Twin excitation/transition states

S. Zilberg, Y. Haas, D. Danovich, S. Shaik, Angew. Chem. Int. Ed. Engl. 37, 1394 (1998). The Twin-Excited State as a Probe for the Transition State in Concerted Unimolecular Reactions. The Semibullvalene Rearrangement. 

Twin-States The ground and excited states that arise from avoided crossing in the VBSCD (see below). Usually, the twin-states correspond to a transition state of a thermal reaction and an excited-state intermediate. This excited-state intermediate can be converted to a funnel (a conical intersection) for the products of a photochemical reaction. See Chapter 6. 

An instructive, albeit trivial, example is the hydrogen exchange reaction, Ha — Hb + He Ha -b Hb — He where the thermal transition state has a collinear geometry, Ha — Hb — He. In the linear structure, the ground state is the resonating combination of R and P, and it constitutes the transition state for the thermal reaction, while the twin-excited state P is the corresponding antiresonant combination that forms the photochemical funnel ... 

In addition, the twin-excited state should possess a real and greatly increased frequency of the reaction coordinate mode. Thus, since the twin-pair has coincident geometry, a spectroscopic characterization of F will provide complementary information on the transition state F and will enable resolution of the transition state structure. 

VB structures, and hence enables one to make predictions in a systematic manner. Other important applications of the twin states concern the possibility of spectroscopic probing or accessing the twin excited state that lies directly above the transition state of a thermal reaction. Thus, much like the foregoing story of benzene, any chemical reaction will possess a transition state, and a twin excited state, j ost cases, albeit not all, the twin excited... 

As a proof of principle, the twin states were characterized for the semi-bullvalene rearrangement and found to possess virtually identical geometries. As shown in Figure 19, the twin excited state possesses symmetry as the symmetry of the reaction coordinate of the thermal process. And the transition state mode, bx, which is imaginary for (Ai) was shown to be real for These calculations match the intriguing findings of Quast... 

The Twin-Excited State as a Probe for the Transition State in Concerted Unimolecular Reactions. The Semibullvalene Rearrangement. 

---