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.