The free energy profile of Mol1 (Table ) is shown in Figure together with the PBE optimized 3D structures of the NH 3 and N 2 intermediates. The energy of the former is indicated by the lower green bar in the column marked NH 3 NH 3 N 2 that is connected to the preceding green bar by a red line (indicating reduction). The energy of this structure is 11.44 kcal/mol higher than the N 2 structure, whereas the corresponding structure for HIPT substituent is 7.86 kcal/mol lower. The likely reason is that the region around the NH 3 is sterically crowded compared to the N 2 complex. For example, H atoms on the NH 3 are as close as 1.95 Å to the H atoms on the nearby methyl groups, whereas the corresponding H-N distance for the N 2 complex is 2.45 Å. For comparison the closest H-H distance between NH 3 and the HIPT substituent is 2.32 Å, which is also consistent with a less sterically crowded environment around the NH 3 and a comparatively lower energy for this intermediate.