As recently explicated by Wang and Borden using NIPE spectroscopy and computations, the potential energy surface of cyclopropyl-1,2,3-trione 1 is remarkably complex.¹ (U)CCSD(T)//aug-cc-pVTZ computations of the D_3h singlet (the ¹A_1’ state shown in Figure 1) is actually a hilltop, possessing two imaginary frequencies. Distorting the structure as indicated by these imaginary frequencies and then optimizing the structure leads directly to dissociation to three CO molecules. Thus, (CO)₃ does not exist as a stable minima on the singlet surface.

The D_3h triplet (the ³E” state shown in Figure 1) is not a critical point on the surface; due to the Jahn-Teller effect is distorts into two different states: the ³B₁ state which is a local energy minimum, and the ³A₂ state which is a transition state between the symmetry-related ³B₁ states.

So, this implies the possibility of a very interesting NIPE experiment. If the radical anion (CO)₃^-.
loses an electron and goes to the singlet surface, it lands at a hilltop(!) and should have a very short lifetime. If it goes to the triplet surface, it lands at either a transition state (³A₂) and again should have a short lifetime, or it can land at the ³B₁ state and perhaps have some lifetime before it dissociates by losing one CO molecule.

1-.
1A1’	3E”
3B1	3A2

Figure 1. (U)CCSD(T)//aug-cc-pVTZ optimized geometries of 1 and its radical anion.

The NIPE spectrum identifies three transitions. By comparing the energies of the electron loss seen in the experiment with the computations, along with calculating the Franck-Condon factors using the computed geometries and vibrational frequencies, the lowest energy transition is to the ¹A_1’ state, and the second transition is part of the vibrational progression also to the ¹A_1’ state. This is the first identification of vibrational frequencies associated with a hilltop structure. The third transition is to the ³A₂ state. No transition to the ³B₁ state is found due to the large geometric difference between the radical anion and the ³B₁ state; the Franck-Condon factors are zero due to no overlap of their wavefunctions.

Once again, the power of the symbiotic relationship between experiment and computation is amply demonstrated in this paper.

References

(1) Chen, B.; Hrovat, D. A.; West, R.; Deng, S. H. M.; Wang, X.-B.; Borden, W. T. "The Negative Ion Photoelectron Spectrum of Cyclopropane-1,2,3-Trione Radical Anion, (CO)₃^•– — A Joint Experimental and Computational Study," J. Am. Chem. Soc. 2014, 136, 12345-12354, DOI: 10.1021/ja505582k.

InChIs

1: InChI=1S/C3O3/c4-1-2(5)3(1)6
InChIKey=RONYDRNIQQTADL-UHFFFAOYSA-N