Study of Hydroxyl Reactions with Methanol and Ethanol by Laser-induced Fluorescence

Analytical potential energy surface and dynamics for the OH + CH3OH reaction

Using as functional form a combination of valence bond and mechanic molecular terms a new full-dimensional potential energy surface was developed for the title reaction, named PES-2022, which was fitted to high-level ab initio calculations at the coupled-cluster singles, doubles, and perturbative triples-F12 explicitly correlated level on a representative number of points describing the reactive system. This surface simultaneously describes the two reaction channels, hydrogen abstraction from the methyl group [(R1) path] and from the alcohol group [(R2) path] of methanol to form water. PES-2022 is a smooth and continuous surface, which reasonably describes the topology of this reactive system from reactants to products, including the intermediate complexes present in the system. Based on PES-2022 an exhaustive dynamics study was performed using quasi-classical trajectory calculations under two different initial conditions: at a fixed room temperature, for direct comparison with the experimental evidence and at different collision energies, to analyze possible mechanisms of reaction. In the first case, the available energy was mostly deposited as water vibrational energy, with the vibrational population inverted in the stretching modes and not inverted in the bending modes, reproducing the experimental evidence. In the second case, the analysis of different dynamics magnitudes (excitation functions, product energy partitioning, and product scattering distributions), allows us to suggest different mechanisms for both (R1) and (R2) paths: a direct mechanism for the (R2) path vs an indirect one, related with "nearly trapped" trajectories in the intermediate complexes, for the (R1) path.

Temperature and Pressure Dependent Rate Coefficients for the Reaction of Ketene with Hydroxyl Radical

The reaction of ketene with hydroxyl radical is drawing growing attention, for it is found to constitute an important step during the combustion of hydrocarbon and oxygenated hydrocarbon fuels, e.g., acetylene, propyne, allene, acetone, gasoline, diesel, jet fuels, and biofuels. We studied the potential energy surface (PES) of this reaction using B2PLYP-D3/cc-PVTZ for geometry optimization and composite methods based on CCSD(T)-F12/cc-PVTZ-F12 for energy calculations. From this PES, temperature- and pressure-dependent rate coefficients and branching ratios at 200-3000 K and 0.01-100 atm were derived using the RRKM/ME approach. The reaction is dominated by four product channels: (i) OH addition on the olefinic carbon of ketene to form CH₂OH + CO, which is the most dominant under all conditions; (ii) H abstraction producing HCCO + H₂O, which is favored at high temperatures; (iii) OH addition on the carbonyl carbon to form CH₃ + CO₂, which is favored at low pressures and high temperatures; and (iv) collisional stabilization of CH₂COOH, which is favored at high pressures and low temperatures. With increasing temperatures, the overall rate constant k_overall exhibit first negative but then positive temperature dependency, with its switching point (also the minimum point) at ∼400 K. Both product channel CH₂OH + CO and HCCO + H₂O are independent of pressure, whereas formation of CH₃ + CO₂ and collisional stabilization of CH₂COOH are highly pressure dependent. Fitted modified Arrhenius expressions of the calculated rate constants are provided for the purpose of combustion modeling.

Investigation of Oxidation Reaction Products of 2-Phenylethanol Using Synchrotron Photoionization

A photolytically Cl-initiated oxidation reaction of 2-phenylethanol (2PE) was carried out at the Advanced Light Source (ALS) in the Lawrence Berkeley National Laboratory. Using the multiplex photoionization mass spectrometer, coupled with the tunable vacuum ultraviolet radiation of the ALS, data were collected at low pressure (4-6 Torr) and temperature (298-550 K) regimes. Data analysis was performed via characterization of the reaction species photoionization spectra and kinetic traces. Products and reaction pathways are also computed using the CBS-QB3 composite method. The present results suggest primary products m/ z = 30 (formaldehyde), 106 (benzaldehyde), and 120 (phenylacetaldehyde) at 298 K, and m/ z = 120 (phenylacetaldehyde) at 550 K. Branching fractions at room temperature are 27 ± 6.5% for formaldehyde, 24 ± 4.5% for benzaldehyde, and 25 ± 5.8% for phenylacetaldehyde and 60 ± 14% for phenylacetaldehyde at 550 K.

The reaction of fluorine atoms with methanol: yield of CH3O/CH2OH and rate constant of the reactions CH3O + CH3O and CH3O + HO2

Measurement and theory of CH₃O + HO₂ and CH₃O + CH₃O reactions, product yields for F + CH₃OH.

High-temperature rate constants for CH3OH + Kr --> products, OH + CH3OH --> products, OH + (CH3)(2)CO --> CH2COCH3 + H2O, and OH + CH3 --> CH) + H2O

The reflected shock tube technique with multipass absorption spectrometric detection of OH radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the dissociation of methanol between 1591 and 2865 K. Rate constants for two product channels [CH3OH + Kr --> CH3 + OH + Kr (1) and CH3OH + Kr --> 1CH2 + H2O + Kr (2)] were determined. During the course of the study, it was necessary to determine several other rate constants that contributed to the profile fits. These include OH + CH3OH --> products, OH + (CH3)2CO --> CH2COCH3 + H2O, and OH + CH3 --> 1,3CH2 + H2O. The derived expressions, in units of cm(3) molecule(-1) s(-1), are k(1) = 9.33 x 10(-9) exp(-30857 K/T) for 1591-2287 K, k(2) = 3.27 x 10(-10) exp(-25946 K/T) for 1734-2287 K, kOH+CH3OH = 2.96 x 10-16T1.4434 exp(-57 K/T) for 210-1710 K, k(OH+(CH3)(2)CO) = (7.3 +/- 0.7) x 10(-12) for 1178-1299 K and k(OH+CH3) = (1.3 +/- 0.2) x 10(-11) for 1000-1200 K. With these values along with other well-established rate constants, a mechanism was used to obtain profile fits that agreed with experiment to within <+/-10%. The values obtained for reactions 1 and 2 are compared with earlier determinations and also with new theoretical calculations that are presented in the preceding article in this issue. These new calculations are in good agreement with the present data for both (1) and (2) and also for OH + CH3 --> products.

Kinetics and Rate Constants of the Reaction CH2OH + O2 → CH2O + HO2 in the Temperature Range of 236−600 K

The kinetics and absolute rate constants of the gas-phase reaction of the hydroxymethyl radical (CH2OH) with molecular oxygen have been studied using laser photolysis/near-IR absorption spectroscopy. The reaction was tracked by monitoring the time-dependent changes in the production of the hydroperoxy radical (HO2) concentration. For sensitive detection of HO2, two-tone frequency modulation absorption spectroscopy was used in combination with a Herriott-type optical multipass absorption cell. Rate constants were determined as a function of temperature (236 K<T<600 K) at 50 Torr of N2. The experimental results exhibit a slight negative temperature dependence in the measured temperature region. Microcanonical variational transition state theory was used to estimate the rate constants of the investigated reaction. The results of the theoretical calculations also suggest a negative temperature dependence of the reaction rate constant in the measured temperature region.

The Reaction of Acetylene with Hydroxyl Radicals

The potential energy surface for the reaction between OH and acetylene has been calculated using the RQCISD(T) method and extrapolated to the complete basis-set limit. Rate coefficients were determined for a wide range of temperatures and pressures, based on this surface and the solution of the one-dimensional and two-dimensional master equations. With a small adjustment to the association energy barrier (1.1 kcal/mol), agreement with experiments is good, considering the discrepancies in such data. The rate coefficient for direct hydrogen abstraction is significantly smaller than that commonly used in combustion models. Also in contrast to previous models, ketene + H is found to be the main product at normal combustion conditions. At low temperatures and high pressures, stabilization of the C2H2OH adduct is the dominant process. Rate coefficient expressions for use in modeling are provided.