The barrier to methane elimination from ionized butan‐2‐one: Theory and experiment

Abstract

AbstractExperimental and theoretical approaches are used to characterize the losses of methyl and methane from ionized butan‐2‐one (a). Formation of a with an energy content near its dissociation threshold by charge exchange demonstrates that even very close to the threshold, methane elimination competes poorly with methyl loss. The fragmentations of a generated by reaction of CH3CH3 with CH2CO+˙ support this conclusion. Ab initio calculations were used to locate a transition state for methane elimination from a though the ion‐neutral complex [CH3CH2CO+ ˙CH3]. These calculations place the top of the barrier to methane elimination 6–7 kJ mol−1 above the threshold for methyl loss and the products of methane elimination about 35 kJ mol−1 below that threshold. This barrier prevents methane elimination from competing strongly with methyl loss. A binding energy of 18 kJ mol−1 was obtained for the complex [CH3CO+ ˙CH2CH3]. The difference between the theoretical energies for the products of methane and methyl loss obtained by including electron correlation are in good agreement with experimental results. However, the calculated difference [ΔHf(CH3CH2CO+) + ΔHf(˙CH3)] minus ΔHf(a) was 23.1 kJ mol−1 smaller than the experimental difference. Although there is clearly a barrier to the H transfer in the elimination of methane from a, whether that transfer takes place by surmounting the barrier or by tunneling through it cannot clearly be determined owing to uncertainties in the barrier height.