Chemical dynamics of the formation of the ethynylsilylidyne radical (SiCCH(X Π2)) in the crossed beam reaction of ground state silicon atoms (Si(P3)) with acetylene (C2H2(X ∑g+1))

Abstract

The reaction dynamics of ground state silicon atoms ͑Si͑ 3 P͒͒ with the acetylene molecule ͑C 2 H 2 ͑X 1 ͚ g + ͒͒ were investigated at a collision energy of 101.6Ϯ 1.6 kJ mol −1 under single collision conditions in a crossed molecular beam machine. We found that the reaction dynamics proceeded via an addition of the silicon atom to the -electrons of the acetylene molecule at a single carbon atom forming a C s symmetric SiC 2 H 2 ͑X 3 AЉ͒ intermediate. The latter either emitted a hydrogen atom leading to the linear SiCCH͑X 2 ⌸͒ product or underwent a hydrogen migration to the SiCCH 2 ͑X 3 A 2 ͒ isomer prior to the decomposition of the latter to SiCCH͑X 2 ⌸͒ plus atomic hydrogen. The overall reaction to form the SiCCH͑X 2 ⌸͒ plus atomic hydrogen products was determined to be highly endoergic by 84Ϯ 6 kJ mol −1 . No evidence of a molecular hydrogen elimination channel was found at this collision energy. Our study predicts that this endoergic neutral-neutral reaction can lead to the formation of an organosilicon transient species, i.e., the linear SiCCH͑X 2 ⌸͒ radical, in high-temperature environments like in the circumstellar envelope of the carbon star IRC+ 10 216, where temperatures of a few 1000 K exist close to the central star.

Untangling the reaction dynamics of the silylidyne radical (SiH; X2Π) with acetylene (C2H2; X1Σg+)

“…Further, these data suggest the absence of a molecular hydrogen loss in the SiH-C 2 H 2 system and the lack of any reactive scattering signal from atomic and/or molecular hydrogen loss in the Si-C 2 H 2 system. The lack of the molecular hydrogen loss in the reaction of silylidyne with acetylene gains also support from a previous examination of this system [19].Likewise, the absence of any reactive scattering signal from ground state silicon atoms with acetylene can be attributed to the highly endoergic (84 kJ mol -1 ) formation of the silapropenylidyne molecule (HCCSi) via atomic hydrogen loss and/or inability of the SiC 2 H 2 intermediate to undergo intersystem crossing followed by molecular hydrogen loss[32]. Considering the best signal-to-noise, we collected a full angular scan of the TOF spectra at m/z = 53 from 13.75° to 33.75°, and the TOF spectra along with the laboratory angular distribution are presented inFigure 2.…”

ON THE FORMATION OF SILACYCLOPROPENYLIDENE (c-SiC2H2) AND ITS ROLE IN THE ORGANOSILICON CHEMISTRY IN THE INTERSTELLAR MEDIUM

“…Smith et al proposed that at these low temperatures, intersystem crossing operates efficiently, and the spin-forbidden exoergic molecular hydrogen loss channels to c-SiC 2 plus molecular hydrogen are open (Smith et al 2006). However, at elevated temperatures close to the photosphere of carbon stars, intersystem crossing is rather inefficient, and c-SiC 2 cannot be formed via the bimolecular reaction of silicon atoms with acetylene (Kaiser & Gu 2009). However, c-SiC 2 can be synthesized upon photolysis and in shocked regions through hydrogen loss from silacyclopropenylidene (c-SiC 2 H 2 ), which in turn can be generated through bimolecular reactions of silylidyne (SiH) with acetylene (C 2 H 2 ) as demonstrated in the present work.…”

AN EXPERIMENTAL AND THEORETICAL STUDY OF THE IONIZATION ENERGIES OF SiC2Hx(x= 0, 1, 2) ISOMERS

“…On the other hand, Smith (2006) proposed that intersystem crossing operates efficiently at lower collision energies. A recent crossed molecular beam experiment by Kaiser & Gu (2009) of ground-state silicon atoms (Si( 3 P)) with acetylene at a collision energy of 101.6 ± 1.6 kJ mol −1 provided experimental evidence that the reaction proceeded via an addition of the silicon atom to the π -electrons of the acetylene molecule at a single carbon atom forming a C s symmetric SiC 2 H 2 (X 3 A") intermediate. The latter either emitted a hydrogen atom leading to the linear HCCSi isomer or underwent a hydrogen migration to the H 2 CCSi isomer prior to the decomposition to linear HCCSi plus atomic hydrogen; minor contributions of the energetically less stable, cyclic SiC 2 H isomer could not be ruled out.…”