Branching Ratios in Reactions of OH Radicals with Methylamine

Jul 29, 2014 - A master equation analysis (using the MESMER package) based on potential energy surfaces from G4 theory was used to demonstrate that th...
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Branching Ratios in Reactions of OH Radicals with Methylamine, Dimethylamine, and Ethylamine Lavinia Onel, Mark Blitz, Matthew Dryden, Lucy Thonger, and Paul Seakins* School of Chemistry, University of Leeds, Leeds, LS2 9JT, U.K. S Supporting Information *

ABSTRACT: The branching ratios for the reaction of the OH radical with the primary and secondary alkylamines: methylamine (MA), dimethylamine (DMA), and ethylamine (EA), have been determined using the technique of pulsed laser photolysis−laserinduced fluorescence. Titration of the carbon-centered radical, formed following the initial OH abstraction, with oxygen to give HO2 and an imine, followed by conversion of HO2 to OH by reaction with NO, resulted in biexponential OH decay traces on a millisecond time scale. Analysis of the biexponential curves gave the HO2 yield, which equaled the branching ratio for abstraction at αC−H position, rαC−H. The technique was validated by reproducing known branching ratios for OH abstraction for methanol and ethanol. For the amines studied in this work (all at 298 K): rαC−H,MA = 0.76 ± 0.08, rαC−H,DMA = 0.59 ± 0.07, and rαC−H,EA = 0.49 ± 0.06 where the errors are a combination in quadrature of statistical errors at the 2σ level and an estimated 10% systematic error. The branching ratios rαC−H for OH reacting with (CH3)2NH and CH3CH2NH2 are in agreement with those obtained for the OD reaction with (CH3)2ND (d-DMA) and CH3CH2ND2 (d-EA): rαC−H,d‑DMA = 0.71 ± 0.12 and rαC−H,d‑EA = 0.54 ± 0.07. A master equation analysis (using the MESMER package) based on potential energy surfaces from G4 theory was used to demonstrate that the experimental determinations are unaffected by formation of stabilized peroxy radicals and to estimate atmospheric pressure yields. The branching ratio for imine formation through the reaction of O2 with α carbon-centered radicals at 1 atm of N2 are estimated as rCH2NH2 = 0.79 ± 0.15, rCH2NHCH3 = 0.72 ± 0.19, and rCH3CHNH2 = 0.50 ± 0.18. The implications of this work on the potential formation of nitrosamines and nitramines are briefly discussed.

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INTRODUCTION One of the most achievable, proposed carbon capture (CC) technologies uses CO2 absorption by aqueous solutions of amine-based compounds, typically monoethanol amine (MEA, HOCH2CH2NH2).1 The technique has been used for some time in natural gas “sweetening” at a relatively small scale. However, large-scale implementation of CC technology will likely result in a significant release of solvent amines and their degradation products, such as methylamine (MA) and dimethylamine (DMA), into the atmosphere.1,2 In addition, MA and DMA are emitted into the atmosphere from a wide range of sources including animal husbandry, food processing, marine sources, and biomass burning.3 Ethylamine has been used as a model compound for biomass combustion.4 Once the amines are released into the atmosphere, there will be a competition between heterogeneous uptake and gas phase oxidation, the latter primarily initiated by the reaction with the OH radical. Previous studies showed that the lifetime with respect to removal by OH is a few hours.5−7 amine + OH → prodcuts

abstraction at the N−H site: RCH 2NHR′ + OH → RCH 2NR′ + H 2O

where R and R′ = H or CH3 depending on the amine. For ethylamine (EA) the β position is an additional potential abstraction site. abstraction at the β site: CH3CH 2NH 2 + OH → CH 2CH 2NH 2 + H 2O

© 2014 American Chemical Society

(R1c)

However, at 298 K, the theoretical study of Galano et al. found that the branching ratio r1c (i.e., fraction of the overall reaction proceeding reaction R1c, k1c/k1) is only 0.004,8 and the experimental chamber study of Nielsen and co-workers reported a conservative upper limit to r1c of 0.1.9 Therefore, reaction R1c is a minor channel, a finding supported by analogy to ethanol where at room temperature OH abstraction at the β position is of minor importance.10 Abstraction from different sites will lead to different products and will dictate the atmospheric impact of amine chemistry. The atmospheric fate and toxicity of amines and their oxidation products has been subject of recent reviews.2,3,11 Of particular concern are the carcinogenic nitrosamines (R2N−NO) and

(R1)

Reaction with OH occurs via H abstraction at both the α position and at the amine group. abstraction at the α site: RCH 2NHR′ + OH → RCHNHR′ + H 2O

(R1b)

Received: Revised: Accepted: Published:

(R1a) 9935

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Scheme 1. Proposed Mechanism for DMA Oxidation Adapted from Nielsen et al.2

nitramines (R2N−NO2)12 which are formed through the channel initiated by abstraction of H from an N−H site (reaction R1b). As shown in a recent study using a chemistry transport model carried out by Karl et al. to assess the environmental impact of monoethanolamine and diethylamine released into atmosphere by a carbon capture plant, the sum of air ground level concentrations of R2N−NO and R2N−NO2 increases almost linearly with branching ratio r1b.13 However, while the overall rate coefficients for the reaction R1 of MA, DMA, and EA are wellknown,5−7 there are only a few, relatively indirect, experimental studies on the branching ratios r1a and r1b. Nielsen et al. performed model simulations to reproduce product data for MA, DMA, and EA and reported that the abstraction of α hydrogens dominates.9,14 The authors found that the abstraction of nitrogen bound hydrogen is a minor channel for primary alkylamines as r1b was 0.25 ± 0.05 for MA and 0.09 ± 0.01 for EA. Nielsen et al. found that abstraction from the amine group is more significant for the secondary amine DMA r1b = 0.42 ± 0.0514 in line with the measurement of Lindley et al., r1b = 0.37 ± 0.0515 and on reviews of relative bond strengths in the amines.16 Beside the branching ratios for the initial OH reaction, Nielsen and co-workers also provided estimations for the branching ratios of the subsequent steps in amine oxidation.2 Scheme 1 shows the major channels and branching ratios in the atmospheric oxidation of DMA proposed by these authors. Previously we found that in the presence of oxygen the reaction of OH with the tertiary amine trimethylamine (TMA) recycled OH, consistent with OH regeneration from a chemically activated peroxy radical.5 In contrast to the OH + TMA reaction, the reaction of OH with the alkylamines possessing N−H bonds DMA, MA, and EA did not regenerate OH when O2 was added. The lack of OH regeneration by OH + MA/DMA/EA in the presence of O2 is in line with the above mechanistic scheme where the radical generated by abstraction from α position, RCHNHR′, yields an HO2 radical by O2 direct abstraction from the N−H site.2 Another alternative is that RCHNHR′ leads to HO2 by an internal abstraction from amine group in an activated peroxy species, R′NHCH(R)O2*.17 The coproduct to HO2 generation will again be an imine: RCHNHR′ + O2 → RCH = NR′ + HO2

Analogies can be made between the reactions of OH + MA/ DMA/EA and the reactions of OH with CH3OH and CH3CH2OH, known to proceed predominantly by α abstraction and generate CH2OH and CH3CHOH, respectively.10,18 Extending the analogy to the reactions between O2 and hydroxymethyl/α-hydroxyethyl radicals, at low pressure reaction R2 will proceed completely through abstraction, generating HO2.10,19 At high pressure the reaction of O2 with RCHNHR′ potentially produces a collisionally stabilized peroxy radical, RCH(O2)NHR′.2,20 RCHNHR′ + O2 + M → RCH(O2 )NHR′ + M

(R2b)

In an atmosphere with sufficient NO, the main stable product following peroxy radical formation (R2b), is an amide, formed via reactions R3 and R4: RCH(O2 )NHR′ + NO → RCH(O)NHR′ + NO2

(R3)

RCH(O)NHR′ + O2 → RC(O)NHR′ + HO2

(R4)

2,9,14

According to Nielsen et al. at atmospheric pressure the branching fraction for peroxy radical formation is ∼45% for CH2NHCH3 and ∼15% for CH2NH2 and CH3CHNH2, respectively. This paper reports determinations of the yield of the HO2 radical in OH + DMA/MA/EA in the presence of oxygen and nitrogen monoxide. The HO2 formed by the product of R1a (RCHNHR′) reacting rapidly with oxygen (R2a), is monitored by conversion to OH by reaction with NO: HO2 + NO → OH + NO2

(R5)

The OH time profile was monitored using laser-induced florescence following the production of OH radicals via laser flash photolysis. The method provides the branching ratios r1a and r1b and was validated by confirming the reported branching ratios for OH reacting with methanol and ethanol. Additionally, the branching ratios r1a and r1b were measured for OD + (CH3)2ND/CH3CH2ND2, to be compared with those for OH + (CH3)2NH/CH3CH2NH2. The competition between the formation of HO2 by O2 abstraction and the generation of a stabilized peroxy species by O2 addition (R2a vs R2b) would

(R2a) 9936

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influence our results. The reactions were investigated as a function of pressure by performing master equation calculations using the MESMER (Master Equation Solver for Multi-Energywell Reactions)21 package to demonstrate that peroxy radical formation was insignificant under the experimental conditions and to estimate atmospheric pressure yields.

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EXPERIMENTAL SECTION Kinetic studies of the OH radical reactions with MA, DMA, and EA were performed by using laser flash photolysis−laser-induced fluorescence apparatus.5,22 The OH precursor, tertiary butylhydroperoxide (TBHP, (CH3)3COOH), amine, nitrogen monoxide (if used, >99.9%, BOC), oxygen (if used, 99.999%, BOC), and the nitrogen carrier gas (99.998%, BOC) flowed through calibrated mass flow controllers into a mixing manifold and introduced into a stainless steel reaction cell. The total pressure in the reaction cell was controlled via a needle valve between the cell and the rotary pump and measured using a capacitance manometer mounted on the reaction cell. The temperature in the cell was measured by using a K-type thermocouple located close to the observation region. Methylamines (all SigmaAldrich, MA ≥98%, DMA ≥99%, TMA ≥99%, EA ≥99%), methanol (Sigma-Aldrich ≥99.9%), TBHP (Sigma-Aldrich, 70% in H2O), and nitrogen monoxide were purified by freeze− pump−thaw cycles and were made up and stored as dilute mixtures in nitrogen in darkened five liter bulbs. OH radicals were generated by laser flash photolysis at 248 nm using a KrF excimer laser (Lambda Physik 210, 5 Hz, typically 5− 15 mJ pulse−1 cm−2) and probed by laser-induced fluorescence. Probe radiation at ∼282 nm was generated by a Nd:YAG pumped dye laser (Powerlite Precision II 8010, Sirah PRSC-DA24, operating with Rhodamine 6G dye). Fluorescence was detected through an interference filter (Andover, 308 ± 10 nm) by a photomultiplier tube (Thorn EMI model 9813 QKB) mounted on top of the reaction cell, perpendicular to the photolysis/probe laser plane. The delay between the photolysis and probe laser was controlled by a delay generator to build up an entire OH temporal profile with typically 6−12 laser shots averaged for each time delay. OH + MA, DMA, and EA Experiments in the Absence of O2/NO. The OH + amine experiments were performed under pseudo-first-order conditions, i.e., the concentration of amine was in large excess over the initial OH concentration ([amine]: [OH]0 >1000:1), hence in the absence of O2/NO the OH radicals decayed according to the single exponential equation: [OH] = [OH]0 exp(−k′OH t )

Figure 1. Bimolecular plot for NO + HO2 reaction. The HO2 radical was generated by OH + MA reaction in the presence of 5 × 1016 molecules cm−3 O2. The inset shows the single exponential OH decay in the absence of NO (red diamonds) and two biexponential decays in the presence of NO concentrations equal to 1.03 × 1014 molecules cm−3 (black triangles) and 3.61 × 1014 molecules cm−3 (blue triangles), respectively. Expanded logarithmic versions of the temporal profiles are presented in the Supporting Information (Figure S1). Studies performed at 298 K and a total pressure of 20 Torr of N2 + O2.

as 0.5−2 × 1011 molecules cm−3. The O2 concentration was ∼100 times higher than the NO concentration and typically [NO] = 1014−1015 molecules cm−3. The analysis of the biexponential OH kinetic traces generated under these conditions is described in the Results section. OD + Deuterated Amines (CH3)2ND and CH3CH2ND2 in the Presence of O2/NO. Deuterated TBHP (CH3)3COOD was used as OD precursor. In order to deuterate the amine and TBHP the carrier gas N2 was passed through a bubbler containing deuterated water (Sigma-Aldrich 99.9 atom % D). The N2 pressure in the bubbler was set just above atmospheric pressure and hence leads to a mixture of ∼0.025 D2O in N2, which then flowed through the mass flow controller. The D2O passivation was carried out continuously overnight before the experiments and during the kinetic measurements. The deuteration occurred by H/D exchange between the amine group of DMA/EA and peroxy group of TBHP with D2O in the 3−4 m of delivery tubing, before the gas entered into the cell. OD and OH signals were monitored in parallel under pseudo-firstorder conditions as in the experiments in the absence of D2O and a typical example is shown in Figure S2. Experiments to quantify the efficiency of deuteration to TBHP (typically 0.80−0.88) and EA (0.81−0.91) are also described in the Supporting Information.

(E1)

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and as the intensity of fluorescence signal, If, is proportional to the [OH], then: If (t ) = If (0) exp( −k′OH t )

MASTER EQUATION (MESMER) MODELING Master equation calculations have been carried out using the program MESMER (Master Equation Solver for Multi-Energywell Reactions)21,24 to investigate the fate of α carbon-centered radicals, RCHNHR, formed by reaction R1a. The required input parameters for the stationary points were obtained from the ab initio G4 calculations.25,26 The problem was set up by dividing the energy of the intermediate RCH(O2)NHR into grains, and the time evolution of the system was then obtained by solving the energy grained master equation,27,28 giving time-dependent concentrations of different species (equations S1 and S2). Rate constants were extracted from the chemically significant eigenvalues using a procedure similar to that described by Bartis and Widom.29 MESMER solves the master equation and the subsequent eigenvalue-eigenvector analysis, and outputs the

(E1a)

Here k′OH = k′1 + kloss, where k′1 is the overall pseudo-first-order rate coefficient for the reaction of OH with amine (R1) and kloss is the pseudo-first-order rate coefficient for OH loss by diffusion and reaction with TBHP. The measurement in the absence of nitrogen monoxide shown in the inset of Figure 1 is an example of such a single exponential decay. OH + MA, DMA, and EA in the Presence of O2/NO. Pseudo-first-order conditions were ensured by using amine, O2, and NO concentrations in large excess over the initial radical concentration, estimated from the photolysis flux, [TBHP] and TBHP absorption cross section at 248 nm of 1.99 × 10−20 cm223 9937

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concentration time-profile for each species together with the phenomenological rate constants of the system. More details on the MESMER calculations can be found in the Supporting Information.

k′5 = k5[NO] + kd

Here k5 is the bimolecular rate coefficient for the reaction of HO2 with NO, R5, and kd is the first-order rate coefficient for diffusional loss of HO2. The bimolecular rate coefficient is the gradient determined from a plot of k′5 as a function of [NO] (Figure 1), (1.11 ± 0.14) × 10−11 cm3 molecule−1 s−1 and has overlapping error bars with the recommended value by Atkinson et al., (8.8 ± 2.4) × 10−12 cm3 molecule−1 s−1, calculated as the mean of a set of absolute rate coefficients.34 We note that our value is ∼25% higher, but it is confirmed by the experiments where HO2 was generated from OH reacting with methanol/O2 and ethanol/O2 described in the next section giving k5 = (1.13 ± 0.11) × 10−11 and (1.05 ± 0.12) × 10−11 cm3 molecule−1 s−1, respectively. Figure S3 in the Supporting Information shows the bimolecular plot for NO reacting with HO2 generated by OH + methanol/O2 as an example. The biexponential parameters allowed the HO2 yield, ΦHO2, and hence r1a and r1b to be assigned:

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RESULTS Methodology for Assigning HO2 Yield. As shown in the Introduction, in the presence of oxygen, both α site abstraction (R1a) and amino group abstraction (R1b) channels generate the HO2 radical. However, the reaction of O2 with the carboncentered radical formed by R1a is fast, with a rate coefficient of ∼10−11 cm3 molecule−1 s−1,30 while the reaction between O2 and the amino radical generated by R1b is much slower, of 10−18− 10−19 cm3 molecule−1 s−1 order of magnitude.26,31 This ratio of reactivity of the carbon and nitrogen atom centered radicals is in qualitative agreement with studies of methanol/ethanol oxidation which show that the reactions of α-hydroxyalkyl radicals with oxygen are about 4 orders of magnitude faster than the reactions of alkoxy radicals with O2 and with rate coefficients of ∼10−11 cm3 molecule−1 s−1.32 Therefore, on our relatively short (ms) experimental time scale, only the channel initiated by R1a generated HO2 through O2 abstraction at N−H site of the carbon-centered radical. The formation of the HO2 radical was probed by adding NO to the system to regenerate OH (reaction R5). The method is illustrated schematically for MA: O2 ,slow

OH, k1b

ΦHO2 = r1a =

OH, k1a

NO

⎯⎯⎯⎯⎯→ CH 2NH + HO2 ⎯⎯→ OH

The inset in Figure 1 shows that the OH single exponential decay in the MA + OH reaction in the absence of nitrogen monoxide becomes biexponential when NO is added, where the long-time component of the biexponential equation is due to OH regeneration. The slow decay constant in the OH profile increased with increasing NO concentration, demonstrating that OH regeneration was governed by HO2 + NO reaction as expected under our experimental conditions where [O2]−[NO] ∼ 100:1. The biexponential OH decays for all OH + amine reactions were fitted to the solution of the rate equations for reactions R1a, R1b, and R5: I f (t ) =

O2 ,slow

O2 ,fast

OD, k1a

NO

The OD signal was analyzed using eq E2, and the obtained pseudo-first-order rate coefficients were used to calculate the yield of DO2 and the branching ratios rOD(a) and rOD(b):

where λ+,− = {−(k′OH + k′5) ± [(k′OH + k′5) − 4k′OH(b)k′5] }/ 2, k′OH = k′1 + kloss and k′OH(b) = k′1b + kloss. Here k′1 and k′1b are pseudo-first-order rate coefficients of reactions R1 and R1b, and kloss is the pseudo-first-order rate coefficient for the slow loss of OH radicals via diffusion, reaction with the OH precursor and R6. OH + NO + M → HONO + M

OD, k1b

⎯⎯⎯⎯⎯→ CH 2NCH3 + DO2 ⎯⎯→ OD

(E2) 2

(E4)

←⎯⎯⎯⎯⎯⎯⎯(CH3)2 N ←⎯⎯⎯⎯⎯⎯ (CH3)2 ND ⎯⎯⎯⎯⎯⎯→ CH 2(CH3)ND

If (0) ((k′OH − λ−) exp(λ+t ) − (k′OH − λ+) λ+ − λ− exp(λ−t ))

k′1a k′ = 1 − 1b = 1 − r1b k′1 k′1

The kinetic analysis using eq E2 was validated by fitting the experimental data using the numerical integrator package Kintecus.35 The numerical analysis used the chemistry scheme presented above for MA and determined the bimolecular rate coefficients of the reactions R1a and R1bwhich were used in eq E4 to calculate r1a and r1b. In contrast to the biexponential analysis the absolute value of k1 was not fixed in the numerical analysis. The results were within 3% of those obtained using the analytical expression E2, and examples of the fits using the two methods are presented in the Supporting Information (Figure S6). Varying the total loss rate for OH (reaction with amine and diffusional loss) by ±10% (the maximum difference from our earlier work5) caused variations in the yields of 0.81 0.98

0.22

0.17

(CH3)2ND

CH3CH2ND2

This work; at the relatively low used pressures, where k2a/k2 ≅ 1, the general expression for ΦHO2/ΦDO2, r(αC−H) × (k2a/k2), becomes r(αC−H). b r(αC−H) was determined using eq E2; errors are calculated using statistical errors at 2σ and estimates of systematic error at 10%. cr(αC−H) was determined using the bimolecular rate coefficients of the reactions R1a and R1b retrieved by fitting the experimental data using the numerical integration package Kintecus.35 The chemistry scheme used in the fitting included the reactions of H with O2 and NO2 and is presented in the Supporting Information. Errors are calculated using statistical errors at 2σ, estimates of systematic error at 10%, and estimates of propagation of error of H atom concentration at 5%. dLindley et al., long-path FTIR study.15 eNielsen et al., experiments in EUPHORE atmospheric chamber.9,14 fGalano and Alvarez-Idaboy, theoretical study.8 gTian et al., theoretical study.38 hOnel et al., theoretical calculations.5 a

A small additional OH signal was found in the system from H atoms reacting with trace impurities of NO2 and from the H + O2 reaction followed by the HO2 + NO reaction (R5).

obtained. Kinetic analysis via eq E2 was performed to retrieve the pseudo-first-order rate coefficients, and the HO2 yield was determined using eq E4. Our result for methanol ΦHO2 = 0.87 ± 0.04 (Supporting Information, Figure S4) is in very good agreement with r(αC−H) = 0.83 ± 0.13 found by Meier et al. using discharge flow−mass spectrometry and differs by ∼14% from r(αC−H) = 0.75 ± 0.08 determined by the same authors using discharge flow−laserinduced fluorescence.18 For ethanol ΦHO2 = 0.92 ± 0.05 at 20 Torr and 0.94 ± 0.05 at 80 Torr of N2 bath gas (Supporting Information, Figure S5), which is in excellent agreement with a branching fraction of 0.92 ± 0.08 for α site abstraction determined by Carr et al. over the pressure range 5−100 Torr, He bath gas, using kinetic isotope effects measured for partially deuterated ethanols.10 A further test to determine the impact of peroxy radicals on the regeneration of OH was via the TMA + OH/NO/O2 reaction. The reaction of OH with TMA in the presence of O2 directly regenerates OH, not HO2, by decomposition of an activated peroxy species in competition with collisional stabilization:5 (CH3)2 NCH 2 + O2 → OH + co‐products

NO2 + H → NO + OH(v = 2, 1, 0)

(R9)

The H atoms were generated in the reaction cell by TMA photolysis.5,37 Separate experiments performed in the same conditions as OH + amine reaction, except that, instead of amine, methylmercaptan (CH3SH) was used as a photolytic H atom precursor at 248 nm which showed that a 2−4% percent conversion of NO to NO2 occurred in the delivery tubing. Note that the cross sections for H atom formation of TMA and DMA at 248 nm are both of 10−20 cm2 orders of magnitude.37 Therefore, the HO2 yield of OH + DMA/O2/NO reaction was recalculated by taking into account the small additional OH signal due to the reactions of H atom generated by DMA photodissociation with O2 and NO2 (Table 1; see Supporting Information for the calculation details). The contribution of H atom reactions to the HO2 yield of OH + MA/EA reaction in the presence of O2/NO was negligible because the cross section for H formation of MA and EA at 248 nm is at least 1 order of magnitude smaller compared to that of DMA.37 Branching Ratios in OH Reactions with Amines in the Presence of O2/NO. According to eq E4 the yield of HO2 does not vary with the concentration of NO and O2 which indeed was confirmed by our experiments. Figure 2 is an example showing that ΦHO2 is independent of NO concentration. Table 1 shows the branching ratio for abstraction of α hydrogens for OH + MA, DMA and EA, and OD + d1-DMA/d2EA at room temperature and at several total pressures. Each value in Table 1 represents the mean of a series of measurements where the concentrations of either amine, NO, or O2 were varied, keeping O2 in excess over NO. Two sets of results are presented for DMA and d1-DMA: r(αC−H) obtained using the simple biexponential analysis and r(αC−H) obtained adding the reactions

(R7)

(CH3)2 NCH 2 + O2 + M → (CH3)2 NCH 2O2 + M (R8)

Experiments were carried out at a sufficiently high pressure, 100 Torr, that the direct OH regeneration in the presence of O2 through R7 occurred to a small extent, 0.13 ± 0.02, calculated using the Stern−Volmer plot obtained fitting the experimental data for OH/TMA/O2 by the solution of the appropriate kinetic equations.5 Therefore, at 100 Torr, the OH/TMA/O2 reaction yields 0.87 ± 0.02 stabilized peroxy species, (CH3)2NCH2O2, through reaction R8. No OH regeneration from (CH3)2NCH2O2/NO/O2 was found, confirming that the peroxy radical does not generate HO2 on the millisecond time scale of our measurements. 9939

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and Alvarez-Idaboy reported (48 C−H):(52 N−H), and we calculated (22 C−H):(78 N−H). We found that the branching ratios changed by ∼85% when the relative barrier heights were changed by 4 kJ mol−1, the typical uncertainty in high level barrier height calculations,5 which suggests that the accurate branching ratios are beyond calculation for the most common quantum chemical methods. Table 1 shows a decrease in r1a from MA to DMA which can be explained by a decrease in the dissociation enthalpy for the N−H bond from primary to secondary amines.16 A similar decrease in the branching ratio for α abstraction is observed on going from MA to EA. The fact that both αC−H and N−H abstraction channels are competitive suggests that only small changes in the energy barriers are required to bring about significant change. However, the energy barrier might only be a partial explanation as significant quantum mechanical tunnelling might be involved in these abstraction processes, especially when there is a significantly bound complex between OH and the amine, similar to that formed by OH and methanol.39 The slightly higher value of r(αC−H) for (CH3)2ND and CH3CH2ND2 compared to (CH3)2NH and CH3CH2NH2, respectively suggests that deuteration of amine group produces a small kinetic isotope effect (KIE) in favor of C−H abstraction. It can be assumed that the dependence of the rate coefficient for α hydrogen abstraction on the deuteration at N−H site is negligible and the observed KIE is due to a decrease in the rate coefficient for abstraction from amine group going from DMA/ EA to d1-DMA/d2-EA. MESMER Calculations. Table 1 shows that the HO2 yield in OH reaction with MA and DMA determined in this work does not vary significantly with total pressure, i.e., CH2NHR + O2 reaction proceeds close to 100% to imine + HO2 in the range of 20−80 Torr. However, a slight decrease in the HO2 yield can be observed for OH + EA reaction in the range of 20−150 Torr, which suggests a few percent collisional stabilization of the peroxy species, CH3CH(O2)NH2 formed by CH3CHNH2 + O2 reaction at 80 and 150 Torr. Master equation calculations were carried out using the MESMER package to verify that low yields of peroxy radical formation are expected under our experimental conditions and to estimate atmospheric branching ratios. Further details and discussion on the calculation can be found in the Supporting Information. The branching ratio between imine + HO2 formation and peroxy radical stabilization from the chemically activated peroxy species is primarily determined by the nature of the potential energy surface, taken from Maguta et al.,26 and the efficiency of collisional stabilization by the N2 bath gas, parametrized as , the average energy transferred in a downward collision. The experimental observation of ∼100% imine formation at the lowest experimental pressures of ∼20 Torr N2 were reproduced in the MESMER calculations for for N2 = 270 cm−1, a value in excellent agreement with our previous studies on peroxy radical stabilization in the TMA/O2 system and with previously reported for N2.30,40,41 A negative pressure dependence of the HO2 yield and a positive pressure dependence of the adduct yield were obtained for all studied reactions between oxygen and α carbon-centered radicals. The negative pressure dependence of the HO2 yield, ΓHO2, is weak for CH2NH2 + O2 and becomes stronger going to CH3NHCH2 + O2 and CH3CHNH2 + O2 (Supporting Information). For CH2NH2 + O2 our result, ΓHO2(1 atm) = 0.79 ± 0.15, is in agreement with ΓHO2(1 bar) = 0.87 found by Maguta et al. using a master equation model similar to that implemented in

Figure 2. HO2 yield as a function of NO concentration for DMA/OH in the presence of NO/O2. Studies carried out at 298 K, using 0.9 Torr of O2 of a total pressure of 20 Torr of N2 + O2. The HO2 yield was calculated using eq E2 followed by eq E4. The horizontal red line corresponds to a HO2 yield equal to 0.71.

of H with O2 and NO2 (2−4%, depending on the experimental conditions) to the chemistry scheme (Supporting Information) and using Kintecus package.35 The analysis performed by Kintecus took into account the small OH signal produced in the OD + d1-DMA experiments by the small fraction of undeuterated TBHP (Figure S2). Therefore, simultaneous fits of kinetic traces of OH/OD such as those presented in Figure S2 were performed. The mean of our measurements for MA over the pressures 20−60 Torr, r1a(MA) = 0.76 ± 0.08 is in very good agreement with the value 0.75 ± 0.05 found by Nielsen et al. using photooxidation experiments carried out in EUPHORE atmospheric chamber.14 The values of r1a(DMA), an average of 0.71 ± 0.08 over 20−80 Torr, were corrected by including the additional OH sources in the DMA experiments in the presence of NO/O2. The mean of the corrected values, 0.59 ± 0.07, is in very good agreement with the measurement of Lindley et al. using longpath FTIR spectroscopy, r1a(DMA) = 0.63 ± 0.0515 and the calculation of Nielsen et al., r1a(DMA) = 0.58 ± 0.05 using time profiles of end-products measured in EUPHORE.2 For the branching ratios in OH + EA reaction the only reported experimental study is that by Nielsen et al. which found r1b(EA) = 0.09 ± 0.01 by modeling the observed time profiles of N-nitro ethylamine.9 In comparison with the previous experimental studies our work used a kinetic method which directly provides the branching ratios. The measured branching ratios for αC−H abstraction are in qualitative agreement with a structure activity relationship (SAR) based on our earlier kinetic studies.5 From the OH + TMA rate coefficient at 298 K ((5.8 ± 0.5) × 10−11 cm3 molecule−1 s−1), a value of the site specific rate coefficient for abstraction from a methyl group, kOH+CH3−N, of (1.9 ± 0.2) × 10−11 cm3 molecule−1 s−1 can be estimated. Combining this SAR value with the experimental total rate coefficients, ktotal, for the reaction of OH with MA and DMA ((2.0 ± 0.1) × 10−11 and (6.3 ± 0.6) × 10−11 cm3 molecule−1 s−1, respectively) gives branching ratios for αC− H abstraction, i.e., r(αC−H) = ((n × kOH+CH3−N)/ktotal), where n = 1 for MA and 2 for DMA, of 0.95 ± 0.15 (MA) and 0.60 ± 0.14 (DMA), in agreement with the current direct measurements. The OH + amine branching ratios in the previous theoretical studies vary significantly. For MA Galano and Alvarez-Idaboy calculated (80 C−H):(20 N−H)8 in good agreement with the ratio of (74 C−H):(26 N−H) found by Tian et al.,38 both studies using a lower level of calculation. Our recent higher level calculations produced (47 C−H):(53 N−H).5 For DMA Galano 9940

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MESMER and the same PES as our calculations.26 The study of Rissanen et al. reported ∼0.20 HO2 formation, ∼0.40 peroxy radical formation and ∼0.40 dissociation back to CH2NH2 and O2 at 1 atm using MultiWell master equation simulations.30 However, both our and Maguta et al.’s studies did not find any contribution of the redissociation channel. For CH3NHCH2 we obtained an HO2 fraction at 1 atm of 0.72 ± 0.19; therefore, the HO2 yield of 0.55 found by Nielsen and co-workers is within our error limits.2 These authors reported that a branching ratio of 0.45 for the stabilization of peroxy radical route gives the best agreement between the simulated and the experimental concentration time profiles of the end-products formaldehyde and N-methylformamide. For CH3CHNH2 our yields at 1 atm 0.50 ± 0.18 HO2 and 0.50 ± 0.18 CH3CH(O2)NH2 agree well with the atmospheric pressure fractions ∼0.6 HO2 and ∼0.4 peroxy radical obtained by Rissanen et al.30 Atmospheric yields from all studies are tabulated in the Supporting Information. Atmospheric Implications. Abstraction from the N−H site potentially leads to carcinogenic nitrosamines and nitramines as can be seen on the right-hand side of Scheme 1.2 Nitrosamines are known to undergo photolysis in sunlight with a typical lifetime of less than an hour.2 EUPHORE experiments detected N-nitroso dimethylamine, but not nitrosamines formed by primary amines MA and EA, CH3NHNO, and CH3CH2NHNO.14 Theoretical studies showed that formation of CH3NHNO is followed by isomerization and subsequent reactions.42,43 However, while Tang et al. found that the subsequent chemistry produces an imine,42 da Silva proposed that CH2NN is formed.43 In contrast to nitrosamines, nitramines are more stable. Their atmospheric lifetime with respect to reaction with OH radicals is more than 3 days, and it is thought that their reactive uptake in the atmospheric aqueous phase may be significant.26 The nitrosamine and nitramine yield depends on the branching fraction for N−H abstraction. While r1b(MA), 0.24 ± 0.08, and r1b(DMA), 0.42 ± 0.07, are in very good agreement with the previous experimental studies,14,15 for ethylamine, r1b = 0.48 ± 0.06 is about five times higher than the value presented by Nielsen et al. in their report,9 suggesting that the carcinogenic compound production for EA is more significant than it was considered before. A major product of both αC−H and N−H abstractions is an imine, RCHNR. The gas phase chemistry of imines is almost unknown. Imines are soluble in water, hence most likely will be quickly removed by aqueous aerosols in troposphere.2 In the aqueous phase imines are known to hydrolyze to form ammonia or an amine and an aldehyde (R10).44 RCHNR + H 2O → RCHO + RNH 2

in the reaction of O2 with CH3CHNH2 where at 1 atm and in the presence of sufficient NO, 0.50 ± 0.18 of the CH3CHNH2 + O2 reaction leads to amide via the stabilized peroxy radical route. The gas phase atmospheric chemistry of amides was reviewed by Barnes et al.45 The small amides are soluble in water; hence uptake by wet aerosols and subsequent hydrolysis is a potential important sink for gas phase amides. Knowledge of the branching ratio between C−H and N−H abstraction by OH is only the first stage in quantifying the potential yields of nitrosamines and nitramines. This paper quantifies these yields, but further direct studies on RNH radicals with O2, NO, and NO2 would clearly be beneficial.

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ASSOCIATED CONTENT

S Supporting Information *

Details on deuteration experiments, MESMER calculations, the validation experiments with methanol and ethanol, and details on the minor corrections accounting for H atom generated OH. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], phone: +44 (0)113 3436568, fax: +44 (0)113 3436565. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from NERC (NE/1013474/ 1) and EPSRC (EP/J010871/1). The authors thank Prof. Nielsen, University of Oslo, for ongoing discussions and providing unpublished results from the Atmospheric Degradation of Amines (ADA) project supported by Masdar, Statoil, Vattenfall, Shell and the CLIMIT program under contracts 193438, 201604, and 208122.

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REFERENCES

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(R10)

In the usual slightly acidic conditions in tropospheric aerosol, the generated amines and ammonia will be mainly protonated and potentially form ammonium and amminium salts, while the aldehyde will partition between gas and liquid phase and participate in secondary chemistry. The imine yield in the route initiated by α hydrogen abstraction depends on the branching ratio in O2 reaction with carbon-centered radical generated by R1a. Our MESMER calculations resulted in 0.79 ± 0.15 and 0.72 ± 0.19 formation of HO2 + imine in the reactions of O2 with CH2NH2 and CH2NHCH3, respectively at atmospheric pressure, suggesting that the formation of amides through the O2 addition channel (R2b−R4) is a minor route in the atmospheric fate of CH2NH2 and CH2NHCH3 radicals. Amide production is more important 9941

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