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

Abstract

We present a combined experimental and theoretical investigation of the bimolecular gas phase reaction of the phenyl radical (C6H5) with silane (SiH4) under single collision conditions to investigate the chemical dynamics of forming phenylsilane (C6H5SiH3) via a bimolecular radical substitution mechanism at a tetra-coordinated silicon atom. Verified by electronic structure and quasiclassical trajectory calculations, the replacement of a single carbon atom in methane by silicon lowers the barrier to substitution thus defying conventional wisdom that tetra-coordinated hydrides undergo preferentially hydrogen abstraction. This reaction mechanism provides fundamental insights into the hitherto unexplored gas phase chemical dynamics of radical substitution reactions of mononuclear main group hydrides under single collision conditions and highlights the distinct reactivity of silicon compared to its isovalent carbon. This mechanism might be also involved in the synthesis of cyanosilane (SiH3CN) and methylsilane (CH3SiH3) probed in the circumstellar envelope of the carbon star IRC+10216. TOC image

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

“…23 Disilene (Si 2 H 4 ) exists as a trans-bent molecule with sp 3 hybridized silicon atoms, but the ethylene (C 2 H 4 ) molecule is planar with both carbon atoms sp 2 hybridized. 24,25 However, whereas an understanding of the chemistry of silane (SiH 4 ) with carbon-bearing reactants in CSEs is beginning to emerge, 22,26,27 the chemistry of disilane (Si 2 H 6 ) has remained largely unexplored. An elucidation of the reaction dynamics of disilane (Si 2 H 6 ) with ground-state atomic carbon (C( 3 P j )) as the simplest "organic" open shell species affords an exceptional opportunity to gauge the activation of disilane through the initial formation of silicon−carbon versus silicon− hydrogen bonds under single collision conditions.…”

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

“…15 Although both the nucleophilic (S N ) and electrophilic (S E ) substitution mechanisms have aided in unraveling the physical organic mechanistic framework driving molecular mass growth processes through carbon−carbon bond coupling, the unraveling of dynamics of radical (S R ) substitution reactions in gas phase, also referred to as homolytic radical substitution reactions, of reactants carrying tetracoordinated carbon or isovalent silicon atoms has been sparse predominantly due to the experimental difficulties in preparing the radical reactants in sufficient concentrations to allow the detection of the nascent reaction products. 16 In pioneering studies, detailed reactive collision mechanisms were revealed through the simplest hydrogen-atom exchange reaction of atomic hydrogen (H) with molecular hydrogen (H 2 ); 17,18 Dixon et al 19 along with Zare et al, 20 Truhlar et al, 21 and Zhao et al 13 unraveled the underlying mechanisms of the reaction of atomic hydrogen (H) with methane (CH 4 ) or D4-methane (CD 4 ). A direct hydrogen abstraction pathway forming molecular hydrogen along with the methyl radical (CH 3 ) was exposed to a barrier of 63 kJ mol −1 , which was energetically preferred to the radical substitution channel leading to methane (CH 4 ) and H with a 159 kJ mol −1 barrier.…”