDO, a Restricted Hartree-Fock description will not properly describe a situation such as the methyl radical. The unpaired electron in this molecule occupies a p-orbital with a node in the plane of the molecule. When an RHF description is used (all the s orbitals have paired electrons), then no spin density exists anywhere in the s system. With a UHF description, however, the spin-up electron in the p-orbital interacts differently with spin-up and spin-down electrons in the s system and the s-orbitals become spatially separate for spin-up and spin-down electrons with resultant spin density in the s system. 

The behavior of methyl and halomethyl radicals in their reactions with the fluoro-olefms (Table 1.2), can thus be rationalized in terms of a more dominant role of polar factors and the nucleophilic or electrophilic character of the radicals involved." Methyl radicals are usually considered to be slightly nucleophilic, trifluoromcthyl and triehloroincthyl radicals arc electrophilic (Tabic 1,4). 

Bernardi, F., Bottom, 1997, Polar Effect in Hydrogen Abstraction Reactions from Halo-Substituted Methanes by Methyl Radical A Comparison Between Hartree-Fock, Perturbation, and Density Functional Theories , J. Phys. Chem., 101, 1912. 

Because the addition steps are generally fast and consequently exothermic chain steps, their transition states should occur early on the reaction coordinate and therefore resemble the starting alkene. This was recently confirmed by ab initio calculations for the attack at ethylene by methyl radicals and fluorene atoms. The relative stability of the adduct radicals therefore should have little influence on reacti-vity 2 ). The analysis of reactivity and regioselectivity for radical addition reactions, however, is even more complex, because polar effects seem to have an important influence. It has been known for some time that electronegative radicals X-prefer to react with ordinary alkenes while nucleophilic alkyl or acyl radicals rather attack electron deficient olefins e.g., cyano or carbonyl substituted olefins The best known example for this behavior is copolymerization This view was supported by different MO-calculation procedures and in particular by the successful FMO-treatment of the regioselectivity and relative reactivity of additions of radicals to a series of alkenes An excellent review of most of the more recent experimental data and their interpretation was published recently by Tedder and... 

The polar effect was at first invoked to explain various directive effects observed in aliphatic systems. Methyl radicals attack propionic acid preferentially at the a-position, ka/kp = 7.8 (per hydrogen), whereas chlorine " prefers to attack at the /3-position, ka/kp = 0.03 (per hydrogen). In an investigation of f-butyl derivatives, a semiquanti-tative relationship was observed between the relative reactivity and the polar effect of the substituents, as evidenced by the pK, of the corresponding acid. In the case of meta- and / ara-substituted toluenes, it has been observed that a very small directive effect exists for some atoms or radicals. When treated by the Hammett relation it is observed that p = —0.1 for H , CeHs , P-CH3C6H4 and CHs . On the contrary, numerous radicals with an appreciable electron affinity show a pronounced polar effect in the reaction with the toluenes. Compilation of Hammett reaction constants and the type of substituent... 

Figure 6.10 In the absence of spin polarization, which corresponds to the ROHF picture, there is zero spin density in the plane containing the atoms of the methyl radical. Accounting for spin polarization, which corresponds to the UHF picture, results in a build-up of negative spin density (represented as a shaded region) in the same plane...

The first quantitative calculation of a high collision efficiency for methyl radical recombination was made by Gorin4 who treated the collision pair as being stabilized by a polarization interaction at relatively large distances. From this point of view the transition state for the reaction corresponds to what might be termed a loose transition state in which there is relatively free libration or rotation of the two methyls relative to each other. 

Two situations are conducive to ipso attack. If polar effects come into play in stabilizing the transition state of the addition of the radical, then frequently ipso attack is encountered. This is clearly brought out in the different behaviour of adamantyl and methyl radicals towards the same substrate. It has been firmly established that while methyl and phenyl radicals are electroneutral, the bridgehead adamantyl radical behaves as a nucleophilic species (80ACR51). If this adamantyl radical is reacted with thiophene substrates made electron deficient by the presence of suitable substituents, then the transition state of the addition step may have the character of a charge-transfer complex the site at which the... 

The rates and orientation of free radical additions to fluoroalkenes depend upon the nature of the attacking radical and the alkene, but polar effects again are important For instance, methyl radical adds 9 5 times faster to tetrafluoroethylene than to ethylene at 164 °C, but the tnfluoromethyl radical adds 10 times taster to ethylene [7551 The more favorable polar transition states combine the nucleophilic radical with the electron deficient olefin 17 and vice versa (18) These polar effects account for the tendency of perfluoroalkenes and alkenes to produce highly regular, alternating copolymers (see Chapter starting on page 1101)... 

Spin Polarization. The methyl radical H3C is a planar species with C3v symmetry. The unpaired electron resides in a carbon 2p orbital and the C-H bonds are sp2 hybrids. The unpaired electron has zero probability in the plane of the hydrogen nuclei, yet the protons display an isotropic splitting of 2.3 mT, so we must rationalize this experimental observation. 

Ionic or polar reactions of alkyl halides rarely are observed in the vapor phase because the energy required to dissociate a carbon-halogen bond heterolyti-cally is almost prohibitively high. For example, while the heat of dissociation of chloromethane to a methyl radical and a chlorine atom is 84 kcal mole-1 (Table 4-6), dissociation to a methyl cation and a chloride ion requires about 227 kcal mole-1 ... 

Polar effects can also be important in atom transfer reactions. 4 In an oft-cited example (Scheme 13), the methyl radical attacks the weaker of the C—H bonds of propionic acid, probably more for reasons of bond strength than polar effects. However, the highly electrophilic chlorine radical attacks the stronger of the C—H bonds to avoid unfavorable polar interactions. As expected, the hydroxy hydrogen remains intact in both reactions. 

Similarly, the p-fragmentation of tertiary alkoxyl radicals [reaction (2)] is a well-known process. Interestingly, this unimolecular decay is speeded up in a polar environment. For example, the decay of the ferf-butoxyl radical into acetone and a methyl radical proceeds in the gas phase at a rate of 103 s 1 (for kinetic details and quantum-mechanical calculations see Fittschen et al. 2000), increases with increasing solvent polarity (Walling and Wagner 1964), and in water it is faster than 106 s 1 (Gilbert et al. 1981 Table 7.2). 

Table 8). The same 2pz function is unoccupied in the calculation of the methyl radical in the presence of a ghost lithium atom using the geometry of CH3Li. This shows that the lithium 2pz orbital acts like a normal valence orbital in the description of the C—Li bond and not, as suggested previously,197 198 as a superposition function. The strong charge donation from Li to C is in line with the difference in electronegativity between these atoms, and with the modern picture of a strongly polar carbon-lithium bond.181-183...

Fig. 17 Total synthesis of methyl curcurbate 76 and methyl epijasmonate 77 via a radical-polar 5-exo cyclization/alkylation sequence...

The only atomic wave-functions that do not have a node at the nucleus are s-functions. The isotropic coupling constant is thus a measure of the s-character of the wave-function of the unpaired electron at the nucleus in question. The coupling constant for an atomic s-electron can be either measured experimentally or calculated from Hartree-Fock atomic wave-functions so that, to a first approximation, the s-electron density may be calculated from the ratio of the experimental and atomic coupling constants. Should the first-order s-character of the wave-function of the unpaired electron be zero, as for example in the planar methyl radical, then a small isotropic coupling usually arises from second-order spin-polarization effects. The ESR spectra of solutions show only isotropic hyperfine coupling. 

Significant changes are also observed for the proton and halogen constants. Thus, the 11 isotropic coupling constant which is negative in the planar methyl radical (spin polarization, Aiso = -23 G), becomes positive when protons are substituted by fluorines133 (Table 15) and a monotonic increase in the 19F isotropic coupling constant is also caused by the... 

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