Carbene 1 R (Figure , R = CF 3 , CN , H, M e, OM e, P h, tBu) reacts with N 2 to form 1A R which is in fact, a pre-complex formed due to the formation of weak hydrogen bonding between a nitrogen atom of N 2 molecule and the nearest hydrogen atom of the carbene molecule. The hydrogen bonding distances of different pre-complexes are within 2.7 Å to 2.9 Å. This step is endergonic for all carbenes. In the very next step, 1A R converts to 1B R through a transition state T S1 R . 1B R is the activated complex that we are looking for. The energy required for the formation of T S1 R from 1A R is the activation energy for this step. In Figure , it is observed that the lowest activation energy is 22.7 kcal/mol which is for the CN substitution and the highest activation energy is 30.1 kcal/mol which is for the OM e substitution. The change in the activation energy of the reaction follows the trend CN < CF 3 < P h < M e < H < tBu < OM e. However, for all carbenes, the free energy change for the formation of 1B R from the starting materials is positive. The order of stability of 1B R follows the trend CN > CF 3 > P h > M e > tBu > H > OM e that is likely correlated to corresponding activation energies.