If I have the reaction C6H5Br = Br + C6H5, can I just get the energies from optimized geometries at TDDFT? I mean, is the procedure different from the gas phase? In the ground state, there is no barrier for this reaction. Should I expect the same in the first excited state?
Your calculation suggests, that bromobenzene (colorless liquid, b.p. 156C) spontaneously decomposes to bromine radical and phenyl radical?
It is not easy to expect the results of excited state dissociation.
First ,you need to find some references. I think there are many papers about excited state dissociation.
I'm not familiar with this kind of calculation but i think that if it is a homolytic dissociation, it should be similar with result of ground state
Hope this help
Martin, the calculations typically carried out at gas phase. The weakest bond is C-Br, so I would think that what happens initially. If you do excited state calculations for any aromatic-Br compound, most likely the C-Br will be significantly elongated in reference to ground state.
I definitely do not object the finding, that C-Br is by far the weakest bond. I am rather surprised, by the non-existence of barrier in ground state. I would expect some barrier even in excited state (first excited singlet). That means - to find the energy as a function of C-Br bond lenght at ground state and excited state.
Hi Sunwoo, I did some search. Most studies discuss proton/H transfer. In most cases, one need to do optimization at the CI level followed by single point at TDDFT to get an energy profile for the fission process. See for example;
TDDFT Potential Energy Functions for Excited State Intramolecular Proton
Transfer of Salicylic Acid, 3-Aminosalicylic Acid, 5-Aminosalicylic Acid,
and 5-Methoxysalicylic Acid, Bull. Korean Chem. Soc. 2007, Vol. 28, No. 12.
They did very nice calculations!
Some recent studies just calculate energy profile directly at TDDFT , but this may be not very accurate procedure.
Martin, maybe there is a barrier in the first excited state but I'm sure there is non in the ground state (though the reaction is very endothermic ~ 80 kcal/mol at 298.15 K).
Dear Mohammednoor, now you really confused me. What is the purpose of your calculations?
Regarding the ground state reaction - according to Hammond's postulate you could expect TS to be very similar to the dissociated complex.
Additionally - do you treat the bond breaking correctly? That means at least broken-symmetry unrestricted Kohn-Sham? Additionally, the final dissociated complex will have triplet ground state.
Dear Martin, you are absolutely correct. I normally do gas phase kinetics. So, it is generally accepted there is no barrier for C-X bond fission in closed shell molecules. For simplicity, we normally assume there is no transition state for this reaction (if you don't consider multireference character method). To get, a an activation energy (to be used in a kinetic model), it is safe to use the reaction energy. I was just trying to get decomposition pathway for bromobenzene in first excited state using TDDFT. It is my first time to do TDDFT, so I am still in the experimentation part of it!
Would you treat C6H5Br as a triplet to get TS for C-Br fission? I can't recall any study found a TS for such reaction using standard DFT, MP2, .etc.
Thanks for the clarification, now I finally understand.
Your last paragraph is actually probably the simplest option to start with. It is not always easy to get correct excited singlet state along the reaction pathway. The ground state triplet could be used as rough, but quite robust approximation to it. So doing C-Br bond scan on triplet could give you some hints.
Definitely avoid any intersystem crossing (singlet to triplet).
With DFT it is not that straightforward to make the proper singlet state behaviour, considering ground and excited state. For qualitatively correct description I would suggest simple MCSCF. In your case of homolytic cleavage it is not fully correct to rely on DFT, as you effectively do several layers of approximation, where any of them can fail.
If you would prefer to stay away of wavefunction methods, than you could recycle the geometries from triplet scan and use them for single point singlet TDDFT. Hopefully it can work somehow reasonably.
I agree with Martin,
If you wanna exactly describe the homolytic dissociation in excited state, you should use high level wavefunction methods such as CASSCF, CASSPT2, EOMCCSD
Please read this paper. I cannot open this paper in my house but it should help.
Computational investigation of the photoinduced homolytic dissociation of water in the pyridine–water complex (http://pubs.rsc.org/en/content/articlelanding/2013/cp/c3cp44585b#!divAbstract)
- Badges
- Science topic