Observation of the slow racemization of isobornyl chloride in a polar solvent in 1923-24 by Meerwein led to the recognition that mechanistic interpretation is the key to understanding chemical reactivity. The hypothesis of ion pairs in which a chloride anion is partnered by a carbocation long ago entered the standard textbooks (see DOI 10.1021/ed800058c and 10.1021/jo100920e for background reading). But the intimate secrets of such ion-pairs are still perhaps not fully recognised. Here, to tease some of them them out, I use the NCI method, which has been the subject of several recent posts.

NCI analysis of the iod-pair transition state for methyl migration in isobornyl chloride. Click for 3D.
The NCI plot below shows the competing 1,6-hydride shift in isobornyl chloride, again involving an ion-pair transition state.

NCI surfaces for the 1,6 hydride migration transition state in isobornyl chloride. Click for 3D.
Modelling is increasingly focusing on these weaker interactions, that probably mediate much (stereo)selectivity in organic reactions. How long before such approaches themselves enter the text-books?
Tags: chemical reactivity, Interesting chemistry, ion pair, isobornyl, Julia Contreras-Garcia, nnon-covalent-interactions, watoc11