Radical Stability—Thermochemical Aspects

Abstract

Reaction enthalpies for hydrogen transfer reactions provide a quantitative basis for the definition of the stability of C‐, S‐, O‐, N‐, and B‐centered radicals. The radical stabilization energies (RSEs) obtained in this way reflect the influence of the substitution pattern on the unpaired spin as well as ground state effects in the respective closed shell parent systems. The combination of theoretically calculated RSEs with experimentally measured bond dissociation energy (BDE) data allows for the construction of global stability scales for radicals of different type and thus the opportunity to predict reaction energies for a wide range of hydrogen transfer reactions.

Biosynthetic Versatility and Coordinated Action of 5′-Deoxyadenosyl Radicals in Deazaflavin Biosynthesis

“…The reversibility also eliminates the possibility of a protein glycyl radical at the CofH active site because the bond dissociation energy of glycine is 358 kJ mol –1 demonstrating that the glycyl radical is also insufficiently reactive to abstract a hydrogen atom from 5′-deoxyadenosine. 33 …”

Transfer Hydrogenation in Open-Shell Nucleotides — A Theoretical Survey

“… a : Defined as RSE = ∆ H = ∆ f H (CH 4 ) + ∆ f H (R•) − ∆ f H (R-H) − ∆ f H (•CH 3 ); from ref. [8]; b : Using following heats of formation:∆ f H 0 (•CH 3 , 32R ) = +146.7 kJ/mol [11] and ∆ f H 0 (CH 4 , 32 ) = +74.6 kJ/mol [5]. ∆ f H 0 of radicals from ref.…”

“…Given the almost identical hydrogenation energies for these two systems (∆ hyd H ( 40R ) = −26.7 kJ/mol vs. (∆ hyd H ( 11R ) = −27.5 kJ/mol) the influence of the C2' hydroxy substituent present in 11R , but not in 40R , appears to be negligible. The small hydrogenation energies for all push/pull-substituted radicals described above reflect the efficient interaction of the alkoxy/hydroxy-donor and carbonyl-acceptor substituents [8,30]. As shown in Scheme 4 for the example of radical 37aR , these can be rationalized with the admixture of charge-transfer configurations such as 37aR-D and 37aR-E to the canonical Lewis structures 37aR-A and 37aR-B .…”

“…This increase in thermochemical driving force of 29.4 kJ/mol for dihydrogen transfer implies that ethanol-2-yl radical 1R is a significantly better dihydrogen donor than its closed shell parent ethanol. On closer inspection of the reactant and product radicals involved it also becomes evident that the increased driving force is exactly identical to the difference in radical stabilization energies (RSE) of the reactant and product radicals ethanol-2-yl radical 1R and acetaldehyde-2-yl radical 4R [8]. In contrast, installation of a radical center in the alkene reaction partner as in allyl radical 14R leads to a substantial reduction of the driving force for transfer hydrogenation with ethanol ( 1 ) to only −4.4 kJ/mol.…”

Insertion of Degradable Thioester Linkages into Styrene and Methacrylate Polymers

“…In DBT, the ortho-alkoxy substituent was hypothesised to stabilise the intermediate radical through lone pair donation (resonance structure 5b in Scheme 1B). 14 Indeed, preliminary ab initio molecular orbital theory and DFT calculations performed at the G3(MP2)-RAD(+) level estimated that the addition of a methyl radical onto DBT was 6.9 kJ/mol more favorable than the addition of a methyl radical onto DOT, (see Figure S10. Notably, both changes (lowering the energy of the intermediate and increasing the energy of the open-ring radical) disfavor ring-opening.…”