Following TS1, r C 1 -O LG increases further, leading to an intermediate state, Inter1. This state adopts an envelope (E3)-like ion conformation with r C 1 -O LG of 2.67 Å. TS2 links the (E 3 )-like and half-chair ( 4 H 3 )-like oxocarbenium (Inter2). Inter2 is 6.8 kcal mol -1 lower in energy than the RS and is stabilised by hydrogen bond interactions between the leaving group and nucleophile (Fig. ). Finally, TS3, corresponding to the addition of the nucleophilic group leading to the final PS, presents an energy barrier of 10.7 kcal mol -1 (measured from Inter2, 3.9 kcal mol -1 higher relative to RS) and is characterised by r C 1 -O LG of 3.94 Å and r C 1 -O Nu of 2.62 Å (Fig. ).