Solvation Dynamics and the Nature of Reaction Barriers and Ion-Pair Intermediates in Carbocation Reactions
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Additions of acids to 1,3-dienes are conventionally understood as involving discrete intermediates that undergo an ordinary competition between subsequent pathways to form the observed products. The combined experimental, computational, and dynamic trajectory study here suggests that this view is incorrect, and that solvation dynamics plays a critical role in the mechanism. While implicit solvent models were inadequate, QM/QM' trajectories in explicit solvent provide an accurate prediction of the experimental selectivity in the addition of HCl to 1,3-pentadiene. Trajectories initiated from a protonation saddle point on the potential of mean force surface are predominantly unproductive due to a gating effect of solvation that allows diene protonation only when the incipient ion pair is neither too solvent-stabilized nor too little. Protonation then leads to relatively unsolvated ion pairs, and a majority of these collapse rapidly to the 1,2-product, without barrier and without achieving equilibrium solvation as intermediates. The remainder decay slowly, at a rate consistent with equilibrium solvation as true intermediates, affording a mixture of addition products. Overall, an accurate description of the nature and pathway selectivity of the ion pair intermediates in carbocation reactions must allow for species lacking equilibrium solvation. Potential reinterpretations of a series of historically notable observations in carbocation reactions are discussed.