Do they occur sequentially or can they occur simultaneously, i.e., in a concerted or co-operative manner?
Two examples of particular interest are coupled electron-proton transfer and double proton transfer.
The figure below shows a carboxylic acid dimer involving two hydrogen bonds
The configuration above has the same energy as the tautomer with the top H moved to the right and the bottom H moved to the left. But, does this reaction occur by simultaneously moving the protons or first moving one and then the second.
For the case of double proton transfer in dimers of a model of a DNA base pair [shown in the picture below] there has been some controversy about whether the process is concerted or sequential. This brief letter by Kwon and Zewail in PNAS gives the relevant references. They stress that some of the controversy seems to stem from confusion about clearly defining what one means by the two options. They argue the weight of the evidence is for sequential.
Another example concerns the role of intermediate electronic states in charge separation processes in molecular photovoltaic materials.
I am interested in exploring simple effective Hamiltonians that might be used to come up with some general criteria and experimental signatures for discriminating between concerted and sequential processes.