Conical intersection fractions

The main difference between these stretched-bipyramidalized conical intersections in rings and substituted ethylenes is the process by which they are reached. As already discussed before (Section 8.4), dynamics calculations [38, 66, 90] showed that an important fraction of trajectories of polar substituted ethylenes undergoes stretching and bipyramidalization in the beginning of the time evolution. Nevertheless, in rings the stretched-bipyramidalized configuration cannot be reached by the direct activation of these modes, but it is obtained indirectly as a consequence of the torsional motion around specific bonds. Despite the fact... 

These simulations are useful for the identification of the types of products to be expected from the deactivation at the ring-opened conical intersection. The obtained fraction of non-cyclic structures, however, cannot be directly compared with the experimental number of fragments coming from ring-opening processes because in the simulations the initial velocities were isotropically generated. In the full dynamics simulation, the history of pyrrole derived from the dynamics on the excited state surface should create a bias towards some specific directions. This is an important point to be investigated in a future work. 

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. 

In addition to this channel, three additional types of trajectories were observed, but their fraction was found to be much lower. In trajectories of type II, the molecules decay to the ground state either via the same C-N conical intersection as in the previous case, followed by back formation of formamide on the So PES or return to the ground state via a non dissociative MXS (Antol et al. 2007). In either case, no dissociation is observed within the simulation time. In trajectories of type III, hoppings to the ground state cause activation of the C-H vibrations and the molecule undergoes C-H bond streching. Full C-H dissociation, however, was observed in only one trajectory. Finally, in 15% trajectories, formamide did not decay to the ground state and did not dissociate within the simulation time. 

Abstract Photoinduced processes in extended molecular systems are often ultrafast and involve strong electron-vibration (vibronic) coupling effects which necessitate a non-perturbative treatment. In the approach presented here, high-dimensional vibrational subspaces are expressed in terms of effective modes, and hierarchical chains of such modes which sequentially resolve the dynamics as a function of time. This permits introducing systematic reduction procedures, both for discretized vibrational distributions and for continuous distributions characterized by spectral densities. In the latter case, a sequence of spectral densities is obtained from a Mori/Rubin-type continued fraction representation. The approach is suitable to describe nonadiabatic processes at conical intersections, excitation energy transfer in molecular aggregates, and related transport phenomena that can be described by generalized spin-boson models. 

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