Conical intersections and singlet-triplet

The preceding discussion reveals a few of the complexities of the photochemistry that is typical of carbonyl and azo compounds. Results on the olefin-carbonyl Paterno-Buchi system,30 and on the photorearrangements of a, (i-enones,31 P, y-enones,32 azo compounds (diazomethane33 and cyclic diazoalkenes104), and acylcyclopropenes34 show similar features. In these examples, one encounters fourfold intersections as well as conical intersections and singlet-triplet crossings. Thus the potential surfaces are more complex than in hydrocarbon photochemistry. 

Location of Conical Intersections and Singlet-Triplet Crossing Points in Solution... 

Figure 6-2. Conical intersection and singlet-triplet crossing location scheme...

Figure 22 DBH ground state equilibrium structure 3 and molecular and electronic structure for the computed low-lying real crossing 10,11. In this system the S1(n-ic )/ S0, T2(n-7ill )/T1(rc-7i 1 ) conical intersections and the T1(Tt-7t ,)/S0 and T2(n-7i,l )/S0 triplet/singlet crossings occur at the same molecular structure. The relevant geometrical parameters are in angstrom units.

A simple example serves to illnstrate the similarities between a reaction mechanism with a conventional intermediate and a reaction mechanism with a conical intersection. Consider Scheme 9.2 for the photochemical di-tt-methane rearrangement. Chemical intnition snggests two possible key intermediate structures, II and III. Computations conhrm that, for the singlet photochemical di-Jt-methane rearrangement, structure III is a conical intersection that divides the excited-state branch of the reaction coordinate from the ground state branch. In contrast, structure II is a conventional biradical intermediate for the triplet reaction. 

Water immediately transforms the POs" into HP04 which presents no further problems. [Mg(OH2)2]+ also spontaneously reacts with water in the presence of a proton in a series of complex steps to finally yield [Mg(OH2)6] + High-level quantum-chemical calculations revealed that the first and most important step in this procedure is a single electron transfer from Mg+ to H+ which occurs in the conical intersection between the singlet and triplet hypersurface of the following reaction ... 

Different topological situations are possible for unavoided crossings between surfaces. One can have intersections between states of different spin multiplicity [an (n - l)-dimensional intersection space in this case, since the interstate coupling vector vanishes by symmetry], or between two singlet surfaces or two triplets [and one has an n - 2)-dimensional conical intersection hyperline in this case]. We have encountered situations in which both types of... 

The selectivity C/D is controlled by the geometry of the conical intersection for the singlet reaction and by the optimal ISC-geometry of the 2-oxatetramethylene biradical (3X) for the triplet reaction. The situation is described for the propionaldehyde/2,3-dihydrofuran photocycloaddition reaction. At low concentrations (0.005 M, triplet conditions), the diastereo-selectivity (endo-159 / exo-159) approaches a maximum endo/exo value of 85 15 (Sch. 56). At high concentrations (0.5 M, singlet conditions), the diastereoselectivity decreased to 52 48. 

The results for the carbon-oxygen attack are summarized schematically in Figure 7.38. The excited-state branch of the reaction path terminates in a conical intersection point at a CO distance of 177 pm before the biradical is fully formed (cf. Figure 7.37a). Thus the system can evolve back to the reactants or produce a transient C,C-biradical intermediate that is isolated by small barriers (< 3 kcal/mol) to fragmentation (TS,) or to rotation and ring closure to oxetane (TS2). The singlet and triplet biradical minima are essentially coincident. 

A schematic representation of the surfaces for the carbon-carbon attack is shown in Figure 7.39. The very flat region of the S surface (barriers of the order of I kcal/mol) corresponds to the C.O-biradical. The biradical has a CC bond length of 156 pm and corresponds to a conical intersection geometry in the case of the singlet, and to a minimum in the case of the triplet. Thus for the singlet photochemistry the decay to So occurs close to the products, and the reaction appears to be concerted. Since, however, the formation of the singlet biradical is also possible from the same funnel, a certain fraction of photoexcited reactant can evolve via a noncon-certed route. 

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