Toluene–water clusters: Ion fragmentation and chemistry

Abstract

Toluene/water cluster ion fragmentation is studied for isolated cold clusters by means of one- and two-color mass resolved excitation spectroscopy, time resolved pump (S1←S0) probe (I←S1) spectroscopy on the nanosecond time scale, and nozzle/laser delay timing experiments. These experiments lead to an identification of parent clusters for all fragment ion clusters observed. Fragmentation reactions depend on cluster size and on the energy deposited in the ion by the two photon I←S1←S0 excitation sequence. Fragments identified by these techniques include (H2O)xH+ (x=3,...,6) and toluene+(H2O)n−1 for toluene(H2O)n clusters and (H2O)xD+, (H2O)xH+, and toluene–d+3(H2O)n−1 for toluene–d3(H2O)n clusters. For n≤3 the preferred cluster fragmentation pathway is loss of a single H2O molecule, while for n≥4 the preferred cluster fragmentation pathway is generation of (H2O)nH+. Cluster ion fragmentation is prevalent in this system because of product stability (i.e., solvated protons and the benzyl radical) and because the I←S1 transition leaves the cluster ion in a very highly excited vibrational state (Δv≫0 for the I←S1 transition). The fragmentation of toluene+(H2O)3 to generate (H2O)3H+ and a benzyl radical takes place by two distinct pathways with generation times τ1<60 ns and τ2=480 ns. The toluene+(H2O)2 fragmentation from toluene+(H2O)3 has a generation time of τ<60 ns. The possible energetics, kinetics, and mechanisms for these fragmentations are discussed.