As an alternative to direct C-H activation, rotation of the bond between the phenyl group and the directing group can take place, via TS2, to present the alternative ortho C-H bond to the iridium catalyst. This transition state was located, via a relaxed potential energy scan, for all substrates except Nphenylpyrazole, for which TS2 could not be located. The energy of TS2 is invariably lower than that for TS1 for each substrate, but ΔΔG varies between 5.1 kcal/mol (oxazoline) and 10.5 kcal/mol (N-methylimidazole). At this stage, an analysis of these energies reveals two trends that might explain the observed experimental behaviour with respect to isotopologue distribution as a function of time; key data are summarised in Table . The energy difference between the complexes with one (VI) and two (V) molecules of coordinated substrate vary considerably, but the two substrates for which the most rapid d2-isotopologue formation is observed, 2-phenylpyridine and 2phenylpyrimidine, show the largest VI vs V energy difference, in favour of the single substrate complex, VI. An associative pathway for substrate exchange in these systems is therefore less feasible here than for other substrates. In addition, and perhaps more importantly, TS2 is considerably lower in energy than intermediate V for these two same substrates, meaning that rotation around the phenyl-directing group bond will take place before associative substrate exchange via V.