Structure of benzene dimer in an vibrationally excited state

It is known that the benzene dimer has few stable structures, T-shaped, parallel-displaced, and sandwich structure.

Now if we excite the dimer such that it goes to a higher vibrational level (which is close to the dissociation limit but the dimer is still bound).

What happened to the structures then? Can we still have different structures or it will be some kind of mixture?

• Presumably you mean the inter benzene vibrational frequency. This will have a very low frequency indeed and several vibrational (and rotational) levels would be excited at room temperature so exciting another level should have a small effect. ($k_BT \approx 210$ cm $^{-1}$ at room temp.) – porphyrin Apr 11 '18 at 19:02

The binding energy of the benzene dimer is really quite small. Ref. [1] reports the parallel-displaced and T-shaped dimers to have a binding energy at about 2.6 kcal/mol. This is about 900 cm$^{-1}$.

So, it is quite likely that if any vibrational mode with more energy than this is excited that upon redistribution of the energy, the dimer will fall apart.

On the other hand, excitation of vibrations less than the binding energy will likely lead to a very rapid interconversion between the T-shaped and parallel-displaced (or maybe sandwich) isomers. I have read that it is believed this happens basically no matter what.

Also, the sandwich isomer has a binding energy closer to 1.5 kcal/mol, so it is potentially possible to excite some low frequency mode which would cause the sandwich isomer to dissociate but not the T-shaped or parallel-displaced isomers.

Note that because this is such a weakly bound dimer, experimental measurement can only be made at very low temperatures. Ref. [2] describes one measurement of the conformers using supersonic jet expansion to form the dimers. This means temperatures close to milliKelvin.

References:

[1]: Janowski, T., & Pulay, P. (2007). High accuracy benchmark calculations on the benzene dimer potential energy surface. Chemical Physics Letters, 447(1-3), 27-32.

[2]: Scherzer, W., Krätzschmar, O., Selzle, H. L., & Schlag, E. W. (1992). Structural isomers of the benzene dimer from mass selective hole-burning spectroscopy. Zeitschrift für Naturforschung A, 47(12), 1248-1252.

• Thank you very much for the answer and nice explanation. – Bikash Apr 13 '18 at 11:02