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1. Directed gas-phase preparation of the elusive phosphinosilylidyne (SiPH 2 , X 2 A′′) and cis / trans phosphinidenesilyl (HSiPH; X 2 A′) radicals under single-collision conditions

   https://doi.org/10.1039/D1CP02812J  

   He, Chao ; Goettl, Shane J. ; Yang, Zhenghai ; Doddipatla, Srinivas ; Kaiser, Ralf I. ; Silva, Mateus Xavier ; Galvão, Breno R. ( September 2021 , Physical Chemistry Chemical Physics)

   The reaction of the D1-silylidyne radical (SiD; X 2 Π) with phosphine (PH 3 ; X 1 A 1 ) was conducted in a crossed molecular beams machine under single collision conditions. Merging of the experimental results with ab initio electronic structure and statistical Rice–Ramsperger–Kassel–Marcus (RRKM) calculations indicates that the reaction is initiated by the barrierless formation of a van der Waals complex (i0) as well as intermediate (i1) formed via the barrierless addition of the SiD radical with its silicon atom to the non-bonding electron pair of phosphorus of the phosphine. Hydrogen shifts from the phosphorous atom to the adjacent silicon atom yield intermediates i2a, i2b, i3; unimolecular decomposition of these intermediates leads eventually to the formation of trans / cis -phosphinidenesilyl (HSiPH, p2/p4) and phosphinosilylidyne (SiPH 2 , p3) via hydrogen deuteride (HD) loss (experiment: 80 ± 11%, RRKM: 68.7%) and d - trans / cis -phosphinidenesilyl (DSiPH, p2′/p4′) plus molecular hydrogen (H 2 ) (experiment: 20 ± 7%, RRKM: 31.3%) through indirect scattering dynamics via tight exit transition states. Overall, the study reveals branching ratios of p2/p4/p2′/p4′ ( trans / cis HSiPH/DSiPH) to p3 (SiPH 2 ) of close to 4 : 1. The present study sheds light on the complex reaction dynamics of the silicon and phosphorous systems involving multiple atomic hydrogen migrations and tight exit transition states, thus opening up a versatile path to access the previously elusive phosphinidenesilyl and phosphinosilylidyne doublet radicals, which represent potential targets of future astronomical searches toward cold molecular clouds (TMC-1), star forming regions (Sgr(B2)), and circumstellar envelopes of carbon rich stars (IRC + 10216). 

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2. A chemical dynamics study of the reaction of the methylidyne radical (CH, X 2 Π) with dimethylacetylene (CH 3 CCCH 3 , X 1 A 1g )

   https://doi.org/10.1039/D1CP04443E  

   He, Chao ; Fujioka, Kazuumi ; Nikolayev, Anatoliy A. ; Zhao, Long ; Doddipatla, Srinivas ; Azyazov, Valeriy N. ; Mebel, Alexander M. ; Sun, Rui ; Kaiser, Ralf I. ( December 2021 , Physical Chemistry Chemical Physics)

   The gas-phase reaction of the methylidyne (CH; X 2 Π) radical with dimethylacetylene (CH 3 CCCH 3 ; X 1 A 1g ) was studied at a collision energy of 20.6 kJ mol −1 under single collision conditions with experimental results merged with ab initio calculations of the potential energy surface (PES) and ab initio molecule dynamics (AIMD) simulations. The crossed molecular beam experiment reveals that the reaction proceeds barrierless via indirect scattering dynamics through long-lived C 5 H 7 reaction intermediate(s) ultimately dissociating to C 5 H 6 isomers along with atomic hydrogen with atomic hydrogen predominantly released from the methyl groups as verified by replacing the methylidyne with the D1-methylidyne reactant. AIMD simulations reveal that the reaction dynamics are statistical leading predominantly to p28 (1-methyl-3-methylenecyclopropene, 13%) and p8 (1-penten-3-yne, 81%) plus atomic hydrogen with a significant amount of available energy being channeled into the internal excitation of the polyatomic reaction products. The dynamics are controlled by addition to the carbon–carbon triple bond with the reaction intermediates eventually eliminating a hydrogen atom from the methyl groups of the dimethylacetylene reactant forming 1-methyl-3-methylenecyclopropene (p28). The dominating pathways reveal an unexpected insertion of methylidyne into one of the six carbon–hydrogen single bonds of the methyl groups of dimethylacetylene leading to the acyclic intermediate, which then decomposes to 1-penten-3-yne (p8). Therefore, the methyl groups of dimethylacetylene effectively ‘screen’ the carbon–carbon triple bond from being attacked by addition thus directing the dynamics to an insertion process as seen exclusively in the reaction of methylidyne with ethane (C 2 H 6 ) forming propylene (CH 3 C 2 H 3 ). Therefore, driven by the screening of the triple bond, one propynyl moiety (CH 3 CC) acts in four out of five trajectories as a spectator thus driving an unexpected, but dominating chemistry in analogy to the methylidyne – ethane system. 

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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)

   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. Directed Gas Phase Formation of the Elusive Silylgermylidyne Radical (H 3 SiGe, X 2 A′′)

   https://doi.org/10.1002/cphc.202000913  

   Yang, Zhenghai ; Doddipatla, Srinivas ; Kaiser, Ralf I. ; Krasnoukhov, Vladislav S. ; Azyazov, Valeriy N. ; Mebel, Alexander M. ( December 2020 , ChemPhysChem)

   The previously unknown silylgermylidyne radical (H₃SiGe; X²A′′) was prepared via the bimolecular gas phase reaction of ground state silylidyne radicals (SiH; X²Π) with germane (GeH₄; X¹A₁) under single collision conditions in crossed molecular beams experiments. This reaction begins with the formation of a van der Waals complex followed by insertion of silylidyne into a germanium‐hydrogen bond forming the germylsilyl radical (H₃GeSiH₂). A hydrogen migration isomerizes this intermediate to the silylgermyl radical (H₂GeSiH₃), which undergoes a hydrogen shift to an exotic, hydrogen‐bridged germylidynesilane intermediate (H₃Si(μ‐H)GeH); this species emits molecular hydrogen forming the silylgermylidyne radical (H₃SiGe). Our study offers a remarkable glance at the complex reaction dynamics and inherent isomerization processes of the silicon‐germanium system, which are quite distinct from those of the isovalent hydrocarbon system (ethyl radical; C₂H₅) eventually affording detailed insights into an exotic chemistry and intriguing chemical bonding of silicon‐germanium species at the microscopic level exploiting crossed molecular beams.

    

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5. Experimental study of the proton-transfer reaction C + H 2 + → CH + + H and its isotopic variant (D 2 + )

   https://doi.org/10.1039/D0CP04810K  

   Hillenbrand, Pierre-Michel ; Bowen, Kyle P. ; Dayou, Fabrice ; Miller, Kenneth A. ; de Ruette, Nathalie ; Urbain, Xavier ; Savin, Daniel W. ( December 2020 , Physical Chemistry Chemical Physics)

   We report absolute integral cross section (ICS) measurements using a dual-source merged-fast-beams apparatus to study the titular reactions over the relative translational energy range of E r ∼ 0.01–10 eV. We used photodetachment of C − to produce a pure beam of atomic C in the ground electronic 3 P term, with statistically populated fine-structure levels. The H 2 + and D 2 + were formed in an electron impact ionization source, with well known vibrational and rotational distributions. The experimental work is complemented by a theoretical study of the CH 2 + electronic system in the reactant and product channels, which helps to clarify the possible reaction mechanisms underlying the ICS measurements. Our measurements provide evidence that the reactions are barrierless and exoergic. They also indicate the apparent absence of an intermolecular isotope effect, to within the total experimental uncertainties. Capture models, taking into account either the charge-induced dipole interaction potential or the combined charge-quadrupole and charge-induced dipole interaction potentials, produce reaction cross sections that lie a factor of ∼4 above the experimental results. Based on our theoretical study, we hypothesize that the reaction is most likely to proceed adiabatically through the 1 4 A′ and 1 4 A′′ states of CH 2 + via the reaction C( 3 P) + H 2 + ( 2 Σ+g) → CH + ( 3 Π) + H( 2 S). We also hypothesize that at low collision energies only H 2 + ( v ≤ 2) and D 2 + ( v ≤ 3) contribute to the titular reactions, due to the onset of dissociative charge transfer for higher vibrational v levels. Incorporating these assumptions into the capture models brings them into better agreement with the experimental results. Still, for energies ≲0.1 eV where capture models are most relevant, the modified charge-induced dipole model yields reaction cross sections with an incorrect energy dependence and lying ∼10% below the experimental results. The capture cross section obtained from the combined charge-quadrupole and charge-induced dipole model better matches the measured energy dependence but lies ∼30–50% above the experimental results. These findings provide important guidance for future quasiclassical trajectory and quantum mechanical treatments of this reaction. 

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