Methyl radical geometry

First examine the geometry of methyl radical. Is it planar or puckered Examine the geometries of 2-methy 1-2-propyl radical, trifluoromethyl radical, trichloromethyl radical and tricyanomethyl radical. Classify each of the substituents (methyl, fluoro, chloro and cyano) as a n-electron donor or as a Tt-electron acceptor (relative to hydrogen). Does replacement of the hydrogens by 7t-donor groups make the radical center more or less puckered Does replacement by Jt-acceptor groups make the radical center more or less puckered Justify your observations. 

Hybrids of the type sp3 are unjustified for disilane. An important conclusion from the above hybridization statement No. 4 is concerned with the contrasting structures of the radicals SiH3 and CH3. The planar geometry of the methyl radical can readily be explained by the (bond-strengthening) sp2-hy-bridization, while the pyramidal silyl radical is thought to be stabilized (with respect to the planar arrangement) through the s-admixture to the lone electron orbital. 

Table 13 B3LYP/6-31G energies of optimized geometries for reaction of methyl radical with methoxyamidyl S0 102e...

We can also determine the relative stability of cis- and trans-l, 3-butadiene by evaluating the stabilization resulting from the union of allyl and methyl radical fragments in a cis and trans geometry. The dissection of 1,3-butadiene is shown below. It is predicted that the trans isomer will be more stable. 

Based on data from competition experiments, trapping of vinyl radicals occurs via a cr-type intermediate, which is lower in energy than the alternative jt-radical structure [55, 56], Stabilization of cr-radicals via hyperconjugation is small, which causes vinyl radicals to be more reactive than e.g. the methyl radical. /Z-Isomerization of a strained cr-vinyl radical proceeds with a rate constant k 3 x 108-1010 s-1 to provide the thermodynamically most favorable geometry [56],... 

Table 9.4 compares to experiment the isotropic h.f.s. values computed for and H in the methyl radical at the UMP2/6-311G(d,p) level both (i) at the UHF/6-31G(d) equilibrium geometry and (ii) as the expectation value over the umbrella mode vibrational wave function computed at this level. Also included are data for the monofluoromethyl radical CHiF, which is even more affected by vibrational averaging because it has a very shallow doublewell potential along the umbrella mode (i.e., the equilibrium structme is pyramidal, but the barrier to inversion is less than 1 kcal mol ), so that its vibrational wave function has large amplitude around a planar structure with smaller h.f.s. than for the equilibrium structure. 

As expected, fluorine substitution has some consequences on structure and stability of the radicals, which are different from the hydrocarbon counterparts. a-F radicals prefer the pyramidal structure because of minimizing 1 repulsion. The trifluoromethyl radical F3C is essentially tetrahedral and has a significant barrier to inversion of about 25 kcal mol - .39 In contrast, the methyl radical H3C itself is planar. Fluorine /J to the radical site is of minor structural consequence. Thus, the pcrfluoro-/er/-butyl radical exhibits a more planar geometry. 

Examine the structures of the two transition states (chlorine atom+methane and chlorine+methyl radical). For each, characterize the transition state as early (close to the geometry of the reactants) or as late (close to the geometry of the products) In light of the thermodynamics of the individual steps, are your results anticipated by the Hammond Postulate Explain. 

ESR spectroscopy is perhaps the best method for the unequivocal detection and observation of free radicals, and ESR 13C hyperfine splitting (hfs) constants are considered to be a very useful indicator of a radical s geometry because non-planarity introduces s character into the orbital that contains the unpaired electron. The methyl radical s 13Ca value of 38 G is consistent with a planar structure. Fluoromethyl radicals exhibit increased 13Ca values, as shown in Table 1, thus indicating increasing non-planarity, with trifluoromethyl radical s value of 272 G lying close to that expected for its sp3 hybridization [4]. 

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...

G ) and the relatively large positive value of (95.9 G for methyl radical Qq = 38 G ) confirm the pyramidal geometry at C and provide a rough estimate of the inversion barrier . 

Methane BDE, 76, 113 geometry of, 32 orbital energies, 26 point group of, 6 reaction with methyl radical, 149 total energy, 29... 

Figure 2. MP2/DZP calculated tramition stale geometries for the delivery of hydrogen atom from trimethylstannane to methyl radical. (UHF/DZP date in parentheses) [AM 1 data in square brackets]...

With this information in hand, it seemed reasonable to attempt to use force field methods to model the transition states of more complex, chiral systems. To that end, transition state.s for the delivery of hydrogen atom from stannanes 69 71 derived from cholic acid to the 2.2,.3-trimethy 1-3-pentyl radical 72 (which was chosen as the prototypical prochiral alkyl radical) were modeled in a similar manner to that published for intramolecular free-radical addition reactions (Beckwith-Schicsscr model) and that for intramolecular homolytic substitution at selenium [32]. The array of reacting centers in each transition state 73 75 was fixed at the geometry of the transition state determined by ah initio (MP2/DZP) molecular orbital calculations for the attack of methyl radical at trimethyltin hydride (viz. rsn-n = 1 Si A rc-H = i -69 A 6 sn-H-C = 180°) [33]. The remainder of each structure 73-75 was optimized using molecular mechanics (MM2) in the usual way. In all, three transition state conformations were considered for each mode of attack (re or ) in structures 73-75 (Scheme 14). In general, the force field method described overestimates experimentally determined enantioseleclivities (Scheme 15), and the development of a flexible model is now being considered [33]. 

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