What have we learnt about heavy carbenes through laser flash photolysis studies?

Abstract

Time resolved gas-phase kinetic studies have contributed a great deal of fundamental information about the reactions and reactivity of heavy carbenes (silylenes, germylenes and stannylenes) during the past two decades. In this article we trace the development of our understanding through the mechanistic themes of intermediate complexes, third body assisted associations, catalysed reactions, non-observed reactions and substituent effects. Ab initio (quantum chemical) calculations have substantially assisted mechanistic interpretation and are discussed where appropriate. Trends in reactivity are identified and some signposts to future studies are indicated. This review, although detailed, is not comprehensive.

Gas‐Phase Kinetics of Chlorosilylene Reactions II. ClSiH + C2H4: Absolute Rate Measurements and Quantum Chemical and RRKM Calculations for the Prototype π Addition Reaction

“…[13] The study of temperaTime-resolved studies of chlorosilylene, ClSiH, generated by the 193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, are carried out to obtain rate constants for its bimolecular reaction with ethene, C 2 H 4 , in the gas-phase. The reaction is studied over the pressure range 0.13-13.3 kPa (with added SF 6 ) at five temperatures in the range 296-562 K. ture dependence will enable us to obtain the activation energy of reaction (1) for the first time. At pressures of 1.3 kPa (in SF 6 ) and at room temperature SiH 2 reacts 3.2 times faster than ClSiH with C 2 H 4 .…”

“…The reaction is studied over the pressure range 0.13-13.3 kPa (with added SF 6 ) at five temperatures in the range 296-562 K. ture dependence will enable us to obtain the activation energy of reaction (1) for the first time. At pressures of 1.3 kPa (in SF 6 ) and at room temperature SiH 2 reacts 3.2 times faster than ClSiH with C 2 H 4 . [9] Of interest will be to see how rate constants compare at infinite pressure, the true bimolecular limit.…”

“…Its purity was checked by gas chromatographic (GC). Sulfur hexafluoride, SF 6 (no GC-detectable impurities) was from Cambrian Gases. All gases used in this work were frozen at 77 K and pumped thoroughly to remove all traces of air prior to use.…”

“…Relatively little attention has been paid to the monosubstituted silylenes, XSiH, although we have carried out studies of SiÀH insertion reactions for both MeSiH [7] and PhSiH [8] which showed only small rate reductions for Me and Ph substitution in the silylene. More recently, [9] using a new photochemical precursor, we studied the kinetics of ClSiH with thirteen different substrate molecules at room temperature at 1.3 kPa total pressure (in SF 6 ). Comparisons with reactions of SiH 2 with eight of the same substrates showed a wide variation in relative rate constants, with Cl-for-H substitution producing rate reduction factors of between 2 and 1300.…”

“…[2] Time-resolved studies, carried out in recent years [3][4][5][6] have helped establish a growing database of gas-phase rate constants for these species. While the parent silylene, SiH 2 , reacts very rapidly (at close to the collisional rate) with many different molecules, substituted silylenes such as SiMe 2 and SiCl 2 react more slowly, indicating the significantly deactivating effects of both methyl-and chloro-substitution.…”