Gas-Phase Formation of 1-Methylcyclopropene and 3-Methylcyclopropene via the Reaction of the Methylidyne Radical (CH; X²Π) with Propylene (CH₃CHCH₂; X¹A′)

Abstract

The crossed molecular beam reactions of the methylidyne radical (CH; X2Π) with propylene (CH3CHCH2; X1A′) along with (partially) substituted reactants were conducted at collision energies of 19.3 kJ mol–1. Combining our experimental data with ab initio electronic structure and statistical calculations, the methylidyne radical is revealed to add barrierlessly to the carbon–carbon double bond of propylene reactant resulting in a cyclic doublet C4H7 intermediate with a lifetime longer than its rotation period. These adducts undergo a nonstatistical unimolecular decomposition via atomic hydrogen loss through tight exit transition states forming the cyclic products 1-methylcyclopropene and 3-methylcyclopropene with overall reaction exoergicities of 168 ± 25 kJ mol–1. These C4H6 isomers are predicted to exist even in low-temperature environments such as cold molecular clouds like TMC-1, since the reaction is barrierless and exoergic, all transition states are below the energy of the separated reactants, and both the methylidyne radical (CH; X2Π) and propylene reactant were detected in cold molecular clouds such as TMC-1.

Gas‐Phase Synthesis of 3‐Vinylcyclopropene via the Crossed Beam Reaction of the Methylidyne Radical (CH; X2Π) with 1,3‐Butadiene (CH2CHCHCH2; X1Ag)

“…However, the formation of p5 can only be accessed via hydrogen emission from the CH moiety of 1,3-butadiene reactant; for the cis-3-vinyl-cyclopropene (p6) and 1-methyl-1,3cyclobutadiene (p12) products formed via the pathways described in this section, the hydrogen loss may originate not only from the CH 2 group of 1,3-butaidene, but also from the methylidyne (CH) radical reactant and the CH moiety of 1,3-butadiene reactant, respectively; this is inconsistent with the results of our isotopic substitution experiments. Therefore, we can conclude that pathways ( 20)- (26) are likely not open under our experimental conditions.…”

“…Typical rotational temperatures of the methylidyne radical were determined to be 14 ± 1 K exploiting laser induced fluorescence (LIF). 26 A supersonic beam of neat 1,3-butadiene (Aldrich Chemistry, 99%+) at a backing pressure of 550 Torr with v p = 777  12 m s -1 and S = 9.5  0.3 was produced in the secondary chamber; this beam crossed perpendicularly with the section of the methylidyne beam resulting in a collision energy E C of 20.8  0.4 kJ mol -1 and a center of mass (CM) angle Θ CM of 60.4  0.3. Each supersonic beam was generated via a piezoelectric pulse valve, which was operated at a repetition rate of 60 Hz, a pulse width of 80 s and peak voltage of -400 V. The neat 1,3butadiene gas was triggered 90 s prior to the methylidyne beam.…”

“…This behavior was also detected in the related reactions of methylidyne with methylacetylene/allene and propylene. 24,26 Here, the barrierless cycloaddition of the methylidyne radical to the carboncarbon triple/double and double bond of methylacetylene/allene and propylene leads eventually leading to triafulvene with atomic hydrogen elimination from the methyl moiety of the methylacetylene reactant and from the CH 2 group of allene; likewise, 1-methylcyclopropene and 3-methylcyclopropene along with atomic hydrogen emission from the vinyl moiety are formed in the methylidyne -propylene system. For the methylidyne-methylacetylene/allene systems, 24 the initial cycloaddition intermediates were located at -334, -322, and -383 kJ mol -1 below the energies of the separated reactants, respectively.…”

“…Similar reaction dynamics also dominate the methylidyne-propylene system. 26 The final products 1-methylcyclopropene (Δ r G = -157 kJ mol -1 ) and 3-methylcyclopropene (Δ r G = -143 kJ mol -1 ) were formed in the bimolecular reaction of methylidyne and propylene with hydrogen atom loss from the initial intermediates via loose exit transition states located 9-15 kJ mol -1 above the separated products. These initial cycloaddition intermediates were located at -345 and -346 kJ mol -1 below the separated reactants, respectively.…”

Gas-Phase Formation of C5H6 Isomers via the Crossed Molecular Beam Reaction of the Methylidyne Radical (CH; X2Π) with 1,2-Butadiene (CH3CHCCH2; X1A′)

“…The rotational temperature of the methylidyne radicals were determined to be 14 ± 1 K via laser-induced fluorescence (LIF). 56 The pulsed (60 Hz) supersonic beam of the 1,2butadiene reactant was generated in the secondary source chamber with v p of 777 ± 12 m s −1 and S of 9.5 ± 0.3; the 1,2butadiene molecular beam crossed perpendicularly with the CH radical beam at a collision energy E C of 20.6 ± 0.4 kJ mol −1 and a center of mass (CM) angle Θ CM of 60.5 ± 0.3°. Each supersonic beam was produced via a piezoelectric pulse valve, which was operated at a repetition rate of 60 Hz, a pulse width of 80 μs, and a peak voltage of −400 V. The secondary pulsed valve was triggered 90 μs prior to the primary pulsed valve to account for the distinct velocities and nozzle-toskimmer distances thus allowing the best overlap and hence highest reactive scattering signal of the methylidyne radicals with 1,2-butadiene.…”