Activation Barriers for Addition of Methyl Radicals to Oxygen-Stabilized Carbocations

Abstract

Activation barriers (DeltaHMe(double dagger)) for adding methyl radicals to ions of the general formula CH3CR=OCH3+ have been measured by looking at the threshold energies for the reverse reaction, dissociative photoionization of ethers of the general formula RC(CH3)2OCH3. Dissociation by loss of a methyl radical has more favorable thermochemistry than loss of R*, yet the onset of R* loss occurs at lower energies than loss of CH3*. In other words, the more endothermic dissociation exhibits a lower appearance energy. Contrathermodynamic ordering of appearance energies is observed for R = Et, nPr, iPr, tBu, and neopentyl. The sum of the appearance energy difference, DeltaAE, and the thermochemical difference (DeltaDeltaH, calculated using G3 theory) gives a lower bound for the barrier for adding methyl radical to CH3CR=OCH3+. More specifically, the difference between that activation barrier and the one for adding R* to (CH3)2C=OCH3+, DeltaHMe(double dagger)-DeltaHR(double dagger), equals DeltaAE + DeltaDeltaH and has values in the range 20-24 kJ mol(-1) for the homologous series investigated. There is no systematic trend with the steric bulk of R, and available evidence suggests that DeltaHR(double dagger) does not have a value >5 kJ mol(-1). The difference in barrier heights, DeltaHMe(double dagger)-DeltaHiPr(double dagger) for CH3* plus iPrC(CH3)=OX+ vs iPr* + (CH3)2C=OX+, has the same value, regardless of whether X = H or CH3. Mixing of higher energy electronic configurations provides a qualitative theoretical explanation for some (but not all) observed trends in barrier heights.