TLDR:
What: Potential energy surface associated with rotation around CC bond in the ring-opened version of 1,2-dioxetane.
How: State average(4)-CASSCF(12,10)/VTZP constrained geometry optimizations keeping OCC'O' and one HCC'O' dihedral angle fixed. Active space is bonding and antibonding CC', OO', CO, and C'O' orbitals in the closed ring structure, plus the two nonbonding orbitals on the oxygens perpendicular to the plane of the ring.
Question: How explain the profiles in Figure 2.
I want to look at the transition states associated with the OCC'O' dihedral torsion in the biradical structure of 1,2-dioxetane (after ring opening). This is similar to the exercises in basic organic chemistry courses, where we look at the anti and gauche conformations of isomers. In Figure 1 you can see one biradical structure with an OCC'O' dihedral of about 70 degrees.
Figure 1. 70 degree dihedral OCC'O' angle minimum structure.
I decided to "map" the PES associated with CC rotation, by doing constrained geometry optimisations. I kept the OCC'O' and one HCC'O' dihedral simultaneously fixed at 10 degree intervals from 0 to 180 degrees, and optimizing the other internal coordinates. I did this with the state-average (4 lowest singlets) complete active space self-consistent field (CASSCF) method, using the atomic natural orbital type basis sets with polarisation at the triple zeta level (ANO-RCC-VTZP), using the Molcas suite. The energies of the singlet states can bee seen in Figure 2.
Figure 2. Mapping the $S_0$, $S_1$, $_2$, and $S_3$ potential energy surfaces, obtained as described above.
Qualitatively, the PES's mainly match what I expected, but with one major change: instead of a transition state close to 120 degrees, I see a minimum.
I am trying to understand why this is what I obtained, but I cannot really explain it. The change is so major, that any errors with my constrained optimization is likely not the cause, I reckon.
Figure 3 shows the C-C bond distance at the various optimized geometries. Here, the main features are present: minimum at 60 degrees and maximum state at 120. This makes sense as in a minimum the structure is more stable, which means shorter bond lengths. However, there is some weird behavior at the 90 degree data point, which probably is due to the same effect as the energy minima around 100 degrees.
Figure 3. C-C bond lengths in Å at the various optimized geometries.
So, my question, can anyone explain the anomalies presented in Figures 2 and 3?
Updates and answers to questions:
Yes, I changed the OCC'O' and noe of the HCCH dihedral angle simultaneously. I only chose one HCCH dihedral as I thought that was enough to keep the structure rigid during the geometry optimization.
The active space is the bonding and antibonding $\sigma$ orbitals of the closed ring structure (CC', OO', CO, and C'O'), plus the two nonbonding p orbitals on the oxygens (perpendicular to the plane of the ring).
I did not use symmetry to speed up the calculations.
For each dihedral angle used, I also measured the CC bond distance. I plotted this as an overlay on the energy profile, to see how the CC bond changed relative to the energy change.
As requested by Brian, the energies of all calculated states are included in the question.
