Electronically excited state reaction paths

Shemesh D, Domcke W (2011) Effect of the Chirality of Residues and gamma-Turns on the Electronic Excitation Spectra, Excited-State Reaction Paths and Conical Intersections of Capped Phenylalanine-Alanine Dipeptides. ChemPhysChem 12 1833-1840... 

The reaction path shows how Xe and Clj react with electrons initially to form Xe cations. These react with Clj or Cl- to give electronically excited-state molecules XeCl, which emit light to return to ground-state XeCI. The latter are not stable and immediately dissociate to give xenon and chlorine. In such gas lasers, translational motion of the excited-state XeCl gives rise to some Doppler shifting in the laser light, so the emission line is not as sharp as it is in solid-state lasers. 

According to the size of the system the MD runs have to be carried out in a way not to notice the actual CC excited state (electronic ground state classical path approximation). However, it seems rather reasonable that any back reaction of the actual excited electronic state should be of minor importance since even in the largest studies complexes only singly excited states (single exciton states) are incorporated. 

Primary photoreactions leading to net oxidation or reduction reactions of coordination compounds are well known and are often the result of decay paths accessible only from CT states. A number of coordination compounds yield photoelectron production in solution, the Ru(2,2 -bipyridine)3+ ion has been shown to be an electron donor from its electronically excited state, and photoreduction of several metal complexes has been studied in detail. Discussion of these three areas should reveal most of the important principles associated with photoredox and CT state chemistry. 

The state-symmetry correlation also indicates that electrocyclic radical interconversion favors a conrotatory path from the first excited state and a disrotatory path from the second excited state. Because of the proximity of the energy levels and the violations of the noncrossing rule, it is probable that the excited state process will not be highly stereoselective. The same detailed considerations must be applied to the five-atom five-electron system and yield the results given in Table 1. Differences between the stereochemical predictions of Table 1 and those of others (Woodward and Hoffmann, 1965a Fukui and Fujimoto, 1966b Zimmerman, 1966) tend to be limited to the excited-state reactions of odd-atom radicals. 

Although photochemical reactions of hydrogen and hydrocarbons proceed easily and with high quantum yields, nitrogen photochemistry is not that straightforward. The triple bond within the N2 molecule is extremely difficult to break (E> 9.7 eV). Furthermore, there are no optically allowed excitation paths into repulsive electronic excited states, and dissociation can occur only via indirect paths. Solar radiation below 100 nm can excite predissociating electronic states and constitutes a minor source of N atoms. Dissociative ionization of N2 by either electron impact or solar extreme UV (10-121 nm) radiation produces one N atom and one N+ ion [9] ... 

It appears that there may be several paths leading to eventual generation of electronically excited state products from luminol. In non-protic solvent, in particularly DMSO, the reaction apparently proceeds through the intermediate luminol dianion [33]. Reaction of the dianion with oxygen results in the formation of 3-aminophthalate in the excited state (42). White and Roswell (1970) have shown that under these conditions the chemiluminescence is due to emission from the excited phthalate. 

If the excess energy is electronic, there is a chance that it may be emitted within a time of 10 to 10 sec as radiation if the transition is possible. Otherwise such electronic energy must be degraded by inelastic collisions to other forms of energy, usually vibrational. Generally it is found that the electronic states produced in chemical reactions are metastable, so that we may expect electronically excited states to follow the path of collisional dcexcitation. The rare cases of allowed transitions result in what are referred to as chemiluminescent reactions. ... 

Elaborate mechanistic schemes have been suggested for the principal rearrangements of cyclohexenone, 2,5-cyclohexadienone, and bicyclo-hexenone systems induced by w - tt excitation which are compatible with the experimental data outlined above. In essence, these mechanisms are based on the common concept that the complicated structural changes are initiated in an electronically excited state. For the appreciably complex ketones considered, reaction initiation in a vibrationally excited ground state produced by adiabatic ir n demotion is expected to be readily suppressed in solution by collisional deactivation. It has been pointed out that by this general concept the rearrangements provide a decay path for electronically excited states which allows transfer of minimal amounts of enei to the environment in each step. 

Recent chemiluminescence studies55,56 have revealed yet another reaction path leading to alkali halide molecules in the lowest electronically excited state... 

In the previous sections it has been implicitly assumed that the unimolecular reaction is electronically adiabatic and, thus, occurs on a single potential energy surface. Electronically excited states (i.e., multiple potential energy surfaces) for unimolecular reactions was discussed in chapter 3 and it is assumed that the reader has read and is familiar with this material (Nikitin, 1974 Hirst, 1985 Steinfeld et al., 1989). Transitions between electronic states are particularly important for the unimolecular decomposition of ions. For example, the following two dissociation paths ... 

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