Kinetics and Product Branching Fractions of Reactions between a Cation and a Radical: Ar⁺ + CH₃ and O₂⁺ + CH₃

Abstract

A novel technique is described for the measurement of rate constants and product branching fractions of thermal reactions between cation and radical species. The technique is a variant of the variable electron and neutral density attachment mass spectrometry (VENDAMS) method, employing a flowing afterglow-Langmuir probe apparatus. A radical species is produced in situ via dissociative electron attachment to a neutral precursor; this allows for a quantitative derivation of the radical concentration and, as a result, a quantitative determination of rate constants. The technique is applied to the reactions of Ar(+) and O2(+) with CH3 at 300 K. The Ar(+) + CH3 reaction proceeds near the collisional rate constant of 1.1 × 10(-9) cm(3) s(-1) and has three product channels: → CH3(+) + Ar (k = 5 ± 2 × 10(-10) cm(3) s(-1)), → CH2(+) + H + Ar (k = 7 ± 2 × 10(-10) cm(3) s(-1)), → CH(+) + H2 + Ar (k = 5 ± 3 × 10(-11) cm(3) s(-1)). The O2(+) + CH3 reaction is also efficient, with direct charge transfer yielding CH3(+) as the primary product channel. Several results needed to support these measurements are reported, including the kinetics of Ar(+) and O2(+) with CH3I, electron attachment to CH3I, and mutual neutralization of CH3(+) and CH2(+) with I(-).

Velocity Map Imaging Study of Charge-Transfer and Proton-Transfer Reactions of CH3 Radicals with H3+

“…Russell et al 5 reported a Fourier transform ion cyclotron resonance study of the reactions of the benzyl cation with allyl radicals produced by flash pyrolysis, providing evidence for the formation of new C− C bonds. Most recently, the Viggiano lab 6 reported on the reactions of Ar + and O 2 + with CH 3 radicals produced by dissociative electron attachment to CH 3 I and monitored by variable electron and neutral density attachment mass spectrometry (VENDAMS). The study yielded accurate rate constants and demonstrated that charge transfer to form CH 3 + and fragments was the dominant process for both ions.…”

Velocity Map Imaging Study of Ion–Radical Chemistry: Charge Transfer and Carbon–Carbon Bond Formation in the Reactions of Allyl Radicals with C+

“…Ellison and Bierbaum and their co-workers 13,14 have measured rates of proton transfer from H 3 O + to allyl radicals and benzyne diradicals in a SIFT (selected ion flow tube) experiment. Most recently, the Viggiano lab 15 showed that reactions of Ar + and O 2 + with CH 3 radicals were dominated by charge transfer, consistent with the low ionization energies of free radicals. A recent publication from our laboratory 16 in which an ion beam was crossed by a beam of free radicals produced by pyrolysis and products were detected by VMI, examined both the proton transfer and charge transfer reaction of H 3 + with CH 3 .…”

A missing link in the nitrogen-rich organic chain on Titan

“…The reactions of ion-molecule involving open-shell species are generally more complex and unpredictable than those of closed-shell molecules. Recently, charge transfer, which occurs on a very short time scale and over long distances, has emerged as the dominant pathway in ion-radical reactions involving the methyl radical (Pei et al 2015;Sawyer et al 2015). Significant bond-forming exit channels have also been observed with sufficient internal energy to cause cleavage of the C-H bonds.…”