Theoretical analysis of the energy barriers for the rotational isomerization of the allyl and the 1-cyano-, 1-hydroxy- and 1-cyano-1-hydroxyallyl radicals

Abstract

An ab initio computational study was carried out on the ground-and transition-state structures for the rotational isomerization of the allyl and the 1-cyano, 1-hydroxy-and 1-cyano-1-hydroxyallyl radical systems in an attempt to gain an understanding of the factors affecting the relative energy barriers for rotational isomerization. The results of ESR rate measurements have indicated that there exists the presence of an 'extra' lowering of the energy barrier for the rotational isomerization of the 1-cyano-1-hydroxylallyl radical relative to the sum of the lowerings of the energy barriers for the rotational isomerization of the 1-cyano-and 1-methoxylallyl radical compared with the energy barrier for the rotational isomerization of the allyl radical. The 'extra' lowering of the energy barrier for the rotational isomerization of the 1-cyano-1-hydroxyallyl radical has been attributed to captodative stabilization of the transition structure for rotational isomerization. Prior calculational studies in the author's and other laboratories have resulted in the suggestion that captodative substitution does not always lead to 'extra' stabilization to a radical center. Recent calculations in the author's laboratories and the results reported in the present paper strongly suggest that ground-state effects can be dominant in determining the rate of a radical-forming reaction. The present paper describes the results of theoretical calculations on the allyl and substituted allyl radical systems that indicate that ground-state effects appear to dominate the relative rates of the rotational isomerization of the substituted allyl radicals.

Impact of Conjugation and Hyperconjugation on the Radical Stability of Allylic and Benzylic Systems: A Theoretical Study

“…The HMO predicts Hűckel energy differences of 0.83β and 0.72β for allyl and benzyl radical, indicating the allyl radical is slightly more stable than the benzyl radical. Delocalization or resonance energies have been used to evaluate the stability of these radicals and are typically quantified through the calculation of CH 2 rotational barriers, − referred to as the resonance stabilization energy (RSE), or through the utilization of isodesmic reactions and calculation of relative bond dissociation energies (BDE). Most computational methods used to investigate these radicals involve the linear combination of atomic orbitals molecular orbital theory (LCAO-MO), however the concepts resonance and “electron pushing” are typically conceptualized through valence bond (VB) theory .…”

Ground- and Triplet Excited-State Properties Correlation: A Computational CASSCF/CASPT2 Approach Based on the Photodissociation of Allylsilanes

“…Return to planarity takes place later during the formation of radicals all the way down to the dissociation limit. The resonance stabilization which for the allyl radical is about 15 kcal·mol –1 , constitutes a large part of the energy released from the transition state down to the separated radicals (Figures and ). We get the impression that the high energy barriers and particularly the high intrinsic barrier ( E a i ≈ 25 kcal·mol –1 ) demonstrate the operation of the PNS that states that “high intrinsic barriers are typically associated with a lack of synchronization between concurrent reaction events such as bond formation/cleavage, solvation/desolvation, development (loss) of resonance, etc.” .…”