Energy values methyl radicals

Formerly, we used for < the value of 11.22 eV, which is commonly employed in closed-shell calculations, but a correct interpretation of ionization potentials requires (34) that Ic be equated to the ionization potential of methyl radical, 9.84 eV. This change, however, does not affect the values of transition energies. 

If the energy of the ionizing electrons is 13 eV (to minimize the secondary fragmentation processes), the intensities of the primary ions peaks with rn/z values 129, 115, and 101 will be 78%, 85%, and 100%, correspondingly. Theoretically calculated intensities should be (101/129) = 0.78 (78%) and (101/115) = 0.88 (88%). As one can see, the resulting values are fairly close to the theoretical ones. The estimation of the intensity of an ion peak resulting due to the loss of methyl radical has a value two to three times higher than the experimental one. 

The reaction enthalpy and thus the RSE will be negative for all radicals, which are more stable than the methyl radical. Equation 1 describes nothing else but the difference in the bond dissociation energies (BDE) of CH3 - H and R - H, but avoids most of the technical complications involved in the determination of absolute BDEs. It can thus be expected that even moderately accurate theoretical methods give reasonable RSE values, while this is not so for the prediction of absolute BDEs. In principle, the isodesmic reaction described in Eq. 1 lends itself to all types of carbon-centered radicals. However, the error compensation responsible for the success of isodesmic equations becomes less effective with increasingly different electronic characteristics of the C - H bond in methane and the R - H bond. As a consequence the stability of a-radicals located at sp2 hybridized carbon atoms may best be described relative to the vinyl radical 3 and ethylene 4 ... 

When considering the stability of spin-delocalized radicals the use of isodesmic reaction Eq. 1 presents one further problem, which can be illustrated using the 1-methyl allyl radical 24. The description of this radical through resonance structures 24a and 24b indicates that 24 may formally be considered to either be a methyl-substituted allyl radical or a methylvinyl-substituted methyl radical. While this discussion is rather pointless for a delocalized, resonance-stabilized radical such as 24, there are indeed two options for the localized closed shell reference compound. When selecting 1-butene (25) as the closed shell parent, C - H abstraction at the C3 position leads to 24 with a radical stabilization energy of - 91.3 kj/mol, while C - H abstraction from the Cl position of trans-2-butene (26) generates the same radical with a RSE value of - 79.5 kj/mol (Scheme 6). The difference between these two values (12 kj/mol) reflects nothing else but the stability difference of the two parents 25 and 26. 

Some stabilization energies are collected in Table 3. The presentation provides an easy overview of the stabilization in singly and doubly substituted methyl radicals. Values in parentheses for doubly substituted radicals represent the sum of the stabilization energies derived from mono-substituted radicals. By comparing the values calculated directly for the doubly substituted radical, information is obtained on antagonistic, additive or synergetic substituent effects. Apart from methyl and ethyl radicals, all other radicals are stabilized. Some points merit comment. 

The temperature dependence of the rate constants of radical addition (k ) is described by the Arrhenius equation (Section 10.2). At a given temperature, rate variations due to the effects of radical and substrate substituents are due to differences in the Arrhenius parameters, the frequency factor, A , and activation energy for addition, . For polyatomic radicals, A values span a narrow range of one to two orders of magnitude [6.5 < log (A /dm3 mol-1 s-1) < 8.5] [2], which implies that large variations in fcj are mainly due to variations in the activation energies, E. This is illustrated by the rate constants and Arrhenius parameters for the addition to ethene of methyl and halogen-substituted methyl radicals shown in Table 10.1. 

The simple model predicts that the highly electronegative CF3- radical should add preferentially to CH2 in vinyl fluoride and CHF in trifluoroethylene. For the weakly electropositive methyl radical, either weak thermodynamic control or even contrathermodynamic control is predicted. The ab initio minimal basis set calculations fall in line with these qualitative predictions, except that for the attack of CH3 on trifluoroethylene no preferential site of attack is found. The activation energy differences calculated by Salem are compared with the experimental results in Table 21. Although the absolute values of the activation energies are too large, the differences reproduce qualitatively the experimental trend. 

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