[t_nuh]

Hey guys

I'm reading a paper that has to do with IVR and fluorescence in trans-stilbene. In the first paragraph, they talk about how there is a reaction barrier that results from an avoided crossing between a singlet B ungerade and a doubly excited singlet A gerade state. It was my understanding that avoided crossing resulted from two states with the same symmetry, which these do no have, so I'm confused as to why they are saying this is avoided? Any help on this would be awesome.

Thanks

[Irlanur]

Can you post the link to the paper?

[t_nuh]

Absolutely. Sorry, I meant to do that in the original question. Here it is http://pubs.acs.org/doi/abs/10.1021/jp960909x

The part I'm referring to is in the first paragraph of the introduction.

Thanks again!

[Corribus]

If you can find yourself a copy of Turro's fantastic photochemistry textbook (Modern Molecular Photochemistry of Organic Molecules) there is a lengthy section on ASRs, particularly as they apply to the twisting of double bonds in the excited state. It's around page 332, section 6.9.

[t_nuh]

Okay awesome. I'll definitely check that out. Thanks!

[Corribus]

One general thing to keep in mind - just because two electronic states are of different symmetry does not mean they cannot couple or interact, because vibrational and rotational states are also involved. Two electronic states may have different symmetries but they can couple through vibrations - the overall symmetry of a state is a product of the symmetries of its respective electronic and vibrational counterparts.

E.g., benzene may be nominally D2h symmetry - with electronic states having symmetries derived from the D2h character table - but the nuclei aren't stationary. During a vibration, the symmetry changes. If there is a certain vibration that can cause the ground and electronic states to temporarily have the same overall symmetry classification, coupling of the states becomes possible. Vibronic coupling is one reason why spectroscopic transitions can often occur between electronic states that have seemingly incompatible symmetries - the transition occurs through a vibrational mode the satisfies the symmetry requirement for the selection rule. Similar vibronic effects can also have an effect on whether avoided crossings and noncrossing impacts excited-state relaxation processes.