Energetics of mechanism of OH-propene reaction at low pressures in inert atmosphere

MP2 calculations with full geometry optimizations were performed with a double- and a triple-zeta basis set to obtain the energy profile for reactions of propene+OH and ethene+OH at low pressures. In these conditions, the β-hydroxyalkyl radicals formed after OH addition to the alkene double bond may go through a rearrangement reaction to form easily identifiable aldehydes and ketones. The theoretical results obtained reproduce the available experimental data well and shed light on the mechanism of reactions characterized by negative activation energies. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 811–819, 1998     

Mechanism and kinetics of the reaction of OH radicals with glyoxal and methylglyoxal: a quantum chemistry + CVT/SCT approach.

A theoretical study of the mechanism and kinetics of the OH hydrogen abstraction from glyoxal and methylglyoxal is presented. Optimum geometries, frequencies, and gradients have been computed at the BHandHLYP/6-311++G(d,p) level of theory for all the stationary points, as well as for 12 additional points along the minimum energy path (MEP). Energies were obtained 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 were calculated for the temperature range 200-500 K by using canonical variational theory (CVT) with small-curvature tunneling (SCT) corrections. Our analysis suggests a stepwise mechanism, which involves the formation of a reactant complex. The overall agreement between the calculated and experimental kinetic data is very good. This agreement supports the reliability of the Arrhenius parameters of the glyoxal + OH reaction that are proposed in this work for the first time. The Arrhenius expressions that best describe the studied reactions are k1 = (9.63 +/- 0.23) x l0(-13)exp[(517 +/- 7)/T] and k2 = (3.93 +/- 0.11) x 10(-13)exp[(1060 +/- 8)/T]cm3 molecule(-1)s(-1) for glyoxal and methylglyoxal, respectively.     

Molecular structure and spectroscopic properties of polyaromatic heterocycles by first principle calculations: spectroscopic shifts with the adsorption of thiophene on phyllosilicate surface

The molecular and electronic structure and spectroscopic properties of thiophene, benzothiophene, and dibenzothiophene molecules have been studied theoretically, using different levels of quantum-mechanical calculations based on Hartree–Fock and density functional theory (DFT) approximations. In all calculation levels used, the molecular structures, dipole moments, thermodynamic properties, and vibration normal modes agree well with the available experimental data and predict the non-available values. The calculated frequencies agree with experimental values within scale factors in the range of 0.94–1.0 for thiophene. Our calculations have refined the mode assignments previously reported. The adsorption of thiophene on a pyrophyllite surface along the 001 plane was also investigated by means of a quantum-mechanical method based on the DFT approximation in a periodical crystal lattice model. The adsorbed thiophene adopts a parallel orientation with respect to the phyllosilicate surface. The shifts observed experimentally in the IR bands of thiophene upon adsorption on pyrophyllite are well reproduced in the thiophene–phyllosilicate complex, finding explanations of this phenomenon and new effects with our calculations.     

Pre-cut dried blood spot (PCDBS): an alternative to dried blood spot (DBS) technique to overcome hematocrit impact

Quantification of analytes by Dried Blood Spots (DBS) on different paper cards has been extensively reported in the past several years. However, some factors limit the robustness of the precision and accuracy of DBS such as: hematocrit level, blood viscosity, analyte nature, spotting technique and spotting conditions. As such, the paper material used for DBS must meet strict quality control criteria to produce reliable quantification of drugs: uniformity, no chemical leaching and no chromatographic effect. To overcome these variables, especially the hematocrit impact, a modification of the traditional DBS, named Pre-Cut Dried Blood Spot (PCDBS), is presented. In contrast to the classical DBS technique, the new PCDBS procedure demonstrates no variation in response, within ±3%, independently of the hematocrit level or of the type of card used. The impact of the hematocrit level on the analyte recovery is discussed for both DBS and PCDBS approaches. Moreover, for quantification of naproxen by liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS), the PCDBS technique was demonstrated to be as precise (%CV ≤3.1%) and accurate (%nominal between 95.4 and 104.4%) as the classical DBS procedure.     

Theoretical Investigation of the OH.‐Initiated Oxidation of Benzaldehyde in the Troposphere

A mechanistic and kinetic study of the OH^.‐initiated oxidation of benzaldehyde is carried out using quantum chemical methods and classical transition state theory. We calculate the rate constant for this reaction within the temperature range of 200–350 K at atmospheric pressure. All possible hydrogen abstraction and OH^. addition channels are considered and branching ratios are obtained. Tunneling corrections are taken into account for abstraction channels, assuming unsymmetrical Eckart barriers. The aldehydic abstraction is by far the most important reaction channel within the entire range of temperatures studied, especially at room temperature and lower—the temperatures relevant to atmospheric chemistry. The relative importance of all the other possible channels increases slightly with temperature. Branching ratios show that addition at the ring and abstraction of an ortho hydrogen contribute about 1 % each at about 300 K, while the branching ratio for the main reaction decreases from 99 % at 200 K to 93 % at 350 K. The results are compared with available experimental measurements.     

On the possible catalytic role of a single water molecule in the acetone + OH gas phase reaction: a theoretical pseudo-second-order kinetics study

In this work, we have revisited the mechanism of the acetone + OH radical reaction assisted by a single water molecule simulating atmospheric conditions. Density functional methods are employed in conjunction with CCSD(T) and large basis sets to explore the potential energy surface of this radical-molecule reaction. Computational kinetics calculations in a pseudo-second order mechanism have been performed, taking into account average atmospheric water concentrations and temperatures. We have used this method recently to study the single-water molecule-assisted H-abstraction in acetaldehyde (Iuga et al. in J Phys Chem Lett 1:3112, 2010) and in glyoxal (Iuga et al. in Chem Phys Lett 501:11, 2010) by OH radicals, and we showed that the initial water complexation step is essential in the rate constant calculation. In both cases, the amount of complex formed is only about 0.01% of the total organic molecule concentration, and as a consequence, water does not accelerate the reaction. In the acetone reaction with OH radicals under atmospheric conditions, we also find that the water–acetone complex concentration is much too small to be relevant, and thus, the rate constant of the water-assisted mechanism is orders of magnitude smaller than the water-free corresponding value.     

Molecular description of indigo oxidation mechanisms initiated by OH and OOH radicals

In this work, we report a quantum chemistry mechanistic study of the hydroxyl (?OH) and hydroperoxyl (?OOH) radicals initiated oxidation of indigo, within the density functional theory framework. All possible hydrogen abstraction and radical addition reaction pathways have been considered. We find that the reaction between a free indigo molecule and an ?OH radical occurs mainly through two competing mechanisms: H-abstraction from an NH site and ?OH addition to the central C═C double bond. Although the latter is favored, both channels occur, the indigo chromophore group structure is modified, and thus the color is changed. This mechanism adequately accounts for the loss of chromophore in urban air, including indoor air such as in museums and in urban areas. Regarding the reactivity of indigo toward ?OOH radicals, only ?OOH-addition to the central double bond is thermodynamically feasible. The corresponding transition state free energy value is about 10 kcal/mol larger than the one for the ?OH initiated oxidation. Therefore, even considering that the ?OOH concentration is considerably larger than the one of ?OH, this reaction is not expected to contribute significantly to indigo oxidation under atmospheric conditions.     

Mechanism and kinetics of the water-assisted formic acid + OH reaction under tropospheric conditions

In this work, we have revisited the mechanism of the formic acid + OH radical reaction assisted by a single water molecule. Density functional methods are employed in conjunction with large basis sets to explore the potential energy surface of this radical-molecule reaction. Computational kinetics calculations in a pseudo-second-order mechanism have been performed, taking into account average atmospheric water concentrations and temperatures. We have used this method recently to study the single water molecule assisted H-abstraction by OH radicals (Iuga, C.; Alvarez-Idaboy, J. R.; Reyes, L.; Vivier-Bunge, A. J. Phys. Chem. Lett. 2010, 1, 3112; Iuga, C.; Alvarez-Idaboy, J. R.; Vivier-Bunge, A. Chem. Phys. Lett. 2010, 501, 11; Iuga, C.; Alvarez-Idaboy, J. R.; Vivier-Bunge, A. Theor. Chem. Acc. 2011, 129, 209), and we showed that the initial water complexation step is essential in the rate constant calculation. In the formic acid reaction with OH radicals, we find that the water-acid complex concentration is small but relevant under atmospheric conditions, and it could in principle be large enough to produce a measurable increase in the overall rate constant. However, the water-assisted process occurs according to a formyl hydrogen abstraction, rather than abstraction of carboxylic hydrogen as in the water-free case. As a result, the overall reaction rate constant is considerably smaller. Products are different in the water-free and water-assisted processes.