Prefulvenic intersection

This intersection is usually referred to as the prefulvenic intersection. As shown in Scheme 12, the intersection is accessible from (i) the Si state by surmounting an energy barrier related to the iS i/iS 2 avoided crossing (see dashed curves), and (ii) from the S2 state through a barrierless path involving a higher-energy S2/S1 conical intersection. 

In the photochemistry of benzene, the so-called channel 3 represents a well-known decay route along which fluorescence is quenched above a vibrational excess of 3000 cm [57], The decay takes place through a prefulvenic conical intersection characterized by an out of plane bending [52,58] and results in the formation of benzvalene and fulvene. The purpose of this study is to find distinct radiationless decay pathways that could be selected by exciting specific combinations of photoactive modes in the initial wavepacket created by a laser pulse. For this, we carry out quantum dynamics simulations on potential energy surfaces of reduced dimension, using the analysis outlined above for the choice of the coordinates. 

The formation of benzvalene is formally an x[2 + 2] cyclo-addition. The S, (Bju) reaction path from benzene toward prefulvene starts at an excited-state minimum with symmetry and proceeds over a transition state to the geometry of prefulvene, where it enters a funnel in S, due to an S,-So conical intersection and continues on the Sg surface, mostly back to benzene, but in part on to benzvalene (Palmer et al., 1993 Sobolewski et al., 1993). At prefulvene geometries, Sg has a flat biradicaloid region of high energy with very shallow minima whose exact location depends on calcula-tional details (Kato, 1988 Palmer, et al., 1993, Sobolewski et al., 1993). Fulvene has been proposed to be formed directly from prefulvene or via secondary isomerization of benzvalene (Bryce-Smith and Gilbert, 1976). Calculations support the former pathway with a carbene intermediate (Dreyerand Klessinger, 1995). 

This was one of the first excited-state problems where theory (conical intersections) and experimental photophysics (radiationless decay) came together to give a unified picture. A fuU analysis of the conical-intersection seam of benzene has been given recentlyHere we will focus on just the prefulvene part. 

Of course the VB theory of benzene is well known. Our purpose now is to show how the prefulvene conical intersection shown in Figures 3.15 and 3.16 can be deduced from the simple VB methods we have been using, but without detailed numerical computation. Unfortunately, the manipulation of VB stmctures and the computation ofVB matrix elements is becoming a lost art. However, all the details can be verified easily from the discussion ofVB theory in the textbooks ofMcWeeny or Eyring. The detailed derivations that form the basis of the current discussion can be found in our recent expositions. ... 

We now explain how the main features of the prefulvene conical intersection in benzene photophysics can be deduced. Referring to Figure 3.15, the ground state of benzene is A - - B (the sum of the two Kekule stmctures) and the vertically excited Si state is just A — B. 

Of course there are many stmctures on the prefulvene-hke conical-intersection seam. In Figure 3.19 we show only two. The full set is presented elsewhere. Further, VB stmctures for 3 electrons and 4 electrons are embedded in the 6-electron case. Thus the atoms 1, 2, and 6 on the... 

Fulvene seems to have become a benchmark molecule for the study of conical intersections. " In recent work we have been able to optimize five geometries on an extended conical intersection. But this is a 6 orbitals with 6 electrons problem, so one might expect that the photochemistry and the conical intersections could be deduced from the previous discussion. In fact it can The two fulvene structures are A (as for benzene Figure 3.5) for the excited state and a structure not too dissimilar to prefulvene for the excited state shown in Figure 3.20 along with the branching-space vectors, which are similar to a combination of the vectors shown in Figure 3.18 for benzene itself... 

Blancafort L, Robb MA. A valence bond description of the prefulvene extended conical intersection seam of benzene. J Chem Theory Comput. 2012 8 4922-4930. Blancafort L, Celani P, Beatpark M, Robb M. A valence-bond-based complete-active-space self-consistent-field method for the evaluation of bonding in organic molecules. Theor Chem Acc. 2003 110 92-99. 

Scheme 7.13 Cut through of the potential energy surfaces of benzene along the prefulvene mode—a vector from the Franck-Condon point to the Sj/Sq conical intersection— calculated using the MOLPRO program at the CASPT2 level with a (6,6) CAS space and a Roos ANO basis set truncated to 6-31G-quality Sj (black). So (red), Tj (green), and T2 (blue). Reprinted with permission from [28, 29]...

Another important photoreaction is the 1,3 cycloaddition of ethene to benzene. Although a 1,2 and a 1,4 photocycloaddition can also take place, the 1,3 photochemical addition is the most important, since it yields a variety of natural products depending on the substitution pattern of the alkene and arene. The reaction mechanism which was obtained by experimental results indicates an exciplex mechanism, whereas a SINDOl calculation favors the prefulvene mechanism.Since the experiments were done in solution and the calculations were performed in a vaccum, this is not at all surprising. According to the SINDOl calculations, there is a conical intersection near the prefulvene minimum, which is very flat. The semiempirical calculations are supported by MCSCF ab initio calculations, which show a remarkable similiarity in the important structures for both the exciplex and the prefulvene mechanisms.This demonstrates that semiempirical programs on the Cl level are well suited for photochemical problems. 

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