The mechanism of the McLafferty rearrangement of the butanal radical cation to ethylene and vinyl alcohol cation is found, by ab initio calculations, to be stepwise. The results of a previous ab initio study are inconclusive because of symmetry restriction in their geometry optimization.
Although there has been accumulating experimental evidence supporting the stepwise character of the McLafferty there is still some controversy"-'2 whether it goes through a stepwise pathway or through a concerted transition state.
Recently, Ha and co-workers" made the fist detailed ab initio calculations on the rearrangement of the butanal radical cation to the vinyl alcohol radical cation and ethylene. They optimized all the geometrical parameters for the structures (A) to (G) in Figure 1 by the Unrestricted Hartree-Fock (UHF) method using the 3 2 1 G basis set.I3 At the critical points, they carried out Unrestricted Moller-Plesset (UMPZ) calculations with the 6-31G * basis.13 From this they concluded that the activation energy of the stepwise pathway is 5.1 kcal/mol higher than that of the concerted one. As pointed out by Bowie,I4 this disagrees with a significant body of experimental evidence. A recent theoretical study on the transition structures for hydrogen atom transfer to oxygen concluded that Ha's transition structure for hydrogen transfer is actually a saddle point becadse of the C , symmetry restriction in their geometry optimization. The true transition structure does not have symmetry and is 1.6 kcal/mol higher in energy than the starting structure at the projected UMP2/6-31G*// UMP2/6-31G* level of theory.I5 This article,I5 published after the present study was underway, concludes that the mechanism for the McLafferty rearrangement of the butanal radical cation involves initial hydrogen transfer followed by CC bond cleavage in subsequent step, but the transition structure and energetics for the CC cleavage were not reported. Since the reaction rate and mechanism are normally determined by the highest energy barrier, it is important to include all possible transition states to draw a conclusion on the mechanism.