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.