Excited state energy barriers

One interesting aspect is the lack of a major excited-state energy barrier. Such a barrier was obtained for an alternative mechanism in which two vinyl bridges bonded initially, followed by scission of two sigma bonds. This alternative mechanism was excluded experimentally. However, the extended Hiickel pictiu-e differs only quantitatively from the SCF-CI computation we carried out many years later (vide infra). 

Photochemistry is controlled in two ways. First, there is the avoidance of excited-state energy barriers, which plays a major role. Second, the placement of conical intersections and avoided crossings determine reaction success vs. radiationless decay. A conical intersection positioned close to the Franck-Condon geometry and before to an appreciable barrier favors decay to the reactant ground state and a low or zero quantum yield. A conical intersection positioned after the first energy barrier on the excited state hypersurface facilitates reaction, epecially if the conical intersection appears close to a product geometry. 

Fig. 16 Schematic of the trends in excited-state energies in [Pt(4 -R-tpy)Cl]+ complexes, showing the mixing of the ILCT state for the amino-substituted complex. AE represents the barrier to deactivation via a3 d-d state. Reproduced from data in [65] with permission from Elsevier...

Chapter 12 ventures into the realm of photochemistry, where structural concepts are applied to following the path from initial excitation to the final reaction product. Although this discussion involves comparison with some familiar intermediates, especially radicals, and offers mechanisms to account for the reactions, photochemistry introduces some new concepts of reaction dynamics. The excited states in photochemical reactions traverse energy surfaces that have small barriers relative to most thermal reactions. Because several excited states can be involved, the mechanism of conversion between excited states is an important topic. The nature of conical intersections, the transition points between excited state energy surfaces is examined. 

Barriers for the BC step in the excited state depend on the steric bulk of the substituents R at the alkyne termini. For R=H, the corresponding photoproduct is formed in up to 35% yield even at approximately 10°C, whereas photolysis of the diphenyl substituted enediyne produced only small amounts (5-8%) of picenoporphyrin even at 125°C. Because no thermal BC has been observed at 125°C with R=Ph, this result suggests that the excited-state cyclization barrier is still lower than the ground-state barrier. Based on the comparison of thermal and photochemical reactivity for the two enediy nes, authors suggested qualitative potential energy surfaces summarized in Scheme 30.16. The presence of the substantial activation barriers at the excited-state hypersurface is consistent with the theoretical analysis of formally forbidden photo-BC. 

According to Kramers model, for flat barrier tops associated with predominantly small barriers, the transition from the low- to the high-damping regime is expected to occur in low-density fluids. This expectation is home out by an extensively studied model reaction, the photoisomerization of tran.s-stilbene and similar compounds [70, 71] involving a small energy barrier in the first excited singlet state whose decay after photoexcitation is directly related to the rate coefficient of tran.s-c/.s-photoisomerization and can be conveniently measured by ultrafast laser spectroscopic teclmiques. 

The vibrationally excited states of H2-OH have enough energy to decay either to H2 and OH or to cross the barrier to reaction. Time-dependent experiments have been carried out to monitor the non-reactive decay (to H2 + OH), which occurs on a timescale of microseconds for H2-OH but nanoseconds for D2-OH [52, 58]. Analogous experiments have also been carried out for complexes in which the H2 vibration is excited [59]. The reactive decay products have not yet been detected, but it is probably only a matter of time. Even if it proves impossible for H2-OH, there are plenty of other pre-reactive complexes that can be produced. There is little doubt that the spectroscopy of such species will be a rich source of infonnation on reactive potential energy surfaces in the fairly near future. 

A kinetic scheme and a potential energy curve picture ia the ground state and the first excited state have been developed to explain photochemical trans—cis isomerization (80). Further iavestigations have concluded that the activation energy of photoisomerization amounts to about 20 kj / mol (4.8 kcal/mol) or less, and the potential barrier of the reaction back to the most stable trans-isomer is about 50—60 kJ/mol (3). 

Fig. 13.11. A schematic drawing of the potential energy surfaces for the photochemical reactions of stilbene. Approximate branching ratios and quantum yields for the important processes are indicated. In this figure, the ground- and excited-state barrier heights are drawn to scale representing the best available values, as are the relative energies of the ground states of Z- and E -stilbene 4a,4b-dihydrophenanthrene (DHP). [Reproduced from R. J. Sension, S. T. Repinec, A. Z. Szarka, and R. M. Hochstrasser, J. Chem. Phys. 98 6291 (1993) by permission of the American Institute of Physics.]...

Such a structure implies that there would be a barrier to rotation about the C(2)—C(3) bond and would explain why the s-trans and s-cis conformers lead to different excited states. Another result that can be explained in terms of the two noninterconverting excited states is the dependence of the ratio of [2 + 2] and [2 + 4] addition products on sensitizer energy. The s-Z geometry is suitable for cyclohexene formation, but the s-E is not. The excitation energy for the s-Z state is slightly lower than that for the s-E. With low-energy sensitizers, therefore, the s-Z excited state is formed preferentially, and the ratio of cyclohexene to cyclobutane product increases. ... 

Isomerization of ( /Z) isomers is another important transformation. Isomerization of ( ) and (Z-) conjugated amides is effected photochemically " (photo-isomerization " ). There is a rather high energy barrier for the excited state required for (E/Z) isomerization. Isomerization of the C=C units in dienes is also induced photochemically. " Isomerization of cyclic alkenes is more difficult but cyclooctene is isomerized photochemically. " Conjugated aldehydes have been isomerized... 

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