Photodissociation of aniline N–H bonds in clusters of different nature

Abstract

We investigated the solvent effects on the N-H bond photodisociation dynamics of aniline (PhNH2) in clusters using velocity map imaging (VMI). The VMI experiment was accompanied by a time-of-flight mass spectrometry after electron ionization to reveal the cluster nature. The H-fragment images were recorded at 243 nm in various expansion regimes corresponding to different species: isolated molecules; small (PhNH2)N, N ≤ 3, clusters; larger (PhNH2)N, N ≥ 10; small mixed PhNH2·(H2O)N, N ≤ 10, clusters; and individual PhNH2 molecules deposited on large (H2O)N, N̄ = 430. The H-fragment kinetic energy distributions exhibit fast fragments around 0.8 eV (A) assigned previously to a direct dissociation along a repulsive πσ* state potential, and slow statistical fragments peaking near 0.2 eV (B). In the aniline clusters the contribution of fast fragments (A) decreases relatively to (B) with increasing cluster size. A similar effect is observed when aniline is solvated with water molecules. The experimental data are interpreted with ab initio calculations. Cluster structures were calculated with both N-H bonds of an aniline molecule participating in hydrogen bonding, as well as the ones with free N-H bonds. The latter ones yield preferentially the fast fragments as the isolated molecule. For N-H engaged in hydrogen bonding a barrier increased along the N-H coordinate on the dissociative πσ* state potential surface, and also the energy of πσ*/S0 conical intersection increased. Thus the fast dissociation channel was closed stabilizing the molecule in clusters. The population could be funnelled through other conical intersections into the hot ground state which decayed statistically, yielding the slow H-fragments.

Biomolecule Analogues 2-Hydroxypyridine and 2-Pyridone Base Pairing on Ice Nanoparticles

“…All the studied molecules coagulated to relatively large clusters on argon nanoparticles, but surprisingly no evidence for any clusters generated on the water–ice nanoparticles was observed. The same behavior (i.e., coagulation on argon and noncoagulation on ice nanoparticles) was observed in our photodissociation studies ,, for other molecules as well. In this article we present an experimental evidence for coagulation of biologically relevant molecules on ice nanoparticles: we observed dimers of hydroxypyridine (HP) and even larger clusters composed of up to about 10 molecules generated on the nanoparticles.…”

“…The clusters have to be generated by HP migration and coagulation on/in the ice nanoparticles prior to the ionization, since the ionization is a process that leads to the cluster fragmentation as discussed above. This is quite a surprising result, since all our previous experiments with pickup of different molecules 8,13,14,65 pointed to a strong clustering on argon nanoparticles but no clustering at all on ice nanoparticles as mentioned in the Introduction. Theoretical calculations within our previous paper 8 justified this lack of clustering by two reasons: first was a low mobility of the adsorbed molecules on ice nanoparticles (kinetic effect), which did not allow them to approach each other; second was a strong bond between water and adsorbed molecules which prevented the molecules from bonding with one another even if they appeared in close proximity (dynamic effect).…”

Different Dynamics of CH3 and Cl Fragments from Photodissociation of CH3Cl in Clusters

“…The strong central features characteristic for photodissociation in clusters were observed in quite a few cases previously. ,,− They result from the fragment caging , as the fragments lose their velocity by collisions with the surrounding environment. The central ”blobs” have isotropic angular distributions contrary to the monomer rings as the fragment collisions reduce the anisotropy.…”

Mass spectrometry of aerosol particle analogues in molecular beam experiments

“…[123][124][125] For water clusters, it was used to deposit different molecules on them including atmospherically relevant species such as N x O y based molecules, 126 hydrogen halides, 99,100,127 and freons 87 for photodissociation studies, and other molecules. 67,128 Also organic and more complex molecules including biomolecules [129][130][131][132][133][134] were deposited on water clusters via the pickup technique. If the beam passes through two (or more) pickup cells in series, different molecules can be deposited on the clusters and chemical reactions could be triggered by photons or electrons on the nanoparticles.…”