Orbital interactions hydrogen atom abstractions

Spiro[2.5]octadienyl radical (146), the radical analog of the phenonium ion (105), is a very short-lived species but was detected by its visible absorption and fluorescence on generation by radical abstraction from 147 prior to facile ring-opening to 148 (equation 29). There appeared to be a modest (two-fold) acceleration compared to 1,3- or 1,4-cyclohexadiene in the rate of abstraction from 147, and this was attributed to stabilization by a favorable interaction between cyclopropyl and the adjacent semioccupied orbital in 146. Kinetic acceleration was also proposed to occur in hydrogen atom abstraction from spiro[2.n]alkanes (149). ... 

Figure 4.17. a) Natural orbital correlation diagram for hydrogen atom abstraction by a carbonyl compound and b) orbital correlations after introducing interactions between orbitals of equal symmetry. 

Figure 4.18. State correlation diagram for hydrogen atom abstraction by a cai nyl compound derived from the natural orbital correlations. The configuration correlations are shown by dotted lines, taking into account the interaction yields the correlations shown by solid and broken lines for singlet and triplet states, respectively.

AEba = —45 kJ mol 1 for the HO + SiH4 reaction and AEba = —43 kJ mol-1 in the reaction of hydrogen atom with water. The repulsion of the electron orbitals of the atoms forming the reaction center AER plays an important role in all the radical abstraction reactions. In the interaction of radicals with molecules the contribution of this repulsion ranges from 25 to 46 kJ mol-1. In reactions of molecules with hydrogen atoms the contribution is naturally smaller, varying from 8 to 16kJ mol-1. 

Abstract—The spectroscopic phenomena of strong hydrogen bonds (band shifts, band broadening, intensity increase) are attributed to the interaction of the n electrons with the hydrogen atom, using the latter s distorted p-orbit. The analogy of the spectra phenomena in the hydrogen-chelated and metal-chelated coinpounds is stressed. 

Except for extreme cases like this, radical reactions are generally not subject to strong polar effects. From the picture of C—H bonding in Chapter 1, we can deduce that the SOMO of a methyl radical is close to halfway between the local cr and cr orbitals of a C—H bond, so that the interactions should be more or less equally the SOMO with the HOMO and with the LUMO. In agreement, methyl radicals abstracting hydrogen atoms are found to be only marginally electrophilic. 

When a radical attacks a C—H or C-halogen bond, the interactions are with a and a orbitals. The latter orbitals are usually high in energy, and we can expect that the major interaction is therefore with the HOMO (Fig. 5-4a), namely the a orbital. Radicals abstracting hydrogen atoms are generally regarded as electrophilic. Reactions of various radicals with p-substituted toluenes have been studied and Hammett plots made (Table 5-1). The p-values are small,... 

Although the regioselectivity is usually high in all these reactions, the relative rates reveal that orbital interactions are important, in addition to the thermodynamic factors favouring the formation of the more stable radical. Thus, a plot of the Hammett p-values for addition to substituted styrenes (Fig. 7.5)987,988 is similar to that for the abstraction of hydrogen atoms in Fig. 7.4. 

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