A combined photoionization time-of-flight mass spectrometry and laser absorption spectrometry flash photolysis apparatus for simultaneous determination of reaction rates and product branching.

In recent years, predictions of product branching for reactions of consequence to both combustion and atmospheric chemistry have outpaced validating experiments. An apparatus is described that aims to fill this void by combining several well-known experimental techniques into one: flash photolysis for radical generation, multiple-pass laser absorption spectrometry (LAS) for overall kinetics measurements, and time-resolved photoionization time-of-flight mass spectrometry (PI TOF-MS) for product branching quantification. The sensitivity of both the LAS and PI TOF-MS detection techniques is shown to be suitable for experiments with initial photolytically generated radical concentrations of ∼1 × 1012 molecules cm-3. As it is fast (µs time resolution) and non-intrusive, LAS is preferred for accurate kinetics (time-dependence) measurements. By contrast, PI TOF-MS is preferred for product quantification because it provides a near-complete picture of the reactor composition in a single mass spectrum. The value of simultaneous LAS and PI TOF-MS detection is demonstrated for the chemically interesting phenyl radical + propene system.

Experimental and Computational Studies of the Kinetics of the Reaction of Atomic Hydrogen with Methanethiol.

The overall rate constant for H + CH3SH has been studied over 296-1007 K in an Ar bath gas using the laser flash photolysis method at 193 nm. H atoms were generated from CH3SH and in some cases NH3. They were detected via time-resolved resonance fluorescence. The results are summarized as k = (3.45 ± 0.19) × 10(-11) cm(3) molecule(-1) s(-1) exp(-6.92 ± 0.16 kJ mol(-1)/RT) where the errors in the Arrhenius parameters are the statistical uncertainties at the 2σ level. Overall error limits of ±9% for k are proposed. In the overlapping temperature range there is very good agreement with the resonance fluorescence measurements of Wine et al. Ab initio data and transition state theory yield moderate accord with the total rate constant, but not with prior mass spectrometry measurements of the main product channels leading to CH3S + H2 and CH3 + H2S by Amano et al.

New pathways for formation of acids and carbonyl products in low-temperature oxidation: the Korcek decomposition of γ-ketohydroperoxides.

We present new reaction pathways relevant to low-temperature oxidation in gaseous and condensed phases. The new pathways originate from γ-ketohydroperoxides (KHP), which are well-known products in low-temperature oxidation and are assumed to react only via homolytic O-O dissociation in existing kinetic models. Our ab initio calculations identify new exothermic reactions of KHP forming a cyclic peroxide isomer, which decomposes via novel concerted reactions into carbonyl and carboxylic acid products. Geometries and frequencies of all stationary points are obtained using the M06-2X/MG3S DFT model chemistry, and energies are refined using RCCSD(T)-F12a/cc-pVTZ-F12 single-point calculations. Thermal rate coefficients are computed using variational transition-state theory (VTST) calculations with multidimensional tunneling contributions based on small-curvature tunneling (SCT). These are combined with multistructural partition functions (Q(MS-T)) to obtain direct dynamics multipath (MP-VTST/SCT) gas-phase rate coefficients. For comparison with liquid-phase measurements, solvent effects are included using continuum dielectric solvation models. The predicted rate coefficients are found to be in excellent agreement with experiment when due consideration is made for acid-catalyzed isomerization. This work provides theoretical confirmation of the 30-year-old hypothesis of Korcek and co-workers that KHPs are precursors to carboxylic acid formation, resolving an open problem in the kinetics of liquid-phase autoxidation. The significance of the new pathways in atmospheric chemistry, low-temperature combustion, and oxidation of biological lipids are discussed.

Dehydration of isobutanol and the elimination of water from fuel alcohols.

Rate coefficients for the dehydration of isobutanol have been determined experimentally from comparative rate single pulse shock tube measurements and calculated via multistructural transition state theory (MS-TST). They are represented by the Arrhenius expression, k(isobutanol → isobutene + H2O)(experimental) = 7.2 × 10(13) exp(-35300 K/T) s(-1). The theoretical work leads to the high pressure rate expression, k(isobutanol → isobutene + H2O)(theory) = 3.5 × 10(13) exp(-35400 K/T) s(-1). Results are thus within a factor of 2 of each other. The experimental results cover the temperature range 1090-1240 K and pressure range 1.5-6 atm, with no discernible pressure effects. Analysis of these results, in combination with earlier single pulse shock tube work, made it possible to derive the governing factors that control the rate coefficients for alcohol dehydration in general. Alcohol dehydration rate constants depend on the location of the hydroxyl group (primary, secondary, and tertiary) and the number of available H-atoms adjacent to the OH group for water elimination. The position of the H-atoms in the hydrocarbon backbone appears to be unimportant except for highly substituted molecules. From these correlations, we have derived k(isopropanol → propene + H2O) = 7.2 × 10(13) exp(-33000 K/T) s(-1). Comparison of experimental determination with theoretical calculations for this dehydration, and those for ethanol show deviations of the same magnitude as for isobutanol. Systematic differences between experiments and theoretical calculations are common.

A study to establish reasonable action limits for patient-specific quality assurance in intensity-modulated radiation therapy.

An effective patient quality assurance (QA) program for intensity-modulated radiation therapy (IMRT) requires accurate and realistic plan acceptance criteria--that is, action limits. Based on dose measurements performed with a commercially available two-dimensional (2D) diode array, we analyzed 747 fluence maps resulting from a routine patient QA program for IMRT plans. The fluence maps were calculated by three different commercially available (ADAC, CMS, Eclipse) treatment planning systems (TPSs) and were delivered using 6-MV X-ray beams produced by linear accelerators. To establish reasonably achievable and clinically acceptable limits for the dose deviations, the agreement between the measured and calculated fluence maps was evaluated in terms of percent dose error (PDE) for a few points and percent of passing points (PPP) for the isodose distribution. The analysis was conducted for each TPS used in the study (365 ADAC, 162 CMS,220 Eclipse), for multiple treatment sites (prostate, pelvis, head and neck, spine, rectum, anus, lung, brain), at the normalization point for 3% percentage difference (%Diff) and 3-mm distance to agreement (DTA) criteria. We investigated the treatment-site dependency of PPP and PDE. The results show that, at 3% and 3-mm criteria, a 95% PPP and 3% PDE can be achieved for prostate treatments and a 90% PPP and 5% PDE are attainable for any treatment site.