The left panel is corresponding to the reactions starting from CR1 to CR4 while the right panel is corresponding to reactions starting from CR5 to CR8. We first analyze the structural parameter changes of transition states in comparison with the starting structures. As can be seen, both C1­C3/C1-C2 and C2­N4/C3­N4 of CR1 to CR8 are shortened in comparison with their corresponding starting structures. For TS1, TS2, TS5 and TS6, the C1­C3 bonds are shortened from 3.00 Å, 3.71 Å, 4.06 Å and 3.58 Å to 2.10 Å, 2.05 Å, 2.09 Å and 2.06 Å and the C2­N4 bonds are shortened to 3.17 Å, 2.89 Å, 2.99 Å and 2.78 Å respectively. For TS3, TS4, TS7 and TS8, the C1­C2 are shortened to 1.92 Å, 2.09 Å, 1.90 Å and 1.97 Å and the C3­C4 are shortened to 2.68 Å, 2.43 Å, 2.73 Å and 2.60 Å. Although both C1­C3 (C1­C2) and C2­N4 (C3­N4) are shortened, the C­C bonds seem have more significant bond length changes than the C­N bond. In order to further elucidate such seemingly contradictory results, we performed IRC calculations from the transition state structures to both forward and reverse directions.
The calculated IRC path that corresponds to path1 is shown in the left panel of Figure 7 while all other IRC paths that lead to various products are depicted in Figure S1­S7. Starting from the corresponding reactant CR1, the IRC reaction path need to firstly overcome a small barrier of ca. 3.2 kcal/mol to reach the transition state structure TS1 with the IRC coordinate changes from ­30/­20 amu1/2Bohr to 0 amu1/2Bohr in path1. However, the energy path that start from TS1 to M1 reveals some interesting patterns: In the IRC coordinates that change from 0 amu1/2Bohr to 5 amu1/2Bohr, the potential energy of the molecule decreases sharply; In the following 5 amu1/2Bohr to 25 amu1/2Bohr region, the potential energy surface seems to be very flat; In the 25 amu1/2Bohr to 30 amu1/2Bohr, however, the energy of the molecule sharply decreases again and finally forms the intermediates M1. Such potential energy changes resulted in an obvious shoulder in this region.