Reactive Pathways in the Chlorobenzene–Ammonia Dimer Cation Radical: New Insights from Experiment and Theory

Abstract

Building upon our recent studies of noncovalent interactions in chlorobenzene and bromobenzene clusters, in this work we focus on interactions of chlorobenzene (PhCl) with a prototypical N atom donor, ammonia (NH3). Thus, we have obtained electronic spectra of PhCl···(NH3)n (n = 1-3) complexes in the region of the PhCl monomer S0 -S1 (ππ*) transition using resonant 2-photon ionization (R2PI) methods combined with time-of-flight mass analysis. Consistent with previous studies, we find that upon ionization the PhCl···NH3 dimer cation radical reacts primarily via Cl atom loss. A second channel, HCl loss, is identified for the first time in R2PI studies of the 1:1 complex, and a third channel, H atom loss, is identified for the first time. While prior studies have assumed the dominance of a π-type complex, we find that the reactive complex corresponds instead to an in-plane σ-type complex. This is supported by electronic structure calculations using density functional theory and post-Hartree-Fock methods and Franck-Condon analysis. The reactive pathways in this system were extensively characterized computationally, and consistent with results from previous calculations, we find two nearly isoenergetic arenium ions (Wheland intermediates; denoted WH1, WH2), which lie energetically below the initially formed dimer cation radical complex. At the energy of our experiment, intermediate WH1, produced from ipso-addition, is not stable with respect to Cl or HCl loss, and the relative branching between these channels observed in our experiment is well reproduced by microcanonical transition state theory calculations based upon the calculated parameters. Intermediate WH2, where NH3 adds ortho to the halogen, decomposes over a large barrier via H atom loss to form protonated o-chloroaniline. This channel is not open at the (2-photon) energy of our experiments, and it is suggested that photodissociation of a long-lived (i.e., several ns) WH2 intermediate leads to the observed products.

π -π stacking vs. C–H/π interaction: Excimer formation and charge resonance stabilization in van der Waals clusters of 9,9′-dimethylfluorene

“…The experimental strategies used in these experiments have been described in detail. 16,17 Brief descriptions are provided here. Resonant 2-photon ionization (R2PI) 18 experiments were conducted in a linear 1 m time-of-flight mass spectrometer (TOFMS), equipped with a heated supersonic molecular beam source.…”

Nucleophilic Aromatic Substitution Between Halogenated Benzene Dopants and Nucleophiles in Atmospheric Pressure Photoionization

“…In APPI, substitution of the halogen with a methoxy group has been reported for halobenzene dopants in the presence of methanol [28]. Several studies exist where density functional theory (DFT) has been utilized to explain the nucleophilic substitution reaction theoretically [29,30].…”

Reaction mechanism of an intracluster SN2 reaction induced by electron capture

“…34 Recently, a π-type intermediate was found in [PhCl–NH 3 ] + systems using the resonant 2-photon ionization method combined with time-of-flight mass spectrometry. 35…”