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

Abstract

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 pisigma* 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 pisigma* 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.

Non-adiabatic dynamics of pyrrole: Dependence of deactivation mechanisms on the excitation energy

“…While at long wavelengths there is a predominance of fast H atoms, at short wavelengths (<218 nm) slow H atom elimination tends to become dominant. Recent theoretical and experimental investigations of pyrrole clusters and complexes demonstrated that in presence of solvent molecules there is a strong reduction of the fast H elimination while keeping the slow H elimination [7,25]. This is evidence that the slow H atoms may be formed by another mechanism than the direct NH-stretching.…”

“…It absorbs UV radiation at about 210 nm and also in a second and stronger band at about 175 nm [2,3]. After excitation, pyrrole shows a rich photochemistry [4][5][6][7], which it still not well understood in its full details. Along the years, pyrrole has become an important molecule to test new methods for excited state calculations [8].…”

Mass spectrometry of hydrogen bonded clusters of heterocyclic molecules: Electron ionization vs. photoionization

“…Briefly, the dissociation of the N-H bond via the * state was identified as a major deactivation pathway at lower wavelengths while internal conversion via * /S 0 conical intersection seems to become more important at larger photon energies. We have recently studied the various effects, which the solvation and hydrogen bonding in clusters can have on the photodissociation dynamics of pyrrole [17] and imidazole [18]. We have concluded that the * dissociation channel is suppressed upon the complexation both for pyrrole and imidazole, even though the mechanisms of closing this reaction channel is somewhat different for these two molecules.…”

Dynamics starting at a conical intersection: Application to the photochemistry of pyrrole

“…Although there is no doubt that this path is the main source of fast H atoms, the source of slow H atoms and of heavier fragments is still a matter of discussion. 13,22,[25][26][27] The second deactivation path consists of N-C ring opening proceeding simultaneously with ring puckering, 26,27 which leads to crossing between the ‫ء‬ state and the ground state ͓see Fig. 1͑b͔͒.…”