Master Equation Model Research Articles

Glycine in a basket: protonated complexes of 1,1,n,n-tetramethyl[n](2,11)teropyrenophane (n = 7, 8, 9) with glycine in the gas-phase.

Protonated complexes composed of a basket-like host molecule 1,1,n,n-tetramethyl[n](2,11)teropyrenophanes (TMnTP) (n = 7, 8, 9) and glycine as a guest were studied in the gas phase by experimental and computational methods. Blackbody infrared radiative dissociation (BIRD) experiments of [(TMnTP)(Gly)]H+ not only provided the observed Arrhenius parameters (activation energies, Eobsa, and frequency factors, A) but also suggested the existence of two populations of isomeric complexes of [(TMnTP)(Gly)]H+, termed fast dissociating (FD) and slow dissociating (SD), due to their relative BIRD rate constants. Master equation modeling was conducted to obtain the threshold dissociation energies E0 of the host-guest complexes. The relative stabilities of the most stable of the n = 7, 8, or 9 [(TMnTP)(Gly)]H+ complexes followed the trend SD-[(TM7TP)(Gly)]H+ > SD-[(TM8TP)(Gly)]H+ > SD-[(TM9TP)(Gly)]H+ by both BIRD and energy resolved sustained off-resonance irradiation collision-induced dissociation experiments (ER-SORI-CID). Computed structures and energies of [(TMnTP)(Gly)]H+ were obtained using B3LYP-D3/6-31+G(d,p) and for all TMnTP molecules, the lowest-energy structures were ones where protonated glycine was within the cavity of the TMnTP, despite the TMnTP molecules having a proton affinity 100 kJ mol-1 higher than glycine. An independent gradient model based on the Hirshfeld partition (IGMH) and natural energy decomposition analysis (NEDA) were applied to visualize and reveal the nature of interactions between hosts and guest. The NEDA analysis suggested that the polarization (POL) component which described interactions between induced multipoles contributed the most to the [(TMnTP)(Gly)]H+ (n = 7, 8, 9) complexes.

Genetic algorithm optimization of a master equation cyclopentane oxidation model against time-resolved speciation experiments

We demonstrate a new approach to constraining rate coefficients in complex chemical reaction mechanisms by master equation (ME) modeling with genetic algorithm optimization, as illustrated for low-temperature cyclopentane oxidation. Our chemical model is based on calculated R• + O2 and •QOOH + O2 potential energy surfaces and is validated against time-resolved speciation experiments, both of which we recently reported (Sheps et al., J. Phys. Chem. A, 125 (2021), 4467). A global sensitivity analysis identifies key parameters of our ME-based sub-mechanism. These parameters are perturbed, and modeled species concentrations are compared with experimental results at 450 – 575 K, to constrain the rate parameters for two major reaction pathways: 1) isomerization of the peroxy radical, ROO•, via H transfer to the carbon-centered •QOOH, radical and 2) HO2 elimination from ROO• to form cyclopentene. The optimization yields temperature-dependent rate coefficients with associated uncertainties for isomerization that are approximately a factor of two lower than our initially calculated values and those from literature. In contrast, the rate coefficients for the HO2 elimination channel are similar to our initial model values and those from the literature at 450 – 800 K but increasingly exceed these values above 800 K. By combining our optimized sub-mechanism with a comprehensive cyclopentane combustion mechanism (Lockachari et al., Combust. Flame, 225 (2021), 255), we find good extrapolation to higher temperatures when tested against our time-resolved speciation data at 600 – 650 K and against literature jet-stirred reactor speciation data at 700 – 1200 K.

Simplified Multiple-Well Approach for the Master Equation Modeling of Blackbody Infrared Radiative Dissociation of Hydrated Carbonate Radical Anions.

Blackbody infrared radiative dissociation (BIRD) in a collision-free environment is a powerful method for the experimental determination of bond dissociation energies. In this work, we investigate temperature-dependent BIRD of CO3·-(H2O)1,2 at 250-330 K to determine water binding energies and assess the influence of multiple isomers on the dissociation kinetics. The ions are trapped in a Fourier-transform ion cyclotron resonance mass spectrometer, mass selected, and their BIRD kinetics are recorded at varying temperatures. Experimental BIRD rates as a function of temperature are fitted with rates obtained from master equation modeling (MEM), using the water binding energy as a fit parameter. MEM accounts for the absorption and emission of photons from black-body radiation, described with harmonic frequencies and infrared intensities from quantum chemical calculations. The dissociation rates as a function of internal energy are calculated by Rice-Ramsperger-Kassel-Marcus theory. Both single-well and multiple-well MEM approaches are used. Dissociation energies derived in this way from the experimental data are 56 ± 6 and 45 ± 3 kJ/mol for the first and second water molecules, respectively. They agree within error limits with the ones predicted by ab initio calculations done at the CCSD(T)/aug-cc-pVQZ//CCSD/aug-cc-pVDZ level of theory. We show that the multiple-well MEM approach described here yields superior results in systems with several low-lying minima, which is the typical situation for hydrated ions.

An experimental and computational investigation of the reaction between pent-1-en-3-yl radicals and oxygen molecules under autoignition conditions

We have measured the kinetics of the reaction between pent-1-en-3-yl (▪) radicals and oxygen molecules using laser-photolysis/photoionization mass spectrometry at temperatures relevant for autoignition (600–710 K). The rate coefficient of the title reaction was found to be relatively large for an allylic radical in the studied temperature range (1.27–1.79×10−15cm3 s−1). With such a large rate coefficient the studied reaction is expected to be an important sink of pent-1-en-3-yl radicals under autoignition conditions. Quantum chemical calculations and master equation simulations were performed to complement the experimental work. Experimental data was used to fix the values of key parameters in the master equation model. The model was then used to investigate the title reaction over a wide range of conditions (200–1500 K and 10−5 – 102 bar). The simulations predict that the title reaction mainly forms (E/Z)-pent-1,3-diene and hydroperoxyl at elevated temperatures (T > 500 K), but non-negligible amounts of (2R/S)-1,2-epoxypent-3-ene and hydroxyl are also formed. We found experimental evidence for both product channels, but it was not conclusive. Arrhenius representations are given for the product channels to facilitate the use of our results in combustion modelling.

Temperature and Pressure-Dependent Rate Constants for the Reaction of the Propargyl Radical with Molecular Oxygen.

Ab initio CCSD(T)/CBS(T,Q,5)//B3LYP/6-311++G(3df,2p) calculations have been conducted to map the C3H3O2 potential energy surface. The temperature- and pressure-dependent reaction rate constants have been calculated using the Rice–Ramsperger–Kassel–Marcus Master Equation model. The calculated results indicate that the prevailing reaction channels lead to CH3CO + CO and CH2CO + HCO products. The branching ratios of CH3CO + CO and CH2CO + HCO increase both from 18 to 29% with reducing temperatures in the range of 300–2000 K, whereas CCCHO + H2O (0–10%) and CHCCO + H2O (0–17%) are significant minor products. The desirable products OH and H2O have been found for the first time. The individual rate constant of the C3H3 + O2 → CH2CO + HCO channel, 4.8 × 10–14 exp[(−2.92 kcal·mol–1)/(RT)], is pressure independent; however, the total rate constant, 2.05 × 10–14 T0.33 exp[(−2.8 ± 0.03 kcal·mol–1)/(RT)], of the C3H3 + O2 reaction leading to the bimolecular products strongly depends on pressure. At P = 0.7–5.56 Torr, the calculated rate constants of the reaction agree closely with the laboratory values measured by Slagle and Gutman [Symp. (Int.) Combust.1988, 21, 875−883] with the uncertainty being less than 7.8%. At T < 500 K, the C3H3 + O2 reaction proceeds by simple addition, making an equilibrium of C3H3 + O2 ⇌ C3H3O2. The calculated equilibrium constants, 2.60 × 10–16–8.52 × 10–16 cm3·molecule–1, were found to be in good agreement with the experimental data, being 2.48 × 10–16–8.36 × 10–16 cm3·molecule–1. The title reaction is concluded to play a substantial role in the oxidation of the five-member radicals and the present results corroborate the assertion that molecular oxygen is an efficient oxidizer of the propargyl radical.

An experimental and theoretical kinetic study of t-butyl radical reaction with molecular oxygen

In this work, a unifying picture of the kinetics of the t-C4H9 + O2 reaction is presented by combining the current and previous experimental results with theory. Direct, time-resolved experiments were performed over a wide temperature range (200–500 K) at low pressures (0.3–6 Torr) using a photoionization mass spectrometry method. The kinetic measurements of the t-butyl + O2 reaction were initiated by laser photolysis of pinacolone at 193 nm or t-butyl bromide at 248 nm to produce t-C4H9 radicals. Energies calculated by quantum chemistry at the CCSD(T)/CBS and CASPT2/CBS levels of theory were used in master equation simulations of the kinetics of the t-C4H9 + O2 reaction. The calculations successfully reproduce the pressure and temperature dependencies of both the current low-pressure experiments and literature kinetic data at about atmospheric pressure as well as the literature kinetic data for the overwhelmingly most important bimolecular reaction channel, t-C4H9 + O2 → i-C4H8 + HO2 in the intermediate temperature range. The experimentally constrained master equation model was utilized to simulate the t-C4H9 + O2 reaction kinetics over wide range of conditions. The results of these simulations are provided in ChemKin compatible PLOG format for later use.

Polarization Spectroscopy Applied to Electromagnetically Induced Transparency in Hot Rydberg Atoms Using a Laguerre–Gaussian Beam

In this work, we have applied polarization spectroscopy to study electromagnetically induced transparency involving hot Rb85 Rydberg state in a vapor cell using a Laguerre–Gaussian mode beam. Such spectroscopy technique generates a dispersive signal, which allows a direct measurement of the transition linewidth. Our results show that the measured transition linewidth for a Laguerre–Gaussian mode control beam is narrower than for a Gaussian mode. Besides, it can be well reproduced by a simplified Lindblad master equation model.

Shock tube study of the pyrolysis kinetics of Di- and trimethoxy methane

The high potential of oxymethylene ethers (OMEs) and related compounds as fuels and fuel-additives motivated a multitude of experimental and theoretical investigations on, e.g., dimethoxy methane (DMM), the smallest member of the OME family. The present work adds to this research by providing combined experimental and theoretical rate coefficients for di- and trimethoxy methane (TMM) pyrolysis. For DMM pyrolysis, the branching ratios between the major dissociation pathways remained elusive in recent studies and is elucidated in the present work using four independent sets of shock tube experiments and master equation modeling. For TMM pyrolysis, the present work provides the very first detailed chemical kinetics model. A key consumption reaction of both compounds, DMM and TMM, is the methoxy-induced H-atom migration, which yields methanol and a singlet diradical. While for DMM this reaction is in direct competition to the C–O bond fission reactions, TMM is found to be a prime example for methoxy-induced H-atom migration, as its pyrolysis chemistry is exclusively governed by this reaction. With the present work, the details of DMM pyrolysis are elucidated and the foundation is laid for detailed chemical kinetics modeling of TMM.

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.

Ab initio and group theoretical study of properties of a carbon trimer defect in hexagonal boron nitride

Hexagonal boron nitride (h-BN) is a promising platform for quantum information processing due to its potential to host optically active defects with attractive optical and spin properties. Recent studies suggest that carbon trimers might be the defect responsible for single-photon emission in the visible spectral range in h-BN. In this theoretical study, we combine group theory together with density-functional theory (DFT) calculations to predict the properties of the neutral ${\mathrm{C}}_{2}{\mathrm{C}}_{\mathrm{N}}$ carbon trimer defect. We find the multi-electron states of this defect along with possible radiative and nonradiative transitions assisted by the spin-orbit and the spin-spin interactions. We also investigate the Hamiltonian for external magnetic-field and ground-state hyperfine interactions. Lastly, we use the results of our investigation in a Lindblad master-equation model to predict an optically detected magnetic resonance signal and the ${g}^{2}(\ensuremath{\tau})$ correlation function. Our findings can have important outcomes in quantum information applications such as quantum repeaters used in quantum networks and quantum sensing.

Using the Autler-Townes and ac Stark effects to optically tune the frequency of indistinguishable single photons from an on-demand source

We describe how a coherent optical drive that is near resonant with the upper rungs of a three-level ladder system, in conjunction with a short-pulse excitation, can be used to provide a frequency-tunable source of on-demand single photons. Using an intuitive master equation model, we identify two distinct regimes of device operation: (i) for a resonant drive, the source operates using the Autler-Townes effect, and (ii) for an off-resonant drive, the source exploits the ac Stark effect. The former regime allows for a large frequency-tuning range but coherence suffers from timing jitter effects, while the latter allows for high indistinguishability and efficiency, but with a restricted tuning bandwidth due to high required drive strengths and detunings. We show how both these negative effects can be mitigated by using an optical cavity to increase the collection rate of the desired photons. We apply our general theory to semiconductor quantum dots, which have proven to be excellent single-photon sources, and find that scattering of acoustic phonons leads to excitation-induced dephasing and increased population of the higher-energy level which limits the bandwidth of frequency tuning achievable while retaining high indistinguishability. Despite this, for realistic cavity and quantum dot parameters, indistinguishabilities of over $90%$ are achievable for energy shifts of up to hundreds of $\ensuremath{\mu}\mathrm{eV}$, and near-unity indistinguishabilities for energy shifts up to tens of $\ensuremath{\mu}\mathrm{eV}$. Additionally, we clarify the often-overlooked differences between an idealized Hong-Ou-Mandel two-photon interference experiment and its usual implementation with an unbalanced Mach-Zehnder interferometer, pointing out the subtle differences in the single-photon visibility associated with these different setups.

Rapid Allylic 1,6 H-Atom Transfer in an Unsaturated Criegee Intermediate.

A novel allylic 1,6 hydrogen-atom-transfer mechanism is established through infrared activation of the 2-butenal oxide Criegee intermediate, resulting in very rapid unimolecular decay to hydroxyl (OH) radical products. A new precursor, Z/E-1,3-diiodobut-1-ene, is synthesized and photolyzed in the presence of oxygen to generate a new four-carbon Criegee intermediate with extended conjugation across the vinyl and carbonyl oxide groups that facilitates rapid allylic 1,6 H-atom transfer. A low-energy reaction pathway involving isomerization of 2-butenal oxide from a lower-energy (tZZ) conformer to a higher-energy (cZZ) conformer followed by 1,6 hydrogen transfer via a seven-membered ring transition state is predicted theoretically and shown experimentally to yield OH products. The low-lying (tZZ) conformer of 2-butenal oxide is identified based on computed anharmonic frequencies and intensities of its conformers. Experimental IR action spectra recorded in the fundamental CH stretch region with OH product detection by UV laser-induced fluorescence reveal a distinctive IR transition of the low-lying (tZZ) conformer at 2996 cm-1 that results in rapid unimolecular decay to OH products. Statistical RRKM calculations involving a combination of conformational isomerization and unimolecular decay via 1,6 H-transfer yield an effective decay rate keff(E) on the order of 108 s-1 at ca. 3000 cm-1 in good accord with the experiment. Unimolecular decay proceeds with significant enhancement due to quantum mechanical tunneling. A rapid thermal decay rate of ca. 106 s-1 is predicted by master-equation modeling of 2-butenal oxide at 298 K, 1 bar. This novel unimolecular decay pathway is expected to increase the nonphotolytic production of OH radicals upon alkene ozonolysis in the troposphere.

Accurate and fast master equation modeling of triplet-triplet annihilation in organic phosphorescent emission layers including correlations

Triplet-triplet annihilation (TTA) in phosphorescent emission layers of modern organic light-emitting diodes compromises their performance and device lifetime. TTA can occur by a F\"orster-type interaction between two triplets, leading to a loss of one of them. The TTA process gives rise to correlations in the positions of the surviving triplets, which complicate its study. These correlations can in principle be accounted for exactly in kinetic Monte Carlo (KMC) simulations, but such simulations are computationally expensive. Here, we present master equation modeling of TTA that accounts for correlations in a computationally efficient way. Cases without and with triplet diffusion, which partly washes out correlations, are considered. We calculate the influence of TTA on transient photoluminescence experiments, where it leads to a deviation from exponential decay, and on steady-state emission efficiency. A comparison with KMC simulations shows that our master equation modeling is an accurate and computationally competitive alternative.

Vibrational-Specific Model of Simultaneous and Relaxation Under Postshock Conditions

The present paper discusses the development of an aerothermochemistry model that can be efficiently coupled with fluid dynamics simulations and retains the accuracy of the quantum-mechanical and molecular dynamic principles from which it is derived. The present model allows simulations of thermochemical nonequilibrium in nitrogen without relying on the existence of a vibrational temperature. A series of verification tests suggest that the vibrational-specific master equation model is sufficiently accurate and computationally efficient when applied to the modeling of dissociating nitrogen flows under adiabatic conditions. Toward this end, a kinetic dataset describing vibrational energy transfer and dissociation in and collisions is obtained on a high-fidelity potential energy surface, and curve fit coefficients are reported. Validation against shock-tube data indicates that the inclusion of electronically excited and ionic species is potentially required when computing and comparing relaxation parameters with the experimental data derived from the radiative signatures of nitrogen shock flows.

Unimolecular isomerisation of 1,5-hexadiyne observed by threshold photoelectron photoion coincidence spectroscopy.

The unimolecular isomerisation of the prompt propargyl + propargyl "head-to-head" adduct, 1,5-hexadiyne, to fulvene and benzene by the 3,4-dimethylenecyclobut-1-ene (DMCB) intermediate (all C6H6) was studied in the high-pressure limit by threshold photoelectron (TPE) spectroscopy. TPE spectra (TPES) were recorded with photoelectron photoion coincidence spectroscopy using synchrotron vacuum ultraviolet radiation. Reference TPES, obtained using pure compounds or judiciously extracted from the pyrolysis data, served as basis functions for pyrolysis quantification. From these spectra, we measured a revised fulvene ionisation energy of 8.401 ± 0.005 eV. Temperature-dependent pyrolysis spectra were decomposed using these basis functions. The basis function coefficients were converted to product yields relying on assumed integral threshold photoionisation cross sections obtained by three, partially mutually exclusive sets of assumptions. Thus, the product yields of DMCB, fulvene, and benzene have been established, as well as their uncertainty. The derived mole fractions are consistent with modeling based on the C6H6 potential and RRKM master equation model of Miller and Klippenstein [J. Phys. Chem. A, 2003, 107, 7783]. Although our results are fully consistent with the parallel isomerisation pathways to benzene and fulvene found by Miller and Klippenstein, we observe the onset of fulvene at a lower temperature than that of the onset of benzene, in agreement with the master equation model but in contrast to the previous experiments of Stein et al. [Proc. Combust. Inst., 1990, 23, 85]. This work promotes the use of photoion mass-selected threshold photoelectron spectroscopy as a rapid, sensitive, isomer-selective, and quantitative detection tool among the panoply of established analytical techniques.

Rotational energy transfer kinetics of optically centrifuged CO molecules investigated through transient IR spectroscopy and master equation simulations.

A combined experimental and theoretical study of quantum state-resolved rotational energy transfer kinetics of optically centrifuged CO molecules is presented. In the experiments, inverted rotational distributions of CO in rotational states up to J = 80 were prepared using two different optical centrifuge traps, one with the full spectral bandwidth of the optical centrifuge pulses, and one with reduced bandwidth. The relaxation kinetics of the high-J tail of the inverted distribution from each optical trap was determined based on high-resolution transient IR absorption measurements. In parallel studies, master equation simulations were performed using state-to-state rate constants for CO-CO collisions in states up to J = 90, based on data from double-resonance experiments for CO with J = 0-29 and a fit to a statistical power exponential gap model. The model is in qualitative agreement with the observed relaxation profiles, but the observed decay rate constants are smaller than the simulated values by as much as a factor of 10. The observed decay rate constants also have a stronger J-dependence than predicted by the model. The results are discussed in terms of angular momentum and energy conservation, and compared to the observed orientational anisotropy decay kinetics of optically centrifuged CO molecules. Models for rotational energy transfer could be improved by including angular momentum effects.

Global Master Equation Analysis of Rate Data for the Reaction C2H4 + H ⇄ C2H5: ΔfH0⊖C2H5.

While forward and reverse rate constants are frequently used to determine enthalpies of reaction and formation, this process is more difficult for pressure-dependent association/dissociation reactions, especially since the forward and reverse reactions are usually studied at very different temperatures. The problems can be overcome by using a data-fitting procedure based on a master equation model. This approach has been applied to existing experimental pressure-dependent forward and reverse rate coefficients for the reaction C2H4 + H ⇄ C2H5 (k1, k-1) using the MESMER code to determine ΔfH0⊖C2H5 from the enthalpy of the reaction. New measurements of k1, k-1 were included in analysis. They are based on laser flash photolysis with direct observation of H atom time profiles by vacuum ultraviolet laser-induced fluorescence under conditions where the approach to equilibrium could be observed. Measurements were made over the temperature range 798-828 K and with [He] from 2.33 to 7.21 × 1018 molecule cm-3. These data were then combined with a wide range of existing experimental data with helium as the bath gas (112 measurements of k1 and k-1, covering the temperature range 285-1094 K, and [He] = 7.1 × 1015-1.9 × 1019 molecule cm-3) and fitted using the master equation solver MESMER. The required vibrational frequencies and rotational constants of the system were obtained from ab initio calculations, and the activation threshold for association (ΔEthresh), enthalpy of reaction (ΔrH0⊖), imaginary frequency (υimag), and helium energy-transfer parameters (⟨ΔE⟩d,298(T/298)n) were optimized. The resulting parameters (errors are 2σ) are ΔEthresh = 11.43 ± 0.34 kJ mol-1, ΔrH0⊖ = -145.34 ± 0.60 kJ mol-1, υimag = 730 ± 130 cm-1, ⟨ΔE⟩d,298 = 54.2 ± 7.6 cm-1, and n = 1.17 ± 0.12. A value of ΔfH298.15⊖(C2H5) = 120.49 ± 0.57 kJ mol-1 is obtained by combining ΔrH0⊖ with standard enthalpies of formation for H and C2H4 and making the appropriate temperature corrections. The dependence of these parameters on how the internal rotor and CH2 inversion modes are treated has been explored. The experimental data for other bath gases have been analyzed, and data sets compatible with the potential energy surface parameters determined above have been identified. The parameters are virtually identical but with slightly smaller error limits. Parameterization of k1, k-1 using the Troe formalization has been used to investigate competition between ethyl decomposition and reaction with oxygen under combustion conditions.

New light on acetone: a master equation model for gas phase photophysics and photochemistry

This paper presents a unified model of gas phase acetone photophysics and photochemistry that is based on experimental data, which are reviewed and discussed in detail, and a detailed microcanonical master equation formulation. The model accounts for experimental measurements in molecular beams as well as in thermal systems, leads to a new interpretation of some experiments, and reveals a fundamental property of quantum yield measurements that are relevant to atmospheric chemistry. It also furnishes a platform for future theoretical and experimental developments.

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 kexp,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 BCH3/BCH2/BNH2 = 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 ΔvapH = 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.

Generalized Haus master equation model for mode-locked class-B lasers.

Using an asymptotic technique, we develop a generalized version of the class-B Haus partial differential equation mode-locking model that accounts for both the slow gain response to the averaged value of the field intensity and the fast gain dynamics on the scale comparable to the pulse duration. We show that unlike the conventional class-B Haus mode-locked model, our model is able to describe not only Q-switched instability of the fundamental mode-locked regime but also the leading edge instability leading to harmonic mode-locked regimes with the increase of the pump power.