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1. Mechanistic molecular motion of transition-metal mediated β-hydrogen transfer: quasiclassical trajectories reveal dynamically ballistic, dynamically unrelaxed, two step, and concerted mechanisms

   https://doi.org/10.1039/D0DT01687J  

   Wheeler, Josh I.; Carlsen, Ryan; Ess, Daniel H. (June 2020, Dalton Transactions)

   null (Ed.)

   The transfer of a β-hydrogen from a metal-alkyl group to ethylene is a fundamental organometallic transformation. Previously proposed mechanisms for this transformation involve either a two-step β-hydrogen elimination and migratory insertion sequence with a metal hydride intermediate or a one-step concerted pathway. Here, we report density functional theory (DFT) quasiclassical direct dynamics trajectories that reveal new dynamical mechanisms for the β-hydrogen transfer of [Cp*Rh III (Et)(ethylene)] + . Despite the DFT energy landscape showing a two-step mechanism with a Rh–H intermediate, quasiclassical trajectories commencing from the β-hydrogen elimination transition state revealed complete dynamical skipping of this intermediate. The skipping occurred either extremely fast (typically <100 femtoseconds (fs)) through a dynamically ballistic mechanism or slower through a dynamically unrelaxed mechanism. Consistent with trajectories begun at the transition state, all trajectories initiated at the Rh–H intermediate show continuation along the reaction coordinate. All of these trajectory outcomes are consistent with the Rh–H intermediate <1 kcal mol −1 stabilized relative to the β-hydrogen elimination and migratory insertion transition states. For Co, which on the energy landscape is a one-step concerted mechanism, trajectories showed extremely fast traversing of the transition-state zone (<50 fs), and this concerted mechanism is dynamically different than the Rh ballistic mechanism. In contrast to Rh, for Ir, in addition to dynamically ballistic and unrelaxed mechanisms, trajectories also stopped at the Ir–H intermediate. This is consistent with an Ir–H intermediate that is stabilized by ∼3 kcal mol −1 relative to the β-hydrogen elimination and migratory insertion transition states. Overall, comparison of Rh to Co and Ir provides understanding of the relationship between the energy surface shape and resulting dynamical mechanisms of an organometallic transformation. 

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2. Mechanistic Aspects on [3+2] Cycloaddition (32CA) Reactions of Azides to Nitroolefins: A Computational and Kinetic Study

   https://doi.org/10.1002/chem.202202294  

   Kawamura, Meire_Y; Alegre‐Requena, Juan_V; Barbosa, Thaís_M; Tormena, Cláudio_F; Paton, Robert_S; Ferreira, Marco_A_B (October 2022, Chemistry – A European Journal)

   Abstract [3+2] cycloadditions of nitroolefins have emerged as a selective and catalyst‐free alternative for the synthesis of 1,2,3‐triazoles from azides. We describe mechanistic studies into the cycloaddition/rearomatization reaction sequence. DFT calculations revealed a rate‐limiting cycloaddition step proceeding via an asynchronous TS with high kinetic selectivity for the 1,5‐triazole. Kinetic studies reveal a second‐order rate law, and¹³C kinetic isotopic effects at natural abundance were measured with a significant normal effect at the conjugated olefinic centers of 1.0158 and 1.0216 at the α and β‐carbons of β‐nitrostyrene. Distortion/interaction‐activation strain and energy decomposition analyses revealed that the major regioisomeric pathway benefits from an earlier and less‐distorted TS, while intermolecular interaction terms dominate the preference for 1,5‐ over 1,4‐cycloadducts. In addition, the major regioisomer also has more favorable electrostatic and dispersion terms. Additionally, while static DFT calculations suggest a concerted but highly asynchronousEi‐type HNO₂elimination mechanism, quasiclassical direct‐dynamics calculations reveal the existence of a dynamic intermediate. 

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3. Combined Crossed Molecular Beams and Ab Initio Study of the Bimolecular Reaction of Ground State Atomic Silicon (Si; ³ P) with Germane (GeH ₄ ; X ¹ A ₁ )

   https://doi.org/10.1002/cphc.202100235  

   Krasnoukhov, Vladislav_S; Azyazov, Valeriy_N; Mebel, Alexander_M; Doddipatla, Srinivas; Yang, Zhenghai; Goettl, Shane; Kaiser, Ralf_I (June 2021, ChemPhysChem)

   Abstract The chemical dynamics of the elementary reaction of ground state atomic silicon (Si;³P) with germane (GeH₄; X¹A₁) were unraveled in the gas phase under single collision condition at a collision energy of 11.8±0.3 kJ mol⁻¹exploiting the crossed molecular beams technique contemplated with electronic structure calculations. The reaction follows indirect scattering dynamics and is initiated through an initial barrierless insertion of the silicon atom into one of the four chemically equivalent germanium‐hydrogen bonds forming a triplet collision complex (HSiGeH₃;³i1). This intermediate underwent facile intersystem crossing (ISC) to the singlet surface (HSiGeH₃;¹i1). The latter isomerized via at least three hydrogen atom migrations involving exotic, hydrogen bridged reaction intermediates eventually leading to the H₃SiGeH isomeri5. This intermediate could undergo unimolecular decomposition yielding the dibridged butterfly‐structured isomer¹p1(Si(μ‐H₂)Ge) plus molecular hydrogen through a tight exit transition state. Alternatively, up to two subsequent hydrogen shifts toi6andi7, followed by fragmentation of each of these intermediates, could also form¹p1(Si(μ‐H₂)Ge) along with molecular hydrogen. The overall non‐adiabatic reaction dynamics provide evidence on the existence of exotic dinuclear hydrides of main group XIV elements, whose carbon analog structures do not exist. 

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4. Kinetic study of the CN radical reaction with 2‐methylfuran

   https://doi.org/10.1002/kin.21403  

   Lee, James; Caster, Kacee; Maddaleno, Trey; Donnellan, Zachery; Selby, Talitha M.; Goulay, Fabien (July 2020, International Journal of Chemical Kinetics)

   Abstract The gas phase reaction of the ground state cyano‐radical (CN (X²∑⁺)) with 2‐methylfuran (2‐MF) is investigated in a quasi‐static reaction cell at pressures ranging from 2.2 to 7.6 Torr and temperatures ranging from 304 to 440 K. The CN radicals are generated in their ground electronic state by pulsed laser photolysis of gaseous cyanogen iodide (ICN) at 266 nm. Their concentration is monitored as a function of reaction time using laser‐induced fluorescence at 387.3 nm on the B²∑⁺(ν′ = 0) ← X²∑⁺(ν″ = 0) vibronic band. The reaction rate coefficient is found to be rapid and independent of pressure and temperature. Over the investigated temperature and pressure ranges, the rate coefficient is measured to be 2.83 (± 0.18) × 10⁻¹⁰cm³molecules s⁻¹. The enthalpies of the stationary points and transition states on the CN + 2‐MF potential energy surface are calculated using the CBS‐QB3 computational method. The kinetic results suggest the lack of a prereactive complex on the reaction entrance channel with either a very small or nonexistent entrance energy barrier. In addition, the potential energy surface calculations reveal only submerged barriers along the minimum energy path. Based on comparisons between previous CN reactions with unsaturated hydrocarbons, the most likely reaction pathway is CN addition onto one of the unsaturated carbons followed by either H or methyl elimination. The implications for the transformation of biomass‐derived fuels in nitrogen‐rich flames is discussed. 

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5. Electric field–catalyzed single-molecule Diels-Alder reaction dynamics

   https://doi.org/10.1126/sciadv.abf0689  

   Yang, Chen; Liu, Zitong; Li, Yanwei; Zhou, Shuyao; Lu, Chenxi; Guo, Yilin; Ramirez, Melissa; Zhang, Qingzhu; Li, Yu; Liu, Zhirong; et al (January 2021, Science Advances)

   null (Ed.)

   Precise time trajectories and detailed reaction pathways of the Diels-Alder reaction were directly observed using accurate single-molecule detection on an in situ label-free single-molecule electrical detection platform. This study demonstrates the well-accepted concerted mechanism and clarifies the role of charge transfer complexes with endo or exo configurations on the reaction path. An unprecedented stepwise pathway was verified at high temperatures in a high-voltage electric field. Experiments and theoretical results revealed an electric field–catalyzed mechanism that shows the presence of a zwitterionic intermediate with one bond formation and variation of concerted and stepwise reactions by the strength of the electric field, thus establishing a previously unidentified approach for mechanistic control by electric field catalysis. 

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