Glycolaldehyde + OH Gas Phase Reaction: A Quantum Chemistry + CVT/SCT Approach

Reaction of HO with Glycolaldehyde, HOCH2CHO: Rate Coefficients (240−362 K) and Mechanism

Absolute rate coefficients for the title reaction, HO+HOCH2CHO-->products (R1), were measured over the temperature range 240-362 K using the technique of pulsed laser photolytic generation of the HO radical coupled to detection by pulsed laser induced fluorescence. Within experimental error, the rate coefficient, k1, is independent of temperature over the range covered and is given by k1(240-362 K)=(8.0+/-0.8)x10(-12) cm3 molecule-1 s-1. The effects of the hydroxy substituent and hydrogen bonding on the rate coefficient are discussed based on theoretical calculations. The present results, which extend the database on the title reaction to a range of temperatures, indicate that R1 is the dominant loss process for HOCH2CHO throughout the troposphere. As part of this work, the absorption cross-section of HOCH2CHO at 184.9 nm was determined to be (3.85+/-0.2)x10(-18) cm2 molecule-1, and the quantum yield of HO formation from the photolysis of HOCH2CHO at 248 nm was found to be (7.0+/-1.5)x10(-2).

Mechanism of the OH-Initiated Oxidation of Glycolaldehyde over the Temperature Range 233−296 K

The mechanism of the gas-phase OH-initiated oxidation of glycolaldehyde (HOCH(2)CHO) was studied in the 233-296 K temperature range using a turbulent flow reactor coupled with a chemical ionization mass spectrometer. In the presence of O2, formaldehyde, CO2, formic acid, and glyoxal were observed at room temperature with the yields of 80, 34, 18, and 14%, respectively. Decrease of temperature to 233 K led to significant changes in the yields of the stable products: those of formaldehyde and glyoxal decreased to 50 and 4%, respectively, whereas that of formic acid increased to 52%. It was also found that the OH + glycolaldehyde + O2 reaction proceeds with considerable reformation of OH radicals (by 25% at 296 K). The observed product yields are explained by a mechanism including formation of short-lived intermediate adducts of the primary radicals with O2. The implication of the obtained results for the HOx budget in the upper troposphere is discussed.

Isopropylcyclopropane + OH Gas Phase Reaction: A Quantum Chemistry + CVT/SCT Approach

A theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from isopropylcyclopropane (IPCP) is presented. Optimum geometries, frequencies and gradients have been computed at the BHandHLYP/6-311++G(d,p) level of theory for all stationary points, as well as for additional points along the minimum energy path (MEP). Energies have been improved by single-point calculations at the above geometries using CCSD(T)/6-311++G(d,p) to produce the potential energy surface. The rate coefficients are calculated for the temperature range 260-350 K by using canonical variational theory (CVT) with small-curvature tunneling (SCT) corrections. Our analysis suggests a stepwise mechanism involving the formation of a reactant complex in the entrance channel and a product complex in the exit channel, for all the modeled paths. The reactant complexes are examined in detail, because they exhibit alkene-like structure. The excellent agreement between the overall calculated and experimental rate coefficients at 298 K supports the reliability of the parameters obtained for the temperature dependence and branching ratios of the IPCP + OH reaction, proposed here for the fist time. The expression that best describes the studied reaction is k(overall) = 6.15 x 10(-13)e1747/RT cm3 x molecule(-1) x s(-1). The predicted activation energy is -0.89 kcal/mol.

Theoretical Investigation of the Hydrogen Abstraction Reaction of the OH Radical with CH3CHF2 (HFC152-a): A Dual Level Direct Density Functional Theory Dynamics Study

The hydrogen abstraction reaction of the OH radical with CH(3)CHF(2) (HFC152-a) has been studied theoretically over a wide temperature range, 200-3000 K. Two different reactive sites of the molecule, CH(3) and CHF(2) groups have been investigated precisely, and results confirm that CHF(2) position of the molecule is a highly reactive site. In this study, three recently developed hybrid density functional theories, namely, MPWB1K, MPW1B95, and MPW1K, are used. The MPWB1K/6-31+G(d,p) method gives the best result for kinetic calculations, including barrier heights, reaction path information and geometry of transition state structures and other stationary points. To refine the barrier height of each channel, a single point energy calculation was performed in MPWB1K/MG3S method. The obtained rate constants by dual level direct dynamics with the interpolated single point energy method (VTST-ISPE) using DFT quantum computational methods, are consistent with available experimental data. The canonical variational transition state theory (CVT) with the zero-curvature and also the small-curvature tunneling correction methods is used to calculate the rate constants. Over the temperature range 200-3000 K, the variation effect, tunneling contribution, branching ratio of each channel are calculated. The rate constants and their temperature dependency in the form of a fitted three-parameter Arrhenius expression are k(1)(T) = 2.00 x 10(-19)(T)(2.24) exp(-1273/T), k(2)(T) = 1.95 x 10(-19)(T)(2.46) exp(-2374/T), and k(T) = 3.13 x 10(-19)(T)(2.47) exp(- 1694/T) cm(3) molecule(-1) s(-1). For the H abstraction from the CHF(2) group, a nonclassical reflection effect is detected as a dominant quantum effect.