Hyperconjugation ethyl radical

The difference between the appearance potentials of the ions from methyl and ethyl radicals indicates that hyperconjugation is less effective in stabilizing an unpaired electron than a positive charge. 

Spin density at a nucleus can also result from a hyperconjugative interaction between a singly occupied p orbital and a bond to a jS-substituent. This is represented for the ]8-protons in the ethyl radical (3) by the contribution of the canonical structure (4). The resulting spin density at the proton is positive, in contrast to the negative spin density... 

The increased importance of hyperconjugation in cations affects rotational barriers, albeit not in a straightforward way. Computations at HF levels with symmetry restraints suggested that the barrier for ethyl cation itself is, in fact, quite small. The rotational barrier in the ethyl radical is also very small. Although initially unexpected, this observation can be explained by different symmetry of orbital interactions involved in negative and positive hyperconjugation. With p-orbitals, the notions of syn- and antiperiplanarity disappear (Figure 6.43) and the equivalence of all periplanar conformations results in a 6-fold symmetry of the rotational profile. 

Representation of the hyperconjugation contribution to the ap value of the ethyl radical. 

We next make the leap in Sec. 9 to spectroscopy of jet cooled hydrocarbon radicals, which are now quite stable with respect to dissociation but nevertheless highly reactive due to their unpaired electrons. These species play an enormously important role in terrestrial combustion chemistry in addition, many examples of such hydrocarbon radicals have already been observed in the interstellar medium. We begin with a brief discussion of the simple yet fundamental methyl radical and halogenated methyl radicals (Sec. 10), followed by the more complex dynamics of internal rotation and hyperconjugation effects in ethyl radical (Sec. 11), as well as tunneling dynamics in cyclopropyl radical (Sec. 12) and vinyl radical (Sec. 13). Indeed,... 

In the foregoing it has been assumed that the relatively large positive coupling to /3-protons in 7r-radicals such as ethyl occurs as a result of hyperconjugation. This is not necessarily the case, although arguments in favour of the concept are considerable (Symons, 1962). Inevitably, there are several alternative descriptions suitable as a starting point for theoretical calculations. Thus, molecular-orbital theory has been used by some (Bersohn, 1956 Chesnut, 1958), and a... 

The second main point in the chapter is the stabilization of a radical center by the presence of alkyl groups attached to the radical carbon. So, ZerZ-butyl radical is more stable than isopropyl, which is better than ethyl methyl radical is the least stable. Hyperconjugation is one concept often used to explain this stabilization. Physically, and electrostatically, the radical carbon can be viewed as somewhat electron deficient (7 valence electrons instead of an octet). Hyperconjugation provides a means for bonds in neighboring alkyl groups to lend" a little electron density to the radical center, thereby making it feel a little less electron-poor. In doing so, the... 

Hyperconjugation in the 1,1-dimethylpropyl radical is the same as in 1 -ethyl-1-methylpropyl [(b) above] in yobr picture, an H should replace one of the end CH3 groups. 

Figure 3-3 Hyperconjugation (green dashed lines) resulting from the donation of electrons in filled sp hybrids to the partly filled p orbital in (A) ethyl and (B) 1-methylethyl and 1,1-dimethylethyl radicals. The resulting delocalization of electron density has a net stabilizing effect.

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