Has the Excited State Proton Transfer Ever Been Observed in Phenol−(NH3)n Molecular Clusters?

A deuterium atom transfer mechanism has been studied in the excited state of perdeuterated phenol−(ND 3 ) n clusters and compared to the hydrogen atom transfer process evidenced in phenol−(NH 3 ) n =1,4 cluster excited state. A strong H/D effect is observed implying a tunneling reaction process. In view of these results, the question of the competition between proton transfer and H transfer is raised. An alternative to the excited-state proton-transfer dynamics paradigm is proposed to explain the present and previous observations. Good agreement with experimental observations can be obtained with the following three propositions: excited-state H atom transfer occurs for n = 1 to 6; ground-state proton transfer takes place for n ≥ 6 and direct excitation of ground-state proton transferred structures leads to fast evaporation/relaxation events in the excited state; excited-state proton transfer, although energetically favored for cluster sizes n ≥ 4, is not observed, probably because its rate is slow compared to the H transfer reaction rate.