Hydrogen transfer in excited pyrrole–ammonia clusters

Abstract

The excited state hydrogen atom transfer reaction (ESHT) has been studied in pyrrole-ammonia clusters [PyH-(NH(3))(n)+hnu-->Py.+.NH(4)(NH(3))(n-1)]. The reaction is clearly evidenced through two-color R2P1 experiments using delayed ionization and presents a threshold around 235 nm (5.3 eV). The cluster dynamics has also been explored by picosecond time scale experiments. The clusters decay in the 10-30 ps range with lifetimes increasing with the cluster size. The appearance times for the reaction products are similar to the decay times of the parent clusters. Evaporation processes are also observed in competition with the reaction, and the cluster lifetime after evaporation is estimated to be around 10 ns. The kinetic energy of the reaction products is fairly large and the energy distribution seems quasi mono kinetic. These experimental results rule out the hypothesis that the reaction proceeds through a direct N-H bond rupture but rather imply the existence of a fairly long-lived intermediate state. Calculations performed at the CASSCF/CASMP2 level confirm the experimental observations, and provide some hints regarding the reaction mechanism.

Proton-transfer and hydrogen-bond interactions determine fluorescence quantum yield and photochemical efficiency of bacteriophytochrome

“…We note that all four pyrrole nitrogens are protonated in the Pr state of plant phytochrome, Cph1 and Bph (16,(23)(24)(25). Recent experimental and computational evidence suggests that pyrroles can efficiently deprotonate in the excited state and deactivate to the ground state through a conical intersection (26,27). Thus, a candidate mechanism for BV excited-state deactivation is a light-induced deprotonation reaction of a BV pyrrole nitrogen through excited-state proton transfer (ESPT).…”

Effects of Amino Substitution on the Excited State Hydrogen Transfer in Phenol: A TDDFT Study

“…The ESHT reaction is contrary to the traditional view that this cluster system will undergo an excited state proton transfer (ESPT) reaction as the pKa of this weak acid is lowered in the electronically excited state. 3 However, the ESHT reaction has been computationally predicted and experimentally confirmed in many systems including phenol, [4][5][6][7] halogenated phenols, 8,9 methylphenol, 10 thiophenol, [11][12][13] pyrrole, 14 indole, 15 and 3-methyl-indole. 16 The ESHT reaction of phenol is driven by a πσ* state that is repulsive along the O-H bond.…”

Rotational spectrum, structure and internal dynamics of the pyrrole·ammonia complex

“…They suggest that the photochemical dynamics is governed by an excited-state hydrogen transfer reaction. Experimental evidence for this proposed mechanism is clearly shown with investigations on clusters of small organic molecules like phenole, indole and pyrrole with water and ammonia [6][7][8][9][10]. In order to describe microsolvation, both experimental and theoretical approaches are thus used to understand the geometry of binary hydrogen-bonded complexes and the internal dynamics of both moieties with respect to each other, besides further properties like the electronic structure.…”