Experimental and Theoretical Evidence for Homogeneous Catalysis in the Gas-Phase Reaction of SiH₂ with H₂O (and D₂O):  A Combined Kinetic and Quantum Chemical Study

Abstract

Time-resolved kinetic studies of the reaction of silylene, SiH(2), with H(2)O and with D(2)O have been carried out in the gas phase at 297 K and at 345 K, using laser flash photolysis to generate and monitor SiH(2). The reaction was studied independently as a function of H(2)O (or D(2)O) and SF(6) (bath gas) pressures. At a fixed pressure of SF(6) (5 Torr), [SiH(2)] decay constants, k(obs), showed a quadratic dependence on [H(2)O] or [D(2)O]. At a fixed pressure of H(2)O or D(2)O, k(obs) values were strongly dependent on [SF(6)]. The combined rate expression is consistent with a mechanism involving the reversible formation of a vibrationally excited zwitterionic donor-acceptor complex, H(2)Si...OH(2) (or H(2)Si...OD(2)). This complex can then either be stabilized by SF(6) or it reacts with a further molecule of H(2)O (or D(2)O) in the rate-determining step. Isotope effects are in the range 1.0-1.5 and are broadly consistent with this mechanism. The mechanism is further supported by RRKM theory, which shows the association reaction to be close to its third-order region of pressure (SF(6)) dependence. Ab initio quantum calculations, carried out at the G3 level, support the existence of a hydrated zwitterion H(2)Si...(OH(2))(2), which can rearrange to hydrated silanol, with an energy barrier below the reaction energy threshold. This is the first example of a gas-phase-catalyzed silylene reaction.

The reaction between silylene and ammonia: Some gas-phase kinetic and quantum chemical studies

“…The true values for the second order rate constants of this reaction are almost certainly smaller than we are observing probably by as much as an order of magnitude (at 10 Torr). It is also probable that a catalysed process, involving two molecules of NH 3 , by analogy with findings in the SiH 2 +H 2 O reaction system [32], would contribute to SiH 2 removal. The non-observation of a quadratic dependence of k obs on [NH 3 ] can therefore be taken as further evidence against the direct reaction of SiH 2 with NH 3 .…”

Silylenes and germylenes: The activation of H–H bond in hydrogen molecule

“…Similar to carbenes, most of silylenes and germylenes can function as Lewis acid or Lewis base, in which the vacant p orbital is responsible for electrophilic character (Lewis acid), while the non-bonding electron pair can act as the nucleophilic (Lewis base) attack region [77]. The activation reactions of some small inorganic and organic compounds by Lewis acid-base addition reactions, such as NO [91,98], NH 3 [103,110], CO [93], CO 2 [94,108], O 2 [91,102], Cl 2 [91] H 2 O [95,96,[103][104][105]110], C 2 H 4 [106,112], and C 2 H 2 [107] with silylenes and germylenes have been investigated experimentally. The insertion reactions of silylenes or germylenes into C-H, C-Cl, C-Br, and O-H bonds of small molecules, for example, CH 4 [110,111], CCl 4 [100,101], and CBr 4 [100] etc., have also been studied by the density functional theory (DFT).…”

“…The reactions of silylenes and germylenes with alcohols have been reported to take place via the activation of O-H bond to generate the corresponding alkoxyhydrosilane [33,35,46]. In fact, activation of O-H of water molecule can also proceed with silylenes [67,95,96,103]. Similar mechanism exists for carbenes as well [20][21][22][23][24][25][26].…”

Gaseous reaction mechanism of C2F radical with water

“…Recently, the first gaseous water‐catalyzed SiH 2 reaction has been experimentally and theoretically shown that addition of another H 2 O can significantly reduce the reaction barrier 9. Herein, we discuss the catalysis effect on the C 2 F + H 2 O reaction.…”