Photodissociation of deuterated pyrrole–ammonia clusters: H-atom transfer or electron coupled proton transfer?

Abstract

Several years ago the discovery of a conical intersection offered an explanation for the ultafast photodissociation of pyrrole. Subsequently, the photodissociation of pyrrole ammonia complexes PyH*(NH₃)_n with n ≥ 3 was studied in the gas phase as a model for a hydrogen-bond forming solvent. Two alternative mechanisms, electron coupled proton transfer (ECPT) and hydrogen atom transfer (HAT, also called the impulsive model, IM), have been proposed. The parent 1 : 1 complex was never studied, due to the short lifetime of the NH₄ radical fragment. Here we report experiments on the deuterated species PyD*(ND₃)_n, including the 1 : 1 complex (n = 1). The velocity distribution of the ND₄ radical is well approximated by a Maxwell–Boltzmann distribution of T ≈ 530 K, with a negative anisotropy parameter of β = −0.3. The impulsive model predicts a much narrower velocity distribution with larger negative anisotropy. The ECPT model predicts a long lived intermediate that should allow thermal equilibration of the vibrational energy but should also destroy the rotational memory of the initially excited state. The average kinetic energy agrees with the prediction of the impulsive model, whereas the wide range of kinetic energies is more in line with ECPT. Hence the mechanism seems to be more complex and requires further theoretical modelling.