Experimental and theoretical study of the pyrrole cluster photochemistry: Closing the πσ* dissociation pathway by complexation

Photolysis of size selected pyrrole clusters has been investigated and compared to the photolysis of an isolated pyrrole molecule. Experimentally, size distributions of different mean cluster sizes (n=3 and n>>5) have been prepared in supersonic expansions and the clusters were photolyzed at 243 and 193 nm. The kinetic energy distributions of the H photofragments have been measured. The distributions exhibit a bimodal character with fast and slow H-fragment peaks similar to the spectra of the bare molecule. However, with increasing cluster size the slow component gains intensity with respect to the fast one. A similar effect is observed with increasing the excitation energy from 243 to 193 nm. Theoretical calculations at the CASSCF/CASPT2 level have been performed for bare and complexed pyrroles (pyrrole is complexed with an argon atom and with another pyrrole unit). Combination of theoretical and experimental approaches leads to the conclusion that the direct dissociative pathway along the πσ* potential energy surface in the N-H stretch coordinate is closed by the presence of the solvent molecule. This pathway is an important channel leading to the fast H atoms in the dissociation of the bare molecule. The solvent molecule influences significantly the electronic structure in the Rydberg-type πσ* state while it has little influence on the valence states. The slow channel is mostly populated by the out-of-plane deformation mode which is also not influenced by solvation. We have also studied other possible reaction channels in pyrrole clusters (hydrogen transfer, dimerization). The present study shows that more insight into the bulk behavior of biologically relevant molecules can be gained from cluster studies