After the hydride transfer, the system relaxes to 3 IM4 which is slightly higher in free energy than 3 IM3' by DG = 5.8 kcal mol -1 . The subsequent barrier for B-N bond formation has a barrier of DG ‡ = 12.3 kcal mol -1 for 3 TS5 with respect to 3 IM3'. The transition state has a small imaginary frequency of i300 cm -1 and has similar features as 3 TS2. The final stage leads to proton transfer from the borane group to the carbon atom of the substrate to give the final product 3 Pr. This barrier is DG = 25.2 kcal mol -1 in free energy with respect to 3 IM5. Consequently, the proton transfer and product release are calculated to be rate-determining.