Infrared and Electronic Spectroscopy of a Model System for the Nucleophilic Substitution Intermediate in the Gas Phase: The C−N Valence Bond Formation in the Benzene−Ammonia Cluster Cation

Growth mechanism of aromatic prebiotic molecules: insights from different processes of ion-molecule reactions in benzonitrile-ammonia and benzonitrile-methylamine clusters

Benzonitrile (BN, C6H5CN) has been detected in the cold molecular cloud Taurus molecular cloud-1 (TMC-1) in 2018, which is suggested to be a precursor in the formation of more complex nitrogen-containing aromatic interstellar compounds. In this study, we utilized mass-selected infrared (IR) photodissociation spectroscopy and quantum chemical calculations to investigate the structures and gaseous ion-molecule reactions of benzonitrile-ammonia (BN-NH3) and benzonitrile-methylamine (BN-MA) clusters. The spectral observations indicate that the cyclic hydrogen bonding structure predominates in both neutral clusters. After VUV (118 nm) single-photon ionization, a new C-N covalent bond formed between BN and NH3 in the (BN-NH3)+ cluster. However, proton sharing constitutes the primary structure of the (BN-MA)+ cluster. The two nitrogen-containing interstellar molecules react with BN to yield distinct products due to difference in charge distribution and molecular polarity in the ionized clusters. The reactions of BN with other molecules contribute to our understanding of the growth mechanisms of complex interstellar molecules.

Reaction mechanism of an intracluster SN2 reaction induced by electron capture

Bimolecular nucleophilic substitution (S_N2) reactions have been widely investigated from both experimental and theoretical points of view because they represent one of the simplest organic reactions. Most studies on S_N2 reactions have been focused on bimolecular collision. In contrast, information on intracluster S_N2 reactions is limited. In this study, an intracluster S_N2 reaction of NF₃-CH₃Cl triggered by electron attachment was investigated using a direct ab initio molecular dynamics (AIMD) method. In the structure of NF₃-CH₃Cl, the N-F bond in NF₃ is oriented collinearly toward the carbon atom of CH₃Cl. After electron capture by NF₃-CH₃Cl, the F^- ion that is generated from the (NF₃)^- moiety collides with the carbon atom of CH₃Cl. The intracluster S_N2 reaction occurs as follows: (NF₃-CH₃Cl)^- (electron capture state) → NF₂-(F^-)-CH₃Cl (pre-reaction complex) → transition state (TS) → NF₂-CH₃F-Cl^- (post-reaction complex) → NF₂ + CH₃F + Cl^- (product state). The reaction energy is efficiently transferred to the translational mode of Cl^-, and the Cl^- ion with a high translational energy is then removed from the system. This energy is significantly larger than that of Cl^- formed in the bimolecular S_N2 reaction (F^- + CH₃Cl). The reaction mechanism is discussed based on the theoretical results.

Reactive pathways in the chlorobenzene-ammonia dimer cation radical: new insights from experiment and theory

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.

Enhancement of a Lewis Acid−Base Interaction via Solvation: Ammonia Molecules and the Benzene Radical Cation

The interaction between ammonia and the benzene radical cation has been investigated by gas-phase studies of mass selected ion clusters {C(6)H(6)-(NH(3))(n=0-8)}(+) via tandem quadrupole mass spectrometry and through calculations. Experiments show a special stability for the cluster ion that contains four ammonias: {C(6)H(6)(NH(3))(4)}(+). Calculations provide evidence that the first ammonia forms a weak dative bond to the cyclohexadienyl radical cation, {C(6)H(6)-NH(3)}(+), where there is a transfer of electrons from ammonia to benzene. Additional solvating ammonia molecules form stabilizing hydrogen bonds to the ring-bound ammonia {C(6)H(6)-NH(3)}(+).(NH(3))(n), which cause cooperative changes in the structure of the cluster complex. Free ammonia is a weak hydrogen bond donor, but electron transfer from NH(3) to the benzene ring that strengthens the dative bond will increase the hydrogen acidity and the strength of the cluster hydrogen bonds to the added ammonia. A progressive "tightening" of this dative bond is observed upon addition of the first, second, and third ammonia to give a cluster stabilized by three N-(+)H x N hydrogen bonds. This shows that the energetic cost of tightening the dative bond is recovered with dividends in the formation of stable cluster hydrogen bonds.