Understanding alkyl substituent effects in R–O bond dissociation reactions in open- and closed-shell systems

The effects of alkyl substituents (R = Me, Et, i-Pr and t-Bu) on the barriers and enthalpies for R–O bond dissociation reactions are examined via high-level ab initio molecular orbital calculations for the open-shell systems, R–OCH2• and R–OC(SCH3)2•. To assist in the interpretation of the results, R–X bond dissociation energies (BDEs) are also examined in the closed-shell systems (X = H, CH3, OCH3, OH and F). The effects of increasing alkylation on the R–O bond dissociation reactions show an unusual and somewhat counterintuitive trend. Specifically, the BDEs broadly increase and the barriers decrease in going from R = Me to R = t-Bu. However, on closer examination, these effects are readily understood in terms of the competing effects of hyperconjugative stabilization of the leaving R• radical and charge-transfer stabilization of the R–O bond. As part of this work, we also show that B3-LYP yields the wrong qualitative trends for the effect of alkyl substituents on the R–O BDEs.