2 research outputs found
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; X<sup>2</sup>Π) with Methylacetylene (CH<sub>3</sub>CCH; X<sup>1</sup>A<sub>1</sub>) and D4-Methylacetylene (CD<sub>3</sub>CCD; X<sup>1</sup>A<sub>1</sub>)
The bimolecular gas-phase reactions
of the ground-state silylidyne
radical (SiH; X<sup>2</sup>Î ) with methylacetylene (CH<sub>3</sub>CCH; X<sup>1</sup>A<sub>1</sub>) and D4-methylacetylene (CD<sub>3</sub>CCD; X<sup>1</sup>A<sub>1</sub>) were explored at collision energies
of 30 kJ mol<sup>–1</sup> under single-collision conditions
exploiting the crossed molecular beam technique and complemented by
electronic structure calculations. These studies reveal that the reactions
follow indirect scattering dynamics, have no entrance barriers, and
are initiated by the addition of the silylidyne radical to the carbon–carbon
triple bond of the methylacetylene molecule either to one carbon atom
(C1; [i1]/[i2]) or to both carbon atoms concurrently (C1–C2;
[i3]). The collision complexes [i1]/[i2] eventually isomerize via
ring-closure to the c-SiC<sub>3</sub>H<sub>5</sub> doublet radical
intermediate [i3], which is identified as the decomposing reaction
intermediate. The hydrogen atom is emitted almost perpendicularly
to the rotational plane of the fragmenting complex resulting in a
sideways scattering dynamics with the reaction being overall exoergic
by −12 ± 11 kJ mol<sup>–1</sup> (experimental)
and −1 ± 3 kJ mol<sup>–1</sup> (computational)
to form the cyclic 2-methyl-1-silacycloprop-2-enylidene molecule (c-SiC<sub>3</sub>H<sub>4</sub>; <b>p1</b>). In line with computational
data, experiments of silylidyne with D4-methylacetylene (CD<sub>3</sub>CCD; X<sup>1</sup>A<sub>1</sub>) depict that the hydrogen is emitted
solely from the silylidyne moiety but not from methylacetylene. The
dynamics are compared to those of the related D1-silylidyne (SiD;
X<sup>2</sup>Π)–acetylene (HCCH; X<sup>1</sup>Σ<sub>g</sub><sup>+</sup>) reaction studied previously in our group, and
from there, we discovered that the methyl group acts primarily as
a spectator in the title reaction. The formation of 2-methyl-1-silacycloprop-2-enylidene
under single-collision conditions via a bimolecular gas-phase reaction
augments our knowledge of the hitherto poorly understood silylidyne
(SiH; X<sup>2</sup>Î ) radical reactions with small hydrocarbon
molecules leading to the synthesis of organosilicon molecules in cold
molecular clouds and in carbon-rich circumstellar envelopes
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; X<sup>2</sup>Π) with Allene (H<sub>2</sub>CCCH<sub>2</sub>; X<sup>1</sup>A<sub>1</sub>) and D4-Allene (D<sub>2</sub>CCCD<sub>2</sub>; X<sup>1</sup>A<sub>1</sub>)
The
crossed molecular beam reactions of the ground-state silylidyne
radical (SiH; X<sup>2</sup>Î ) with allene (H<sub>2</sub>CCCH<sub>2</sub>; X<sup>1</sup>A<sub>1</sub>) and D4-allene (D<sub>2</sub>CCCD<sub>2</sub>; X<sup>1</sup>A<sub>1</sub>) were carried out at
collision energies of 30 kJ mol<sup>–1</sup>. Electronic structure
calculations propose that the reaction of silylidyne with allene has
no entrance barrier and is initiated by silylidyne addition to the
Ï€ 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
SiC<sub>3</sub>H<sub>5</sub> 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<sup>–1</sup>; computationally: −5 ± 3 kJ mol<sup>–1</sup>). This hydrogen loss yields the hitherto elusive
2-methyl-1-silacycloprop-2-enylidene molecule (c-SiC<sub>3</sub>H<sub>4</sub>), which can be derived from the closed-shell cyclopropenylidene
molecule (c-C<sub>3</sub>H<sub>2</sub>) 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