Figure 5. Overlapping lowest unoccupied molecular orbital of1a’ and highest occupied molecular orbital of 2a’ .
The benzene ring of 1a’ rotates, and the C(16)–N(28)–O(29) bond angle decreases gradually from 180.00°. C(6) of 1a’ and N(28) of 2a’ are close to each other, which facilitates their bonding. The reaction proceeds through transition state TS2 at which the C(16)–N(28) distance is 2.050 Å and the C(16)–N(28)–O(29) bond angle is 147.96°. Finally, the C(16)–N(28) single bond forms with a bond length of 1.432 Å and C(16)–N(28)–O(29) bond angle of 117.76°.
IRC calculation proved that TS2 is a first-order saddle point in the potential energy surface of the reaction. The ΔE afor this step is ΔE a2 = 9.76 kcal·mol-1. For the IRC analysis of this process, different algorithms, including HPC, LQA and GS2, were adopted to obtain more accurate reactant and intermediate structures, and the number and size of steps were calculated. By comparison, it is concluded that when IRC is generated for the same transition state with the same step number and step size, LQA takes the shortest time, GS2 takes the second place, HPC takes the longest time, and the number of steps does not converge easily in the HPC algorithm. However, the use of LQA and GS2 algorithms can avoid such errors, because it does not involve the problem of correction step size. The HPC algorithm has the highest accuracy and yields more accurate reactants and products. The accuracy of GS2 is intermediate, while that of LQA is relatively poor. Figure 6 shows the IRC curve obtained by combining the calcall keyword (stepsize = 5, maxpoint = 200) with the HPC algorithm. With product 3a as the reference, the energy level diagram of elementary reaction 2 was obtained and is shown in Figure 6.