The mechanism of the photochemical fragmentation reaction is investigated theoretically using the model system, hydantoin, using the CAS(22,16)/6-31G(d) and MP2-CAS-(22,16)/6-311G(d)//CAS(22,16)/6-31G(d) methods. The model investigation demonstrates that the preferred reaction route for the photofragmentation reaction is as follows: hydantoin → Franck-Condon region → conical intersection → fragment photoproducts (i.e., CO, isocyanic acid, and methylenimine). The theoretical finding additionally suggests that no organic radicals exist during the fragmentation reaction. Moreover, due to the high activation energy, the theoretical evidences suggest that it would be difficult to yield the three fragments under the thermal reaction. All the above theoretical observations are consistent with the available experimental results.
- conical intersection
Hydantoins (or 2,4-imidazolidinediones,
It is well established that most photochemical reactions begin from an excited potential surface and then cross over to a lower surface along the reaction route. They subsequently arrive on the ground state surface through a series of radiationless transitions (i.e., CIs). Eventually, they generate the photoproducts on the ground state surface [3, 4, 5, 6, 7, 8]. That is to say, it is the presence of minima and transition states on the ground and excited state that controls the photochemical reactions [3, 4, 5, 6, 7, 8].
The theoretical results of the
The multireference Møller-Plesset (MP2-CAS) algorithm , which is given in the program package GAUSSIAN 09 , has also been utilized to compute dynamic electron correlations. In this work, the relative energies mentioned in the text are those determined at the MP2-CAS-(22,16)/6-311G(d) level using the CAS(22,16)/6-31G(d) (hereafter designed MP2-CAS and CASSCF, respectively) geometry.
The central feature of the photochemical mechanism of
Figure 2 contains all the relative energies of the key points with respect to the energy of the reactant,
At the beginning,
As seen in Figure 2, after the vertical excitation process,
This work also examined the thermal reaction (Scheme 1) on the ground state (S0) potential energy surface using the same levels of theory. In spite of the fact that photoexcitation elevates
Besides these, one may argue that it is possible to have radical formation in such a photofragmentation reaction. In fact, the CAS(22,16)/6-31G(d) method has been used to calculate the energies of some radicals (such as HN═C═O• and CO•). Their relative energies, however, are higher than that of the
In conclusion, the present theoretical computations demonstrate that upon absorption of a photon of light, the hydantoin (
The author is grateful to the National Center for High-Performance Computing of Taiwan for generous amounts of computing time, and the Ministry of Science and Technology of Taiwan for the financial support. The author also wishes to thank Professor Michael A. Robb, Dr. Michael J. Bearpark, Dr. S. Wilsey, (University of London, UK) and Professor Massimo Olivucci (Universita degli Studi di Siena, Italy) for their encouragement and support during his stay in London.
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