McLafferty rearrangement of the radical cations of butanal and 3-fluorobutanal: a theoretical investigation of the concerted and stepwise mechanisms

The stepwise and concerted pathways for the McLafferty rearrangement of the radical cations of butanal (Bu(+)) and 3-fluorobutanal (3F-Bu(+)) are investigated with density functional theory (DFT) and ab initio methods in conjunction with the 6-311+G(d,p) basis set. A concerted transition structure (TS) for Bu(+), (H), is located with a Gibbs barrier height of 37.7 kcal/mol as computed with CCSD(T)//BHandHLYP. Three pathways for the stepwise rearrangement of Bu(+) have been located, which are all found to involve different complexes. The barrier height for the H(gamma) transfer is found to be 2.2 kcal/mol, while the two most favorable TSs for the C(alpha)-C(beta) cleavage are located 8.9 and 9.2 kcal/mol higher. The energies of the 3F-Bu(+) system have been calculated with the promising hybrid meta-GGA MPWKCIS1K functional of DFT. Interestingly, the fluorine substitution yields a barrier height of only 20.5 kcal/mol for the concerted TS, (3F-H). A smaller computed dipole moment, 12.1 D, for (3F-H) compared with 103.2 D for (H) might explain the stabilization of the substituted TS. The H(gamma) transfer, with a barrier height of 4.9 kcal/mol, is found to be rate-determining for the stepwise McLafferty rearrangement of 3F-Bu(+), in contrast to the unsubstituted case. By inspection of the spin and charge distributions of the stationary points, it is noted that the bond cleavages in the concerted rearrangements are mainly of heterolytic nature, while those in the stepwise channels are found to be homolytic.