On the Chlorobenzene-Ammonia Cluster Reaction: Entrance Channel Dynamics, 2CR2PI Spectroscopy, and Ion Imaging of the Reaction Dynamics

Prior studies of halobenzene-ammonia complexes have shown that the nature of the cationic intermediate (i.e., Wheland-type vs ion-radical) may play a key role in determining the reaction products. To probe this link, we report here the reaction dynamics of the chlorobenzene-ammonia 1:1 complex (PhCl···NH₃) using product ion imaging following two-color resonant two-photon ionization. A threshold value of 8.863 ± 0.008 eV was determined for the appearance of protonated aniline, which accompanies Cl atom loss and is the dominant product channel at energies near threshold. Scanning down to energies close to threshold, we find no evidence for a roaming halogen radical mechanism leading to HCl products, which was evidenced in the related bromobenzene-ammonia complex, and proceeded through an ion-radical intermediate structure. Here, supporting calculations indicate that both types of intermediates are present, but the Wheland-type structure is significantly more stable. Addressing a key question of earlier work, analysis of the PhCl···NH₃ potential energy surface (PES) in the reactant region establishes a complicated entrance channel pathway linking the in-plane σ-type complex to the Wheland intermediate (iWH) on the [PhCl···NH₃]^+• cationic surface. This pathway involves stepwise transition of the weakly bound ammonia from the initial in-plane σ-type complex to an <i>ortho</i> Wheland intermediate, followed by rearrangement to the <i>ipso</i> position. Finally, given that fluorine has been shown to stabilize aromatic ions, we hypothesized that fluorine substitution might alter the structure of the intermediate, favoring the ion-radical intermediate. To test this hypothesis, as an illustrative example the PES of the <i>meta</i>-PhClF-NH₃ system on the cationic surface was computed. Confirming our hypothesis, these calculations show an inversion in stability for the Wheland-type and ion-radical complex intermediates, with the latter preferred energetically at the examined level of theory.