Combined Experimental and Theoretical Study on the Formation of the Elusive 2-Methyl-1-silacycloprop-2-enylidene Molecule under Single Collision Conditions via Reactions of the Silylidyne Radical (SiH; X²Π) with Allene (H₂CCCH₂; X¹A₁) and D4-Allene (D₂CCCD₂; X¹A₁)

Abstract

The crossed molecular beam reactions of the ground-state silylidyne radical (SiH; X(2)Π) with allene (H2CCCH2; X(1)A1) and D4-allene (D2CCCD2; X(1)A1) were carried out at collision energies of 30 kJ mol(-1). Electronic structure calculations propose that the reaction of silylidyne with allene has no entrance barrier and is initiated by silylidyne addition to the π electron density of allene either to one carbon atom (C1/C2) or to both carbon atoms simultaneously via indirect (complex forming) reaction dynamics. The initially formed addition complexes isomerize via two distinct reaction pathways, both leading eventually to a cyclic SiC3H5 intermediate. The latter decomposes through a loose exit transition state via an atomic hydrogen loss perpendicularly to the plane of the decomposing complex (sideways scattering) in an overall exoergic reaction (experimentally: -19 ± 13 kJ mol(-1); computationally: -5 ± 3 kJ mol(-1)). This hydrogen loss yields the hitherto elusive 2-methyl-1-silacycloprop-2-enylidene molecule (c-SiC3H4), which can be derived from the closed-shell cyclopropenylidene molecule (c-C3H2) by replacing a hydrogen atom with a methyl group and the carbene carbon atom by the isovalent silicon atom. The synthesis of the 2-methyl-1-silacycloprop-2-enylidene molecule in the bimolecular gas-phase reaction of silylidyne with allene enriches our understanding toward the formation of organosilicon species in the gas phase of the interstellar medium in particular via exoergic reactions of no entrance barrier. This facile route to 2-methyl-1-silacycloprop-2-enylidene via a silylidyne radical reaction with allene opens up a versatile approach to form hitherto poorly characterized silicon-bearing species in extraterrestrial environments; this reaction class might represent the missing link, leading from silicon-bearing radicals via organosilicon chemistry eventually to silicon-carbon-rich interstellar grains even in cold molecular clouds where temperatures are as low as 10 K.

A Combined Experimental and Theoretical Study on the Formation of the 2-Methyl-1-silacycloprop-2-enylidene Molecule via the Crossed Beam Reactions of the Silylidyne Radical (SiH; X2Π) with Methylacetylene (CH3CCH; X1A1) and D4-Methylacetylene (CD3CCD; X1A1)

“…This distribution is relatively narrow and spread over only about 25 o within the scattering plane as defined by the primary and secondary beam, which indicates a relatively low translational energy release. We also show in Figure 4 108,124 Considering that the hydrogen atom can be emitted from the silylidyne radical or from methylacetylene, we are probing now to what extent the hydrogen atom originates from the hydrocarbon or silylidyne radical. We carried out the crossed beam reaction of the silylidyne…”

“…The bimolecular reactions of the silylidyne radical (SiH; X 2 Π) with methylacetylene (CH 3 CCH; X 1 A 1 ) and D4-methylacetylene (CD 3 CCD; X 1 A 1 ) were studied under single collision conditions in a universal crossed molecular beam machine. 61,[104][105][106][107][108] Briefly, a pulsed supersonic beam of ground state silylidyne radicals (SiH; X 2 Π) was generated exploiting the photolysis of disilane resolved TOF spectra. To gain information on the scattering dynamics, the laboratory data were transformed into the center-of-mass reference frame using a forward-convolution routine.…”

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

“…Briefly, disilane (Si 2 H 6 ) seeded in helium at a fraction of 0.5 % was fed into a piezoelectric valve (Physik Instrumente) operating at 120 Hz, -300 V peak voltage, and 2 atm backing pressure; the supersonic beam was intercepted by the 193 nm ArF output of an excimer laser (Coherent, Inc.) which was focused to a 1 mm × 4 mm spot with a pulse energy of 30 mJ at a repetition rate of 60 Hz. The photodissociation products were probed via an on axis quadrupole mass spectrometer at the time-of-flight (TOF) mode and by laser-induced fluorescence (LIF) technique [26][27][28], and the rotational temperature of the silylidyne radical (SiH; X 2 Π) was determined to be 40 ± 10 K majorly. The pulsed beam then passed through a stainless steel skimmer, a four-slot chopper wheel rotating at a speed of 120 Hz, which selected a portion of the beam with a well-defined peak velocity (v p ) and speed ratio (S) of 1714 ± 13 m s -1 and…”

Gas phase formation of c-SiC 3 molecules in the circumstellar envelope of carbon stars

“…The primary beam, generated via photolysis of 0.5% disilane seeded in helium, contains ground and excited state silicon atoms (Si( 3 P)/Si( 1 D)) along with the silylidyne radical (SiH(X 2 Π)). The reaction dynamics of the silylidyne radical with allene and methylacetylene have been explored previously (27,28), while ground state silicon atoms do not react with allene or methylacetylene (SI Appendix). Therefore, the present study allows us to discriminate scattering signal obtained via the reaction of electronically excited silicon atoms from that obtained via the reaction of the silylidyne radical with allene and methylacetylene, respectively.…”

“…Therefore, the present study allows us to discriminate scattering signal obtained via the reaction of electronically excited silicon atoms from that obtained via the reaction of the silylidyne radical with allene and methylacetylene, respectively. These neutral reaction products were ionized via electron impact at 80 eV within a triply differentially pumped quadrupole mass-spectrometric detector, and then mass and velocity analyzed to record time-of-flight (TOF) spectra of the ionized products (29) ) was found to originate from the reaction of the silylidyne radical (SiH(X 2 Π)) with allene and methylacetylene leading to the formation of 2-methyl-1-silacycloprop-2-enylidene (c-SiC 3 H 4 ) plus atomic hydrogen (27,28). Second, in both reactions, the TOF spectra at m/z of 68 ).…”