Dynamics of Hydrogen Atom Transfer in Indole(NH₃)_n Clusters

Abstract

The hydrogen atom transfer reaction in indole (NH3) n clusters excited at 263 nm to the S1(ππ*) state is studied in pump−probe experiments with femtosecond laser pulses. For small clusters the reaction is characterized by two successive processes on distinct time scales: the time constant of the primary process is in the sub-ps region, whereas the secondary decay time growing with the cluster size reflects a relaxation process within 25 to 150 ps. A preliminary model of the H-transfer reaction is discussed.

Photoinduced hydrogen transfer reaction dynamics in indole–ammonia clusters at different excitation energies

“…Fig. at l pu ¼ 263 nm 6 ) and cannot be understood without a more detailed knowledge of the specific energetic structure of the corresponding cluster. This may be caused by the reduced ionization efficiency of the Table 1 Time constants t 2 , t 3 and t 4 characterizing the dynamics of nondeuterated and deuterated indoleÁ(ammonia) n , n ¼ 1-5, clusters excited at five different pump wavelengths l pu .…”

“…Experimental evidence for this dark state has been found by Zwier and co-workers using excited-state fluorescence-dip infrared spectroscopy. 5 Very recently we have studied the photochemical processes in indole-ammonia 6,7 and indole-water clusters 8 in pumpprobe experiments with fs laser pulses. 4 For indoleÁ(NH 3 ) n complexes it is assumed that a population of the dark state should lead to an intracluster hydrogen atom transfer from the N-H group of the indole molecule to the surrounding (NH 3 ) n moiety.…”

“…3 However, the internal conversion as well as the properties of the ps* potential surface determining the character and dynamics of possible subsequent photochemical processes are expected to depend sensitively on the local environment of the chromophore. 6 The detection of the photoions as well as the coincident photoelectrons with fs time resolution revealed detailed information about both the dynamics and energetics of the dissociative H-transfer reaction in indoleÁ(NH 3 ) n clusters. Indeed, Jouvet et al were able to identify in a ns pump-probe ionization experiment the neutral (NH 3 ) nÀ1 ÁNH 4 dissociation photoproducts which are associated with the H-atom transfer reaction thus giving further evidence for the proposed mechanism.…”

Revealing the role of excited state proton transfer (ESPT) in excited state hydrogen transfer (ESHT): systematic study in phenol–(NH3)n clusters

“…In indole, as for PhOH, the H loss in the free molecule 71 and the hydrogen transfer to the NH 3 cluster have been demonstrated. [72][73][74][75][76][77] In the free molecule, the H loss through the ps* state 71 has been observed but is also contaminated by H atoms generated by one or more (unintended but unavoidable) multiphoton processes. UV excitation of indole-(NH 3 ) n clusters leads to a H atom transfer reaction and the formation of cNH 4 (NH 3 ) n radicals.…”

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.…”