Twisted allyl radical

Figure 5 ROHF and UHF MOs for twisted allyl radical. Note that there is much less spin polarization in 7t of twisted allyl than in Ki of planar allyl (see Figure 2). Numbers denote the coefficients of the p AOs on carbon.

The discussion of cis-trans photoisomerization of alkenes, styrene, stilbene, and dienes has served to introduce some important ideas about the interpretation of photochemical reactions. We see that thermal barriers are usually low, so that reactions are very fast. Because excited states are open-shell species, they present new kinds of structures, such as the twisted and pyramidalized CIs that are associated with both isomerization and rearrangement of alkenes. However, we will also see familiar structural units as we continue our discussion of photochemical reactions. Thus the triplet diradical involved in photosensitized isomerization of dienes is not an unanticipated species, given what we have learned about the stabilization of allylic radicals. 

The number of electrons changes stability in a more complex way in three-center systems, i.e. the allyl and related species. In this case, delocalization of charge is much more important than delocalization of spin. For example, rotation around the C-C bond becomes much more difBcult in the allyl cation (-38 kcal/mol) compared to the allyl radical (-13 (calculated), 15.7 (experimental)kcal/mol). Allylic anions have a lower rotation barrier relative to the cation (-23 vs. -38kcal/mol). In the case of anions, additional stabilization to the twisted form (-8-14 kcal/mol) is provided by rehybridization, which partially offsets the lower efficiency of hyperconjugation in the twisted anion than in the twisted cation. The calculated barriers for the allyl system depend strongly on the methods employed, but the trend of cation > anion > radical remains. The same trend is observed for the rotation barriers in the benzyl radical and cation (Figure 3.10). ... 

The rotational barrier in allyl radicals also illustrates the effect of the number of orbitals in the conjugated array on the conjugation. Removing one p-orbital from conjugation via the 90° twist in allyl radical imposes the energy penalty of only 15kcal/mol - considerably less costly than the penalty for the analogous rotation in ethylene (-65 kcal/mol). 

However, even if one is not interested in modeling ESR spectra, preventing electrons of opposite spin from having different spatial distributions imposes a constraint on ROHF wavefunctions that has energetic consequences. For example, if one compares the ROHF energy of the planar allyl radical in C2J, symmetry (cf. below), where spin polarization is quite important, to that of the twisted species, where spin polarization of the electrons in the n bond is almost absent, one obtains a rotational barrier that is far too low (see Table 1), compared to the experimental value of 15 kcal/mol. 

FIGURE 5.1 Electronic structure of the allyl ligand and some features of metal-allyl bonding. Nodes are shown as dotted lines in (a). Electron occupation in the allyl radical is shown in (b). The canting of the allyl is seen in (c), and the twisting of the CH2 groups in (d). 

I McLachlan calculation of spin densities, discussion of twisting at C(2)-C(8) and C(2 )-C(8) bonds radical with all positions of the benzoring deuterated also prepared. " ) INDO calculation of coupling constants for optimized structure. - Mixtures of exo- and enatom abstraction from allyl cyanide at various temperatures in CjHCI). - Determination of rotational barrier. ... 

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