Letter pubs.acs.org/JPCL
Pressure-Dependent I‑Atom Yield in the Reaction of CH2I with O2 Shows a Remarkable Apparent Third-Body Efficiency for O2 Haifeng Huang, Arkke J. Eskola, and Craig A. Taatjes* Combustion Research Facility, Mail Stop 9055, Sandia National Laboratories, Livermore, California 94551-0969, United States S Supporting Information *
ABSTRACT: The formation of I atom and Criegee intermediate (CH2OO) in the reaction of CH2I with O2 has potential relevance for aerosol and organic acid production in the marine boundary layer. We report measurements of the absolute yield of I atom as a function of pressure for N2, He, and O2 buffer at 298 K. Although the overall rate coefficient is pressureindependent, the I-atom yield, correlated with CH2OO, decreases with total pressure, presumably because of increased stabilization of CH2IOO. The extrapolated yield of the I + Criegee channel under tropospheric conditions is small but nonzero, ∼0.04. The zero-pressure limiting I-atom yield is unity, within experimental error, implying negligible branching to IO + CH2O. The apparent collision efficiency of O2 in stabilizing CH2IOO is a remarkable factor of 13 larger than that of N2, which suggests unusually strong interaction or possible reaction between the chemically activated CH2IOO# and O2. SECTION: Kinetics and Dynamics
T
(R1c) is at most a very minor channel. Instead, Eskola et al.8 found iodine atom to be the dominant product, although with their photoionization mass spectrometer they were unable to determine the isomeric identity of the CH2OO coproduct. The rate coefficient of the reaction (R1) has been found to be pressure-independent from 4 to 15 Torr in N23 and from 0.6 to 45 Torr in He.8 The work of Gravestock et al.6 implies that IO is mainly produced through secondary chemistry, and Welz et al.2 found that generation of IO initiated by reaction (R1) was slower than that of I atoms and Criegee radicals. The formation of I and CH2OO in reaction (R1) could affect models of aerosol formation in the marine boundary layer,10 where I atom is thought to be an important initiator of aerosols, but the yield of (I + CH2OO) in reaction (R1) and its dependence on pressure of different colliders are still unknown. In this letter, we measure the pressure-dependent I-atom yield in reaction (R1) by monitoring the absorption of generated I atoms (see the Experimental Methods section). Figure 1 shows typical I atom absorption signals. There are two time-resolved transient absorption traces shown in the plot: one taken with O2 and the other without O2. Both traces have had the off-resonance background subtracted (see the Supporting Information (SI) for more details). Whereas both show a near-instantaneous rise (faster than the 1 μs instrumental time resolution) of I-atom absorption upon photolysis, only the trace with O2 has an additional slower rising absorption, which corresponds to the I atom generated through channel (R1b). This slow rise is exponential, consistent with Scheme 1 for pseudo-first-order conditions in [O2]. The
ropospheric reactions of Criegee intermediates play a role in the budgets of sulfates, nitrates, and organic acids. Criegee intermediates are principally formed in ozonolysis,1 but recent experiments2 showed that the simplest Criegee intermediate, formaldehyde oxide CH2OO, is generated in the reaction CH2I + O2. There are extensive studies on this reaction3−9 because of its importance in tropospheric halogenated alkane chemistry.7−10 The proposed reaction mechanism is shown in Scheme 1. The overall second-order Scheme 1. Reaction (R1)
rate constant of reaction (R1) is defined to be k1. The initial chemically excited CH2IOO# complex can be stabilized by a third-body collider M through channel (R1a). The Criegee intermediate is generated through channel (R1b), with coproduct iodine (I) atom. Reaction channel (R1c) refers to any bimolecular channels that do not form I atoms, for example, production of formaldehyde and IO. Despite the body of research on CH2I + O2, an apparent discrepancy exists among the results from different experiments. Using cavity ring-down spectroscopic methods, Enami et al.5 claimed that the IO yield is substantial and that IO is a primary product of reaction (R1). However, Eskola et al.8 and Dillon et al.9 report that direct IO generation through reaction © 2012 American Chemical Society
Received: October 4, 2012 Accepted: November 5, 2012 Published: November 5, 2012 3399
dx.doi.org/10.1021/jz301585c | J. Phys. Chem. Lett. 2012, 3, 3399−3403
The Journal of Physical Chemistry Letters
Letter
1/yield = 1 + +
absorption signals of both traces decrease slowly at longer time because of diffusion and secondary reactions. Each I atom generated by photolysis corresponds to a CH2I radical, and each I atom of the slowly rising signal corresponds to the formation of a CH2OO, identified elsewhere as the Criegee intermediate CH2OO.2 Therefore, the absorption signal of generated I atoms is internally calibrated,11 and the ratio between the amplitudes of the slowly rising absorption signal and of the step-function rise is equal to the yield of I atom (and by inference, Criegee intermediate) in reaction (R1). In Figure 1, the solid curve is the fit of a kinetic model to the trace with O2. The data is modeled with a two-exponential function11 derived from the governing kinetic equations (see SI):
S1 S0
k2
[CH 2I 2]
(3)
Figure 2. Inverse of I-atom yield versus total density in He (blue solid square) and in N2 (green solid circle) buffer gases. For both measurements, the mole fractions of gas components in the mixture are fixed to 0.013% CH2I2, 20.006% O2, and 79.981% buffer gas. The two linear fits are: (0.91 ± 0.12) + (9.6 ± 1.8) × 10−19[total] (in He) and (0.98 ± 0.14) + (10.6 ± 2.4) × 10−19[total] (in N2), with [total] denoting total concentration. Both slopes are in units of cm3 molecule−1. In this article and the Supporting Information, unless specified, the errors of fitted parameters in an equation are of ±2σ. The error bars in the plot are at the ±1σ level, and all fits have been properly weighted by the uncertainties.
(1)
Here s(t) is time-resolved absorption signal, H(t) is the unit step (Heaviside) function, S0 is the amplitude of the stepfunction rise, S1 is the amplitude of the slowly rising component, and k′ is the pseudo-first-order rate constant of reaction (R1). The effective first-order rate coefficient kloss captures the slow loss of I atoms shown in Figure 1. Both k′ and kloss are in units of s−1. The second-order rate constant of reaction (R1) k1 is equal to the slope of a plot of k′ versus [O2]. On the basis of the simple Lindemann picture of Scheme 1 and a steady-state approximation for CH2IOO#, k′ can be expressed as the pseudo-first-order forward rate constant kf[O2] multiplied by the total branching ratio to channels (R1a), (R1b), and (R1c) (see the SI). The yield of I atom in reaction (R1), which is a function of pressure and colliders, is defined to be yield =
k′CH2I2
Equation 3 explicitly contains stabilization contributions from all colliders (with M standing for buffer gas He or N2). Following eq 3, the inverse of the yield has a linear dependence on the concentration of each collider. The coefficients in front of all concentrations in eq 3 are proportional to the collision efficiency of different colliders in stabilizing the CH2IOO complex. The zero-pressure intercept in eq 3, 1 + k3/k2, is unity plus the relative branching fraction to other bimolecular channel, that is, (R1c). Figure 2 displays the results of I atom yield versus total density, with mole fractions of all gas components in the
Figure 1. Time-resolved I-atom absorption signal at 10 Torr pressure in helium buffer gas. The unfilled squares are data with no O2, showing an instantaneous generation of I atoms from photolysis. The unfilled circles are taken with [O2] = 8.8 × 1015 molecule/cm3 and clearly display an additional slow rise of I-atom absorption after photolysis. For both traces, [CH2I2] = 6.5 × 1013 molecule/cm3. The solid line is the fit to eq 1.
⎛ ⎞ S1k′ (e−klosst − e−k ′ t )⎟ s(t ) = H(t )⎜S0e−klosst + k′ − kloss ⎝ ⎠
k′O2 k3 k′ + M [M] + [O2 ] k2 k2 k2
mixture fixed (0.013% CH2I2, 20.006% (≈ 1/5) O2, and 79.981% (≈ 4/5) He/N2 buffer). Figures S1 and S2 in the SI show similar plots where the number density of CH2I2 is held constant. The slopes in all three cases contain contributions of a combination of colliders; however, because the concentration of CH2I2 is small, the change in its concentration makes a negligible change in the slope and the intercept in a plot of inverse yield versus total concentration, [total]. The intercepts in all plots (given in the captions) are all very close to unity, implying that k3/k2 is a small number and that direct generation of IO (R1c) is a very minor (
