Radicals geometry

An informative IR spectrum of the t-butyl radical, containing 18 bands, has been recorded after freezing of the products of vacuum pyrolysis of azoisobutane [110] and 2-nitrosoisobutane [111] in an argon matrix at 10 K (Pacansky and Chang, 1981). This spectrum is in agreement with a pyramidal structure of the radical (CH3)3C (symmetry C3v) which has elongated CH bonds in positions trans to the radical centre. On the basis of experimental vibrational frequencies and ab initio calculations of the radical geometry the enthalpy value [// (300)] of its formation has been calculated as 44 kJ moP. ... 

In summary, we must conclude that the inferences about radical geometry and stereochemistry that may be drawn from the available experimental information, both spectroscopic and chemical, are less firm than one might hope. It is reasonably clear that CH3 is planar or close to it and that CF3 (and probably also CHF2) is nonplanar for other radicals, particularly tertiary ones, the data are not entirely conclusive, but at least demonstrate that if the radicals are nonplanar they invert rapidly, probably faster than 10 sec-1. 

Reaction steps Initiation, propogation and termination. Radical geometries tend to be planar (sp2 hybrid). Example halogenation of alkane or alkene... 

Changes of the radical geometry affect their chemical properties. Thus, the strength of the Si-H bond (enthalpy (A //0(Si-I I)) of process (HO)3Si + II =>(IIO)3Si-II) formed upon addition of an H atom to the radical, as well as the frequency (v(Si-H)) of Si-H valence vibration for (HO)3Si-H molecule depend on the spatial structure of the radical and the product (see Table 7.5). The larger is the a so(29Sia) value in the radical, the stronger this bond becomes (the stretching vibration frequency also increases synchronously). 

Lott WB, Chagovetz AM, Grissom CB. Alkyl radical geometry controls geminate cage recombination in alkylcobalamins../ Am Chem Soc 1995 117 12194-201. 

Here it is interesting to continue with the discussion of using the ENDOR data to discern radical geometry. Results for the cytosine reduction product observed in irradiated single crystals of l-MethylCytosine 5-FluoroUracil are shown in Table 18-2 [34]. First one notes the three principal values of the hyperfine coupling tensor. For an ordinary rr-electron radical with unit spin density on the central... 

The dominant role of non-Lewis delocalizations on vinoxy geometry and vibrational properties coidd also be assessed by deleting these interactions (using the DEL keylist, a standard NBO program option ) and recalculating the optimized radical geometry in the absence of these interactions. An example of such deletions will be presented in Section 7.4.4 for the vinoxy-water complex. 

Radical geometry is apparently subject to strong substituent effects. It is believed that these effects are a result of interactions between occupied orbitals of the substituent groups. When two substituents are present, an antibonding interaction between electrons in the substituent groups is minimized by a nonplanar geometry. Thus, the tendency toward nonplanarity increases with the number and TT-donor character of the substituents. For some common radical substituents, this order is CF3[Pg.513]

Rearrangements.—Cyclepropyl-Allyl Rearrangement. Further kinetic studies of this isomerization in the gas phase have been reported, and analytical least-motion forms have been applied to the reaction. An orbital description of a concerted pathway for the conversion of cyclopropane into propene has been supplied by Baldwin and Grayston, who have noted that in dehydroadamantanes the reaction proceeds smoothly even though orthogonal trimethylene radical geometry cannot be achieved. 

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