Chapter 5


(1)       Question: What factors influence the free rotor effect?


Answer: Some set of atoms in the excited state must be able to experience a rotational motion that couples to an electronic deactivation..  Rotation is not enough; the rotational motion must be coupled to mixing the excited state and the ground state.  Such coupling cannot be predicted apriori, but comes from computational chemistry. The most common example is the rotation about a C=C bond.


(2)       Question: Does molecular rigidity inhibit radiative or radiationless processes?

Answer:  Rigidity more generally inhibits radiationless processes because radiationless processes require some vibronic or rotational coupling which mixes the excited and ground surfaces.  The more rigid the system the fewer the number of effective vibronic or rotational couplings that can trigger radiationless transitions.


(3)       Question: In Figure 6.10, why does the alkyl ketone have a faster rate constant for intersystem crossing than pyrenealdehyde?

Answer: The alkyl ketone possesses a S1(n,p*) state which possesses more spin orbit coupling than a S1(p,p*) state.  For pyrene aldehyde the lowest state is on the pyrene, i.e., S1 is a p,p* state and undergoes slow intersystem crossing, just like pyrene.

(4)       Question: When is the transfer of electronic energy into excess vibrational energy a rapid process?

Answer; When the coupling is effective in promoting electronic mixing of the excited and ground states at achievable geometries on the excited state surface.  Again there are no simple ways to predict such geometries without computation or other guidelines.  Remember, the valence structures that we draw for molecules only apply to the energetics of the ground state surface.  We need to think about valence bond structures that would have the same energy in the excited state AND the ground state.  At these geometries, the energies of R and *R would be close and vibrations that mix these states would be effective.  To be able to do this effectively, you have to become conversant with valence bond structure for excited states.


(5)       Question: Explain the difference between the deuterium isotope effect on Sn®S1 (internal conversion) versus T1®S0 (intersystem crossing).


Answer: Both transitions require Franck-Condon overlap and follow the Golden Rule for radiationless transitions (exponential dependence on energy gap). The Sn®S1 (internal conversion) transitions usually involve electronic states that are close in energy because a large number of electronic transitions from various configurations exists as one goes above the S1 state.  In addition, the Sn®S1 (internal conversion) does not require a spin change.  Therefore, such processes are typically limited only by vibronic coupling which is commonly of the order of a few vibrational cycles.

The T1®S0 (intersystem crossing) has a large energy gap and requires a mechanism for both vibrational energy relaxation and spin mixing.  This slows down the process enormously compared to internal conversion. 

What do you predict in general for the rate of Sn®S1 (internal conversion) versus S1®S0 (internal conversion)?  Which is better to compare with T1®S0 (intersystem crossing): Sn®S1 (internal conversion) or S1®S0 (internal conversion)?


(6)       Question: Is there an experimental technique which can distinguish between triplet sublevels?

Answer:  Absolutely. In a magnetic field the three levels are energetically split so one can use light to go from one level to another.  This is a form of Electron Paramagnetic Spectroscopy.  Usually one needs to go to very low temperatures or us a solid matrix to perform the experiment because the levels are strongly mixed by rotational motion in a magnetic field.  Many magnetic effects on photochemical reactions result from the selective intersystem crossing into one of the three sublevels.  You’ll see more of this in Chapter 6 from section 6.15 and following.