A new method for determining the intrinsic resistance energy of H-atom transfer reaction and structure–activity relationship of H-donating ability

Abstract

The intrinsic resistance energy ΔG^≠_XH/X of H-donor XH in hydrogen atom transfer (HAT) reaction is usually used to evaluate the kinetic H-donating ability. In this article, a new method for determining ΔG^≠_XH/X of H-donor in HAT reaction was proposed by the definition of thermo-kinetic parameter ΔG^≠o (XH) = 1/2[ΔG^≠_XH/X + ΔG^o (XH)]. ΔG^≠o (XH) is the characteristic physical parameter of XH to describe the H-donating ability in a chemical reaction during a certain reaction time, ΔG^o (XH) is the bond dissociation free energy. The kinetic studies of HATs between 20 alcohols, ethers, alkanes and amines (XH) with cumyloxyl radical [PhC (CH₃)₂O^•, CumO^•] were researched, and ΔG^o (XH), ΔG^≠_XH/X, and ΔG^≠o (XH) were used to investigate the H-donating abilities of the studied substrates in thermodynamics, kinetics, and HAT reaction. The effect of structures of XH on these three parameters, the structure–activity relationship, such as the influence of electronic, steric, polar and stereoelectronic effects of substituents, heteroatoms insertion in cycloalkanes, and cis–trans configurational isomerism on the H-donating abilities were discussed in detail. This study not only provides a new method for determining ΔG^≠_XH/X but also systematically studies the factors affecting the H-donating ability, laying a foundation for the selection, design, and synthesis of more efficient antioxidants.