Reactions of Photoionization-Induced CO-H2O Cluster: Direct Ab Initio Molecular Dynamics Study

C-C Bond Formation Reaction Catalyzed by a Lithium Atom: Benzene-to-Biphenyl Coupling

Transition-metal-catalyzed carbon-carbon (C-C) bond formation is an important reaction in pharmaceutical and organic chemistry. However, the reaction process is composed of multiple steps and is expensive owing to the presence of transition metals. This study proposes a lithium-catalyzed C-C coupling reaction of two benzene molecules (Bz) to form a biphenyl molecule, which is a transition-metal-free reaction, based on ab initio and direct ab initio molecular dynamics (AIMD) calculations. The static ab initio calculations indicate that the reaction of two Bz molecules with Li^- ions (reactant state, RC) can form a stable sandwiched complex (precomplex), where the Li^- ion is sandwiched by two Bz molecules. The complex formation reaction can be expressed as 2Bz + Li ^- → Bz(Li ^-)Bz, where the C-C distance between the Bz rings is 2.449 Å. This complex moves to the transition state (TS) via the structural deformation of Bz(Li^-)Bz, where the C-C distance is shortened to 2.118 Å. The barrier height was calculated to be -9.9 kcal/mol (relative to RC) at the MP2/6-311++G(d,p) level. After TS, the C(sp³)-C(sp³) single bond was completely formed between the Bz rings (the C-C bond distance was 1.635 Å) (late complex). After the dissociation of H₂ from the late complex, a biphenyl molecule was formed: the C(sp²)-C(sp²) bond. The calculations suggest that the C-C bond coupling of Bz occurred spontaneously from 2Bz + Li^-, and biphenyl molecules were directly formed without an activation barrier. Direct AIMD calculations show that the C-C coupling reaction also takes place under electron attachment to Li(Bz)₂: Li(Bz)₂ + e^- → [Li^-(Bz)₂]_ver → precomplex → TS → late complex, where [Li^-(Bz)₂]_ver is the vertical electron capture species of Li(Bz)₂. Namely, the C-C coupling reaction spontaneously occurred in Li(Bz)₂ owing to electron attachment. Similar C-C coupling reactions were also observed for halogen-substituted benzene molecules (Bz-X, X = F and Cl). Furthermore, this study discusses the mechanism of C-C bond formation in electron capture based on the theoretical results.

Intracluster Reaction Dynamics of Ionized Micro-Hydrated Hydrogen Peroxide (H2O2): A Direct Ab Initio Molecular Dynamics Study

Hydrogen peroxide (H₂O₂) is a unique molecule that is applied in various fields, including energy chemistry, astrophysics, and medicine. H₂O₂ readily forms clusters with water molecules. In the present study, the reactions of ionized H₂O₂-water clusters, H₂O₂ ⁺(H₂O) _n , after vertical ionization of the parent neutral cluster were investigated using the direct ab initio molecular dynamics (AIMD) method to elucidate the reaction mechanism. Clusters with one to five water molecules, H₂O₂-(H₂O) _n (n = 1-5), were examined, and the reaction of [H₂O₂ ⁺(H₂O) _n ]_ver was tracked from the vertical ionization point to the product state, where [H₂O₂ ⁺(H₂O) _n ]_ver is the vertical ionization state (hole is localized on H₂O₂). After ionization, fast proton transfer (PT) from H₂O₂ ⁺ to the water cluster (H₂O) _n was observed in all clusters. The HOO radical and H₃O⁺(H₂O) _n-1 were formed as products. The PT reaction proceeds directly without an activation barrier. The PT times for n = 1-5 were calculated to be 36.0, 9.8, 8.3, 7.7, and 7.1 fs, respectively, at the MP2/6-311++G(d,p) level, indicating that PT in these clusters is a very fast process, and the PT time is not dependent on the cluster size (n), except in the case of n = 1, where the PT time was slightly longer because the bond distance and angle of the hydrogen bond in n = 1 were deformed from the standard structure. The reaction mechanism was discussed based on these results.