New insights for mesospheric OH: multi-quantum vibrational relaxation as a driver for non-local thermodynamic equilibrium.

The question of whether mesospheric OH(υ) rotational population distributions are in equilibrium with the local kinetic temperature has been debated over several decades. Despite several indications for the existence of non-equilibrium effects, the general consensus has been that emissions originating from low rotational levels are thermalized. Sky spectra simultaneously observing several vibrational levels demonstrated reproducible trends in the extracted OH(υ) rotational temperatures as a function of vibrational excitation. Laboratory experiments provided information on rotational energy transfer and direct evidence for fast multi-quantum OH(high-υ) vibrational relaxation by O atoms. We examine the relationship of the new relaxation pathways with the behavior exhibited by OH(υ) rotational population distributions. Rapid OH(high-υ) + O multi-quantum vibrational relaxation connects high and low vibrational levels and enhances the hot tail of the OH(low-υ) rotational distributions. The effective rotational temperatures of mesospheric OH(υ) are found to deviate from local thermodynamic equilibrium for all observed vibrational levels.

Collision dynamics of O(3P) + DMMP using a specific reaction parameters potential form.

Starting from previous benchmark CBS-QB3 electronic structure calculations (Conforti, P. F.; Braunstein, M.; Dodd, J. A. J. Phys. Chem. A 2009, 113, 13752), we develop two global potential energy surfaces for O((3)P) + DMMP collisions, using the specific reaction parameters approach. Each surface is simultaneously fit along the three major reaction pathways: hydrogen abstraction, hydrogen elimination, and methyl elimination. We then use these surfaces in classical dynamics simulations and compute reactive cross sections from 4 to 10 km s(-1) collision velocity. We examine the energy disposal and angular distributions of the reactive and nonreactive products. We find that for reactive collisions, an unusually large amount of the initial collision energy is transformed into internal energy. We analyze the nonreactive and reactive product internal energy distributions, many of which fit Boltzmann temperatures up to ~2000 K.

Energetics and dynamics of the reactions of O(3P) with dimethyl methylphosphonate and sarin.

Electronic structure and molecular dynamics calculations were performed on the reaction systems O((3)P) + sarin and O((3)P) + dimethyl methylphosphonate (DMMP), a sarin simulant. Transition state geometries, energies, and heats of reaction for the major reaction pathways were determined at several levels of theory, including AM1, B3LYP/6-311+G(d,p), and CBS-QB3. The major reaction pathways for both systems are similar and include H-atom abstraction, H-atom elimination, and methyl elimination, in rough order from low to high energy. The H-atom abstraction channels have fairly low barriers (approximately 10 kcal mol(-1)) and are close to thermoneutral, while the other channels have relatively high energy barriers (>40 kcal mol(-1)) and a wide range of reaction enthalpies. We have also found a two-step pathway leading to methyl elimination through O-atom attack on the phosphorus atom for DMMP and sarin. For sarin, the two-step methyl elimination pathway is significantly lower in energy than the single-step pathway. We also present results of O((3)P) + sarin and O((3)P) + DMMP reaction cross sections over a broad range of collision energies (2-10 km s(-1) collision velocities) obtained using the direct dynamics method with an AM1 semiempirical potential. These excitation functions are intended as an approximate guide to future hyperthermal measurements, which to our knowledge have not yet examined either of these systems. The reaction barriers, reaction enthalpies, transition state structures, and excitation functions are generally similar for DMMP and sarin, with some moderate differences for methyl elimination energetics, which indicates DMMP will likely be a good substitute for sarin in many O((3)P) chemical investigations.

Hyperthermal atomic oxygen source for near-space simulation experiments.

A hyperthermal atomic oxygen (AO) beam facility has been developed to investigate the collisions of high-velocity AO atoms with vapor-phase counterflow. Application of 4.5 kW, 2.4 GHz microwave power in the source chamber creates a continuous discharge in flowing O(2) gas. The O(2) feedstock is introduced into the source chamber in a vortex flow to constrain the plasma to the center region, with the chamber geometry promoting resonant excitation of the TM(011) mode to localize the energy deposition in the vicinity of the aluminum nitride (AlN) expansion nozzle. The approximately 3500 K environment serves to dissociate the O(2), resulting in an effluent consisting of 40% AO by number density. Downstream of the nozzle, a silicon carbide (SiC) skimmer selects the center portion of the discharge effluent, prior to the expansion reaching the first shock front and rethermalizing, creating a beam with a derived 2.5 km s(-1) velocity. Differential pumping of the skimmer chamber, an optional intermediate chamber and reaction chamber maintains a reaction chamber pressure in the mid-10(-6) to mid-10(-5) Torr range. The beam has been characterized with regard to total AO beam flux, O(2) dissociation fraction, and AO spatial profile using time-of-flight mass spectrometric and Kapton-H erosion measurements. A series of reactions AO+C(n)H(2n) (n=2-4) has been studied under single-collision conditions using mass spectrometric product detection, and at higher background pressure detecting dispersed IR emissions from primary and secondary products using a step-scan Michelson interferometer. In a more recent AO crossed-beam experiment, number densities and predicted IR emission intensities have been modeled using the direct simulation Monte Carlo technique. The results have been used to guide the experimental conditions. IR emission intensity predictions are compared to detected signal levels to estimate absolute reaction cross sections.

Ab initio energies and product branching ratios for the O+C3H6 reaction.

Intermediate and transition-state energies have been calculated for the O+C3H6 (propene) reaction using the compound ab initio CBS-QB3 and G3 methods in combination with density functional theory. The lowest-lying triplet and singlet potential energy surfaces of the O-C3H6 system were investigated. RRKM statistical theory was used to predict product branching fractions over the 300-3000 K temperature and 0.001-760 Torr pressure ranges. The oxygen atom adds to the C3H6 terminal olefinic carbon in the primary step to generate a nascent triplet biradical, CH3CHCH2O. On the triplet surface, unimolecular dissociation of CH3CHCH2O to yield H+CH3CHCHO is favored over the entire temperature range, although the competing H2CO+CH3CH product channel becomes significant at high temperature. Rearrangement of triplet CH3CHCH2O to CH3CH2CHO (propanal) via a 1,2 H-atom shift has a barrier of 122.3 kJ mol(-1), largely blocking this reaction channel and any subsequent dissociation products. Intersystem crossing of triplet CH3CHCH2O to the singlet surface, however, leads to facile rearrangement to singlet CH3CH2CHO, which dissociates via numerous product channels. Pressure was found to have little influence over the branching ratios under most conditions, suggesting that the vibrational self-relaxation rates for p