Theoretical and Experimental Study on the Reaction of tert-Butylamine with OH Radicals in the Atmosphere

Kinetics of the Simplest Criegee Intermediate CH2OO Reaction with tert-Butylamine

The kinetics of the simplest Criegee intermediate (CH₂OO) reaction with tert-butylamine ((CH₃)₃CNH₂) was studied under pseudo-first-order conditions with the OH laser-induced fluorescence (LIF) method at the temperature range of 283-318 K and the pressure range of 5-75 Torr. Our pressure-dependent measurement showed that at 5 Torr─the lowest pressure measured in the current experiment─this reaction was under the high-pressure limit condition. At 298 K, the reaction rate coefficient was measured to be (4.95 ± 0.64) × 10^-12 cm³ molecule^-1 s^-1. The title reaction was observed to be negative temperature-dependent; the activation energy of (-2.82 ± 0.37) kcal mol^-1 and the pre-exponential factor of (4.21 ± 0.55) × 10^-14 cm³ molecule^-1 s^-1 were derived from the Arrhenius equation. The rate coefficient of the title reaction is slightly larger than (4.3 ± 0.5) × 10^-12 cm³ molecule^-1 s^-1 of the CH₂OO reaction with methylamine; the electron inductive effect and the steric hindrance effect might play a role in contributing to such difference.

Atmospheric Chemistry of N-Methylmethanimine (CH3N═CH2): A Theoretical and Experimental Study

The OH-initiated photo-oxidation of N-methylmethanimine, CH₃N=CH₂, was investigated in the 200 m³ EUPHORE atmospheric simulation chamber and in a 240 L stainless steel photochemical reactor employing time-resolved online FTIR and high-resolution PTR-ToF-MS instrumentation and in theoretical calculations based on quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations forecast the OH reaction to primarily proceed via H-abstraction from the =CH₂ group and π-system C-addition, whereas H-abstraction from the −CH₃ group is a minor route and forecast that N-addition can be disregarded under atmospheric conditions. Theoretical studies of CH₃N=CH₂ photolysis and the CH₃N=CH₂ + O₃ reaction show that these removal processes are too slow to be important in the troposphere. A detailed mechanism for OH-initiated atmospheric degradation of CH₃N=CH₂ was obtained as part of the theoretical study. The photo-oxidation experiments, obstructed in part by the CH₃N=CH₂ monomer–trimer equilibrium, surface reactions, and particle formation, find CH₂=NCHO and CH₃N=CHOH/CH₂=NCH₂OH as the major primary products in a ratio 18:82 ± 3 (3σ-limit). Alignment of the theoretical results to the experimental product distribution results in a rate coefficient, showing a minor pressure dependency under tropospheric conditions and that can be parametrized k(T) = 5.70 × 10^–14 × (T/298 K)^3.18 × exp(1245 K/T) cm³ molecule^–1 s^–1 with k₂₉₈ = 3.7 × 10^–12 cm³ molecule^–1 s^–1. The atmospheric fate of CH₃N=CH₂ is discussed, and it is concluded that, on a global scale, hydrolysis in the atmospheric aqueous phase to give CH₃NH₂ + CH₂O will constitute a dominant loss process. N₂O will not be formed in the atmospheric gas phase degradation, and there are no indications of nitrosamines and nitramines formed as primary products.

OH Kinetics with a Range of Nitrogen-Containing Compounds: N-Methylformamide, t-Butylamine, and N-Methyl-propane Diamine

Emissions of amines and amides to the atmosphere are significant from both anthropogenic and natural sources, and amides can be formed as secondary pollutants. Relatively little kinetic data exist on overall rate coefficients with OH, the most important tropospheric oxidant, and even less on site-specific data which control the product distribution. Structure-activity relationships (SARs) can be used to estimate both quantities. Rate coefficients for the reaction of OH with t-butylamine (k₁), N-methyl-1,3-propanediamine (k₂), and N-methylformamide (k₃) have been measured using laser flash photolysis coupled with laser-induced fluorescence. Proton-transfer-reaction mass spectrometry (PTR-MS) has been used to ensure the reliable introduction of these low-vapor pressure N-containing compounds and to give qualitative information on products. Supporting ab initio calculations are presented for the t-butylamine system. The following rate coefficients have been determined: k_1,298K= (1.66 ± 0.20) × 10^-11 cm³ molecule^-1 s^-1, k(T)₁ = 1.65 × 10^-11 (T/300)^-0.69 cm³ molecule^-1 s^-1, k_2,293K = (7.09 ± 0.22) × 10^-11 cm³ molecule^-1 s^-1, and k_3,298K = (1.03 ± 0.23) × 10^-11 cm³ molecule^-1 s^-1. For OH + t-butylamine, ab initio calculations predict that the fraction of N-H abstraction is 0.87. The dominance of this channel was qualitatively confirmed using end-product analysis. The reaction of OH with N-methyl-1,3-propanediamine also had a negative temperature dependence, but the reduction in the rate coefficient was complicated by reagent loss. The measured rate coefficient for reaction 3 is in good agreement with a recent relative rate study. The results of this work and the literature data are compared with the recent SAR estimates for the reaction of OH with reduced nitrogen compounds. Although the SARs reproduce the overall rate coefficients for reactions, site-specific agreement with this work and other literature studies is less strong.

Atmospheric Chemistry of 2-Amino-2-methyl-1-propanol: A Theoretical and Experimental Study of the OH-Initiated Degradation under Simulated Atmospheric Conditions

The OH-initiated degradation of 2-amino-2-methyl-1-propanol [CH₃C(NH₂)(CH₃)CH₂OH, AMP] was investigated in a large atmospheric simulation chamber, employing time-resolved online high-resolution proton-transfer reaction-time-of-flight mass spectrometry (PTR-ToF-MS) and chemical analysis of aerosol online PTR-ToF-MS (CHARON-PTR-ToF-MS) instrumentation, and by theoretical calculations based on M06-2X/aug-cc-pVTZ quantum chemistry results and master equation modeling of the pivotal reaction steps. The quantum chemistry calculations reproduce the experimental rate coefficient of the AMP + OH reaction, aligning k(T) = 5.2 × 10^–12 × exp (505/T) cm³ molecule^–1 s^–1 to the experimental value k_exp,300K = 2.8 × 10^–11 cm³ molecule^–1 s^–1. The theoretical calculations predict that the AMP + OH reaction proceeds via hydrogen abstraction from the −CH₃ groups (5–10%), −CH₂– group, (>70%) and −NH₂ group (5–20%), whereas hydrogen abstraction from the −OH group can be disregarded under atmospheric conditions. A detailed mechanism for atmospheric AMP degradation was obtained as part of the theoretical study. The photo-oxidation experiments show 2-amino-2-methylpropanal [CH₃C(NH₂)(CH₃)CHO] as the major gas-phase product and propan-2-imine [(CH₃)₂C=NH], 2-iminopropanol [(CH₃)(CH₂OH)C=NH], acetamide [CH₃C(O)NH₂], formaldehyde (CH₂O), and nitramine 2-methyl-2-(nitroamino)-1-propanol [AMPNO₂, CH₃C(CH₃)(NHNO₂)CH₂OH] as minor primary products; there is no experimental evidence of nitrosamine formation. The branching in the initial H abstraction by OH radicals was derived in analyses of the temporal gas-phase product profiles to be B_CH3/B_CH2/B_NH2 = 6:70:24. Secondary photo-oxidation products and products resulting from particle and surface processing of the primary gas-phase products were also observed and quantified. All the photo-oxidation experiments were accompanied by extensive particle formation that was initiated by the reaction of AMP with nitric acid and that mainly consisted of this salt. Minor amounts of the gas-phase photo-oxidation products, including AMPNO₂, were detected in the particles by CHARON-PTR-ToF-MS and GC×GC-NCD. Volatility measurements of laboratory-generated AMP nitrate nanoparticles gave Δ_vapH = 80 ± 16 kJ mol^–1 and an estimated vapor pressure of (1.3 ± 0.3) × 10^–5 Pa at 298 K. The atmospheric chemistry of AMP is evaluated and a validated chemistry model for implementation in dispersion models is presented.

New Mechanistic Insights into Atmospheric Oxidation of Aniline Initiated by OH Radicals

This study theoretically reports the comprehensive kinetic mechanism of the aniline + OH reaction in the range of 200-2000 K and 0.76-7600 Torr. The temperature- and pressure-dependent behaviors, including time-resolved species profiles and rate coefficients, were studied within the stochastic RRKM-based master equation framework with the reaction energy profile, together with molecular properties of the species involved, characterized at the M06-2X/aug-cc-pVTZ level. Hindered internal rotation and Eckart tunneling treatments were included. The H-abstraction from the -NH₂ moiety (to form C₆H₅NH (P1)) is found to prevail over the OH-addition on the C atom at the ortho site of aniline (to form 6-hydroxy-1-methylcyclohexa-2,4-dien-1-yl (I2)) with the atmospheric rate expressions (in cm³/molecule/s) as k^abstraction(P1) = 3.41 × 10¹ × T^-4.56 × exp (-255.2 K/T) for 200-2000 K and k^addition(I2) = 3.68 × 10⁹ × T^-7.39 × exp (-1163.9 K/T) for 200-800 K. The U-shaped temperature-dependent characteristics and weakly positive pressure dependence at low temperatures (e.g., T ≤ 800 K and P = 760 Torr) of k_total(T) are also observed. The disagreement in k_total(T) between the previous calculations and experimental studies is also resolved, and atmospheric aniline is found to be primarily removed by OH radicals (τ_OH ∼ 1.1 h) in the daytime. Also, via TD-DFT simulations, it is recommended to include P1 and I2 in any atmospheric photolysis-related model.

Experimental and Theoretical Study of the OH-Initiated Degradation of Piperazine under Simulated Atmospheric Conditions

The OH-initiated photo-oxidation of piperazine and 1-nitropiperazine as well as the photolysis of 1-nitrosopiperazine were investigated in a large atmospheric simulation chamber. The rate coefficient for the reaction of piperazine with OH radicals was determined by the relative rate method to be k_{OH-piperazine} = (2.8 ± 0.6) × 10^–10 cm³ molecule^–1 s^–1 at 307 ± 2 K and 1014 ± 2 hPa. Product studies showed the piperazine + OH reaction to proceed both via C–H and N–H abstraction, resulting in the formation of 1,2,3,6-tetrahydropyrazine as the major product and in 1-nitropiperazine and 1-nitrosopiperazine as minor products. The branching in the piperazinyl radical reactions with NO, NO₂, and O₂ was obtained from 1-nitrosopiperazine photolysis experiments and employed analyses of the 1-nitropiperazine and 1-nitrosopiperazine temporal profiles observed during piperazine photo-oxidation. The derived initial branching between N–H and C–H abstraction by OH radicals, k_N–H/(k_N–H + k_C–H), was 0.18 ± 0.04. All experiments were accompanied by substantial aerosol formation that was initiated by the reaction of piperazine with nitric acid. Both primary and secondary photo-oxidation products including 1-nitropiperazine and 1,4-dinitropiperazine were detected in the aerosol particles formed. Corroborating atmospheric photo-oxidation schemes for piperazine and 1-nitropiperazine were derived from M06-2X/aug-cc-pVTZ quantum chemistry calculations and master equation modeling of the pivotal reaction steps. The atmospheric chemistry of piperazine is evaluated, and a validated chemical mechanism for implementation in dispersion models is presented.

Bulk Organic Aerosol Analysis by Proton-Transfer-Reaction Mass Spectrometry: An Improved Methodology for the Determination of Total Organic Mass, O:C and H:C Elemental Ratios, and the Average Molecular Formula

We have recently shown in this journal (Müller et al. Anal. Chem. 2017 , 89 , 10889 - 10897 ) how a proton-transfer-reaction mass spectrometry (PTR-MS) analyzer measured particulate organic matter in urban atmospheres using the "Chemical Analysis of Aerosol Online" (CHARON) inlet. Our initial CHARON studies did not take into account fragmentation of protonated analyte molecules, which introduced a small but significant negative bias in the determination of bulk organic aerosol parameters. Herein, we studied the ionic fragmentation of 26 oxidized organic compounds typically found in atmospheric particles. This allowed us to derive a correction algorithm for the determination of the bulk organic mass concentration, m_OA, the bulk-average hydrogen-to-carbon ratio, (H:C)_bulk, the bulk-average oxygen-to-carbon ratio, (O:C)_bulk, and the bulk-average molecular formula, MF_bulk. The correction algorithm was validated against AMS data using two sets of published data. Finally, we determined MF_bulk of particles generated from the reaction of α-pinene and ozone and compared and discussed the results in relation to the literature.

Piperazine Enhancing Sulfuric Acid-Based New Particle Formation: Implications for the Atmospheric Fate of Piperazine

Piperazine (PZ), a cyclic diamine, is one of 160 detected atmospheric amines and an alternative solvent to the widely used monoethanolamine in post-combustion CO₂ capture. Participating in H₂SO₄ (sulfuric acid, SA)-based new particle formation (NPF) could be an important removal pathway for PZ. Here, we employed quantum chemical calculations and kinetics modeling to evaluate the enhancing potential of PZ on SA-based NPF by examining the formation of PZ-SA clusters. The results indicate that PZ behaves more like a monoamine in stabilizing SA and can enhance SA-based NPF at the parts per trillion (ppt) level. The enhancing potential of PZ is less than that of the chainlike diamine putrescine and greater than that of dimethylamine, which is one of the strongest enhancing agents confirmed by ambient observations and experiments. After the initial formation of the (PZ)₁(SA)₁ cluster, the cluster mainly grows by gradual addition of SA or PZ monomer, followed by addition of (PZ)₁(SA)₁ cluster. We find that the ratio of PZ removal by NPF to that by the combination of NPF and oxidations is 0.5-0.97 at 278.15 K. As a result, we conclude that participation in the NPF pathway could significantly alter the environmental impact of PZ compared to only considering oxidation pathways.

Detailed kinetics of tetrafluoroethene ozonolysis

The C2F4 + O3 reaction plays an important role in the oxidation process of perfluoroalkenes in the atmosphere. The detailed reaction mechanism was explored using the accurate electronic structure method, CCSD(T)/CBS//B3LYP/aug-cc-pVTZ. The 1,3-cycloaddition of O3 with C2F4 to form the primary ozonide was found to be the rate-determining step of the oxidation process with a small barrier (i.e., 7.3 kcal mol-1 at 0 K). The temperature- and pressure-dependent behaviors of the title reaction were characterized in the range of 200-1000 K & 0.1-760 Torr using the integrated deterministic and stochastic master equation/Rice-Ramsperger-Kassel-Marcus (ME/RRKM) rate model with the inclusion of the corrections for anharmonicity and tunneling treatments. It is found that the anharmonic effect plays a role in the kinetic behaviors (e.g., lower the rate by a factor of ∼ two at 298 K) while the tunneling correction is insignificant. The total rate constants were found to be pressure-independent under the considered conditions, shown as ktot(T) = 4.80 × 10-23 × T2.69 × exp(-2983.4 K/T) (cm3 per molecule per s), which confirms the latest experimental data by Acerboni et al. (G. Acerboni, N. R. Jensen, B. Rindone and J. Hjorth, Chem. Phys. Lett., 1999, 309, 364-368); thus this study helps to resolve a long-term controversy among the previous measurements. The sensitivity analyses on the derived rate coefficients and time-resolved species mole fraction with respect to the ab initio input parameters were also performed to further understand as well as quantify the kinetic behaviors for the title reaction.