Bimolecular Reaction Dynamics in the Phenyl-Silane System: Exploring the Prototype of a Radical Substitution Mechanism

Gas-phase preparation of silylacetylene (SiH3CCH) through a counterintuitive ethynyl radical (C2H) insertion

Elementary reaction mechanisms constitute a fundamental infrastructure for chemical processes as a whole. However, while these mechanisms are well understood for second-period elements, involving those of the third period and beyond can introduce unorthodox reactivity. Combining crossed molecular beam experiments with electronic structure calculations and molecular dynamics simulations, we provide compelling evidence on an exotic insertion of an unsaturated sigma doublet radical into a silicon-hydrogen bond as observed in the barrierless gas-phase reaction of the D1-ethynyl radical (C2D) with silane (SiH4). This pathway, which leads to the D1-silylacetylene (SiH3CCD) product via atomic hydrogen loss, challenges the prerequisite and fundamental concept that two reactive electrons and an empty orbital are required for the open shell, unsaturated radical reactant to insert into a single bond.

Competing Si2CH4-H2 and SiCH2-SiH4 Channels in the Bimolecular Reaction of Ground-State Atomic Carbon (C(3Pj)) with Disilane (Si2H6, X1A1g) under Single Collision Conditions

The bimolecular gas-phase reaction of ground-state atomic carbon (C(³P_j)) with disilane (Si₂H₆, X¹A_1g) was explored under single-collision conditions in a crossed molecular beam machine at a collision energy of 36.6 ± 4.5 kJ mol^-1. Two channels were observed: a molecular hydrogen elimination plus Si₂CH₄ (reaction 1) pathway and a silane loss channel along with the formation of SiCH₂ (reaction 2), with branching ratios of 20 ± 3 and 80 ± 4%, respectively. Both channels involved indirect scattering dynamics via long-lived Si₂CH₆ reaction intermediate(s); the latter eject molecular hydrogen and silane in "molecular" elimination channels within the rotational plane of the fragmenting intermediate nearly perpendicularly to the total angular momentum vector. These molecular elimination channels are associated with tight exit transition states as reflected in a significant electron rearrangement as visible from the chemical bonding in the light reaction products molecular hydrogen and silane. Once these hydrogenated silicon-carbide clusters are formed within the inner envelope of carbon stars such as of IRC + 10216, the stellar wind can drive both Si₂CH₄ and SiCH₂ to the outside sections of the envelope, where they can be photolyzed. This is of particular importance to unravel potential formation pathways to disilicon monocarbide (Si₂C) observed recently in the circumstellar shell of IRC + 10216.

Gas-Phase Preparation of Silyl Cyanide (SiH3CN) via a Radical Substitution Mechanism

The silyl cyanide (SiH₃CN) molecule, the simplest representative of a fully saturated silacyanide, was prepared in the gas phase under single-collision conditions via a radical substitution mechanism. The chemical dynamics were direct and revealed a pronounced backward scattering as a consequence of a transition state with a pentacoordinated silicon atom and almost colinear geometry of the attacking cyano radical and leaving hydrogen. Compared to the isovalent cyano (CN)-methane (CH₄) system, the CN-SiH₄ system dramatically reduces the energy of the transition state to silyl cyanide by nearly 100 kJ mol^-1, which reveals a profound effect on the chemical bonding and reaction mechanism. In extreme high-temperature environments including circumstellar envelopes of IRC +10216, this versatile radical substitution mechanism may synthesize organosilicon molecules via reactions of silane with doublet radicals. Overall, this study provides rare insights into the exotic reaction mechanisms of main-group XIV elements in extreme environments and affords deeper insights into fundamental molecular mass growth processes involving silicon in our universe.

Experimental and Theoretical Study on the Gas-Phase Reactions of Germyl Radicals with NF3: Homolytic Substitution at the Nitrogen Atom vs Fluorine Abstraction

In this paper, we report on the unexplored reaction mechanisms of bimolecular homolytic substitution (S_H2) between GeH₃ radicals and the nitrogen atom of NF₃. The S_H2 reactions are studied both experimentally and theoretically with ab initio and density functional theory (DFT) calculations. The experimental results of X-ray irradiation of mixtures of GeH₄ and NF₃ show the formation of GeH₃-NF₂ and GeH₃-F. The trend of product yields as a function of the increase in GeH₄ partial pressure in the irradiated mixtures evidences the predominant role of GeH₃ radicals. Particularly, the S_H2 mechanism can be hypothesized for the reaction between GeH₃ radicals and NF₃ molecules leading to GeH₃-NF₂. This mechanism is further confirmed by the increase in GeH₃-NF₂ yield observed if O₂ is added, as a radical scavenger, to the reaction mixture. In agreement with the experimental data, from the calculations performed at the CCSD(T) and G3B3 levels of theory, we observe that the GeH₃-NF₂ product actually occurs from a bimolecular homolytic substitution by the GeH₃ radical, which attacks the N atom of NF₃, and this reaction is in competition with the fluorine abstraction reaction leading to GeH₃F, even if other mechanisms may be involved in the formation of this product.