Article pubs.acs.org/JPCA
Kinetics and Product Branching Fractions of Reactions between a Cation and a Radical: Ar+ + CH3 and O2+ + CH3 Jordan C. Sawyer, Nicholas S. Shuman, Justin P. Wiens, and Albert A. Viggiano* Air Force Research Laboratory, Space Vehicles Directorate, Kirtland AFB, New Mexico 87117, United States 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 cm3 s−1 and has three product channels: → CH3+ + Ar (k = 5 ± 2 × 10−10 cm3 s−1), → CH2+ + H + Ar (k = 7 ± 2 × 10−10 cm3 s−1), → CH+ + H2 + Ar (k = 5 ± 3 × 10−11 cm3 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−.
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INTRODUCTION Ions and neutral radicals are often both produced in energetic plasma environments, for example, in combustion, etching, reentry, and ionospheric plasmas. These species are generally reactive, and the chemistry of ions with stable neutrals and of stable neutrals with radicals has been extensively explored; however, excepting reactions involving stable radicals (e.g., NO) or atomic species (e.g., H, N, O) very few studies have investigated reactions between ions and neutral radicals. In fact, in compilations of thermal ion−neutral reactions containing several thousand reactions from over 2000 studies through 2003,1−3 only a single reaction involving a polyatomic, unstable radical (OH) had been reported, and that only with an upper limit on the rate constant.4 Subsequent to those compilations, several groups have worked to fill this void in the literature. The Bierbaum and Ellison groups at the University of Colorado, Boulder, reported a series of experiments using a flowing afterglow-selected ion flow tube (FA-SIFT) to measure the reaction of H3O+ and OH− with organic radicals produced using a Chen-type5 supersonic pyrolysis nozzle.6,7 The experiments resulted in interesting mechanistic insights; however, the kinetics were hard to quantify due to the difficulty of determining the radical concentrations, and rate constants were reported to an order of magnitude. Similarly, product identification, mechanistic insight, and rough determination of rate constants were achieved by the Gross group using a Chen nozzle coupled to a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer for reactions of the benzyl radical cation with a variety of pyrolysis products including the allyl radical.8 Relatedly, ion-radical reactions have been used in chemical detection schemes. For instance, proton transfer from water-solvated H3O+ has been investigated as a detection scheme for peroxy radicals, producing the radicals by photolyzation in a flow reactor, with again rough estimates of the reaction rates being determined.9 Other chemical ionization mass spectrometry This article not subject to U.S. Copyright. Published XXXX by the American Chemical Society
schemes have been used to identify concentrations of atmospheric neutrals, some of which are radicals.10 The Chen nozzle used as a radical source in the prior kinetics studies has the advantages of cleanly producing radicals at a high intensity (∼1013 cm−3), but uncertainty in the absolute concentration limits the uncertainty of derived rate constants. Here, we present an alternative approach to generating radicals in a flow tube apparatus, resulting in lower densities (∼1010 cm−3), but with better quantification of the absolute concentration. The technique is a variant of the variable electron and neutral density attachment mass spectrometry (VENDAMS) method, described elsewhere for the measurement of electron attachment to radical species, mutual neutralization, and dissociative recombination processes.11 Briefly, a neutral precursor (e.g., CH3I) is introduced into a flowing afterglow (i.e., a weakly ionized plasma) and then rapidly undergoes dissociative electron attachment CH3I + e → CH3 + I−
0 ΔHr,298 = − 0.59 eV
(1)
The electron density, measured just before the point of precursor introduction using a Langmuir probe, is converted quickly to I−, which is produced in an equal amount to the methyl radical. The CH3 radical concentration is equal to the amount of electron depletion after accounting for the minor effects of diffusion and side reactions. This concentration of CH3 radicals is then available for subsequent kinetics measurements inside the flow tube. The experiments here do not directly vary the stable neutral concentration as is typical in flow tube studies, but rather use the VENDAMS method of varying the plasma density (and subsequently the radical concentration) and deriving kinetic information from the resulting product ion concentrations. Received: November 17, 2014 Revised: January 12, 2015
A
DOI: 10.1021/jp511500k J. Phys. Chem. A XXXX, XXX, XXX−XXX
The Journal of Physical Chemistry A
Article
recombination have been described elsewhere.11,15 Details of the VENDAMS technique to measure ion-radical reactions have not been reported previously. Production of the afterglow in the FALP follows a tried-andtrue recipe: a large (∼10−15 std. L min−1; all gases are added to the flow tube using mass flow control valves (MKS)) flow of helium (99.999%; Matheson) passes through a 1″ diameter Evenson-style microwave discharge cavity16 yielding He+, He2+, He*, and e− in concentrations of a few ppm. A smaller flow (200 std. cm3 min−1) of Ar (99.999%; Matheson) is added downstream, converting He2+ and He* into Ar+, resulting in a weakly ionized Ar+/e− plasma, with trace amounts of remnant He+ being minimized at higher pressures. The afterglow is carried by the neutral helium gas down a 1 m-long, 7 cmdiameter, stainless steel-lined glass flow tube. The center axis of the afterglow is sampled through a pinhole aperture at the terminus of the flow tube leading to a mass spectrometer comprising an ion lens stack, quadrupole mass filter, and electron multipler; the remainder of the gas is evacuated through a Roots pump. The absolute electron density in the afterglow is measured using a cylindrical Langmuir probe17 (consisting of a 7.6 mm length of 0.025 mm tungsten wire centered on the center axis of the flow tube) that is movable from 15 cm before to 40 cm past a gas inlet located near the center of the length of the flow tube. The electron density at the gas inlet is variable from ∼108 cm−3 (the lower limit reliably measured using the Langmuir probe) to ∼5 × 1010 cm−3, adjusted primarily by translating the microwave source further from or closer to the inlet. Both the ion velocity and the ambipolar diffusion rate are measured using the Langmuir probe. The velocity is measured by pulsing the microwave discharge and noting the time of arrival of the disturbance on the probe as a function of distance along the flow tube; a typical velocity at 300 K and 2 Torr is 50 m s−1 resulting in a reaction time of 10 ms. The diffusion rate is measured by monitoring the decay of the electron concentration as a function of distance along the flow tube; the typical ambipolar diffusion rate constant at 300 K and 2 Torr is 200 s−1. Without additional manipulation of the afterglow, Ar+ serves as the cation reactant. Other cations may be prepared by the addition of an appropriate precursor to undergo charge exchange with Ar+ (or an alternate noble gas). Here, studies were also done adding O2 through a reactant gas inlet (located 44 cm before the end of the flow tube) in sufficiently large concentrations to quickly (i.e., within 0.5 ms) convert the Ar+ to O2+. The radical reactant is prepared via dissociative electron attachment to a suitable neutral precursor added to the afterglow through the same gas inlet, although a separate mass flow control valve is used. The dissociative attachment must be relatively rapid (greater than ∼10−8 cm3 s−1) to produce a sufficient quantity of radical from a neutral concentration that is low enough to minimize the competing reaction between the neutral and the cation of interest. In this case CH3I (Sigma-Aldrich; >99%) is used as a precursor to yield I− and CH3 via reaction 1. To achieve sufficiently low concentrations of CH3I, a 0.1% mixture in helium is prepared, and that mixture is added to the afterglow at a flow of between 1 to 3 std. cm3 min−1, resulting in concentrations on the order of 1010 cm−3. A schematic representation of the FALP instrument along with modeled ion concentrations for a typical measurement of reaction 2 are shown in Figure 1.
The advantage of this method is that the shape of the product curves for individual species is highly dependent on the chemistry, giving a means to confirm that the assumed mechanism of production is correct. If the modeled shape does not match the data, the assumed chemistry must be wrong; conversely, matching the shape is an excellent indicator that the assumed chemistry is correct. The current work focuses on two reactions involving the methyl radical, each with multiple possible exothermic product channels. The first involving Ar+ is the simplest reaction amenable to study through this VENDAMS scheme and will be used to illustrate the experimental method and data analysis (ΔH0r,298 for eq 2 are taken from refs 12 and 13). 0 = − 5.92 eV Ar + + CH3 → Ar + CH3+, ΔHr,298 +
→ Ar + CH 2 + H,
0 ΔHr,298
(2.1)
= − 0.57 eV
(2.2)
0 → Ar + CH+ + H 2 , ΔHr,298 = − 0.46 eV
(2.3)
The initial aim of this work, through a series of measurements involving both the target radical and the neutral precursor, is to derive accurate kinetics for reactions 2.1−2.3. A unique feature of VENDAMS is that accurate partial rate constants are directly measured, the total rate constant being the sum of the partials. The VENDAMS technique also allows products, such as CH3+, from reaction 2 to be distinguished from the same species produced by the reaction between Ar+ and the precursor molecule, CH3I, by the shape of the curve. An analogous scheme may be applied to a variety of radicals, as discussed below. The cation species may also be varied, and is illustrated here through reaction with O2+, which has a large number of possible product channels: 0 = − 2.23 eV O2+ + CH3 → CH3+ + O2 , ΔHr,298 0 → HCO+ + H 2O, ΔHr,298 = −7.54 eV +
→ H 2CO + OH,
0 ΔHr,298
(3.1) (3.2)
= − 3.45 eV
(3.3)
0 → CH 2OH+ + O, ΔHr,298 = − 3.54 eV
(3.4)
+
→ CO + H 2O + H,
0 ΔHr,298
= − 3.22 eV
0 → H 2O+ + HCO, ΔHr,298 = −3.04 eV 0 = − 8.54 eV O2+ + CH3 → H3O+ + CO, ΔHr,298
(3.5) (3.6) (3.7)
0 → H 2+ + CO2 + H, ΔHr,298 = − 2.24 eV
(3.8)
0 → CO2+ + H 2 + H, ΔHr,298 = − 1.62 eV
(3.9)
+
→ COOH + H 2 ,
0 ΔHr,298
= − 7.29 eV
0 → HCOOH+ + H, ΔHr,298 = − 3.93 eV
→
CH(OH)+2 ,
0 ΔHr,298
