Montana Forestry Notes, June 1964

This is issue 1: Soil Temperatures in the Lubrecht Experimental Foresthttps://scholarworks.umt.edu/montana_forestry_notes/1000/thumbnail.jp

Direct Detection of Products from the Pyrolysis of 2-Phenethyl Phenyl Ether

The pyrolysis of 2-phenethyl phenyl ether (PPE, C_6H_5C_2H_4OC_6H_5) in a hyperthermal nozzle (300-1350 °C) was studied to determine the importance of concerted and homolytic unimolecular decomposition pathways. Short residence times (<100 μs) and low concentrations in this reactor allowed the direct detection of the initial reaction products from thermolysis. Reactants, radicals, and most products were detected with photoionization (10.5 eV) time-of-flight mass spectrometry (PIMS). Detection of phenoxy radical, cyclopentadienyl radical, benzyl radical, and benzene suggest the formation of product by the homolytic scission of the C_6H_5C_2H_4-OC_6H_5 and C_6H_5CH_2-CH_2OC_6H_5 bonds. The detection of phenol and styrene suggests decomposition by a concerted reaction mechanism. Phenyl ethyl ether (PEE, C_6H_5OC_2H_5) pyrolysis was also studied using PIMS and using cryogenic matrix-isolated infrared spectroscopy (matrix-IR). The results for PEE also indicate the presence of both homolytic bond breaking and concerted decomposition reactions. Quantum mechanical calculations using CBS-QB3 were conducted, and the results were used with transition state theory (TST) to estimate the rate constants for the different reaction pathways. The results are consistent with the experimental measurements and suggest that the concerted retro-ene and Maccoll reactions are dominant at low temperatures (below 1000 °C), whereas the contribution of the C_6H_5C_2H_4-OC_6H_5 homolytic bond scission reaction increases at higher temperatures (above 1000 °C)

Developing improved MD codes for understanding processive cellulases

"The mechanism of action of cellulose-degrading enzymes is illuminated through a multidisciplinary collaboration that uses molecular dynamics (MD) simulations and expands the capabilities of MD codes to allow simulations of enzymes and substrates on petascale computational facilities. There is a class of glycoside hydrolase enzymes called cellulases that are thought to decrystallize and processively depolymerize cellulose using biochemical processes that are largely not understood. Understanding the mechanisms involved and improving the efficiency of this hydrolysis process through computational models and protein engineering presents a compelling grand challenge. A detailed understanding of cellulose structure, dynamics and enzyme function at the molecular level is required to direct protein engineers to the right modifications or to understand if natural thermodynamic or kinetic limits are in play. Much can be learned about processivity by conducting carefully designed molecular dynamics (MD) simulations of the binding and catalytic domains of cellulases with various substrate configurations, solvation models and thermodynamic protocols. Most of these numerical experiments, however, will require significant modification of existing code and algorithms in order to efficiently use current (terascale) and future (petascale) hardware to the degree of parallelism necessary to simulate a system of the size proposed here. This work will develop MD codes that can efficiently use terascale and petascale systems, not just for simple classical MD simulations, but also for more advanced methods, including umbrella sampling with complex restraints and reaction coordinates, transition path sampling, steered molecular dynamics, and quantum mechanical/molecular mechanical simulations of systems the size of cellulose degrading enzymes acting on cellulose."http://deepblue.lib.umich.edu/bitstream/2027.42/64203/1/jpconf8_125_012049.pd

Negative-Ion Photoelectron Spectroscopy, Gas-Phase Acidity, and Thermochemistry of the Peroxyl Radicals CH_3OO and CH_3CH_2OO

Methyl, methyl-d3, and ethyl hydroperoxide anions (CH_3OO-, CD_3OO-, and CH_3CH_2OO-) have been prepared by deprotonation of their respective hydroperoxides in a stream of helium buffer gas. Photodetachment with 364 nm (3.408 eV) radiation was used to measure the adiabatic electron affinities:  EA[CH_3OO, X̃^2A‘‘] = 1.161 ± 0.005 eV, EA[CD_3OO, X̃^2A‘‘] = 1.154 ± 0.004 eV, and EA[CH_3CH_2OO, X̃^2A‘‘] = 1.186 ± 0.004 eV. The photoelectron spectra yield values for the term energies:  ΔE(X̃^2A‘‘−Ã^2A‘)[CH_3OO] = 0.914 ± 0.005 eV, ΔE(X̃^2A‘‘−Ã^2A‘)[CD_3OO] = 0.913 ± 0.004 eV, and ΔE(X̃^2A‘‘−Ã^2A‘)[CH_3CH_2OO] = 0.938 ± 0.004 eV. A localized RO−O stretching mode was observed near 1100 cm^(-1) for the ground state of all three radicals, and low-frequency R−O−O bending modes are also reported. Proton-transfer kinetics of the hydroperoxides have been measured in a tandem flowing afterglow−selected ion flow tube (FA-SIFT) to determine the gas-phase acidity of the parent hydroperoxides: Δ_(acid)G_(298)(CH_3OOH) = 367.6 ± 0.7 kcal mol^(-1), Δ_(acid)G_(298)(CD_3OOH) = 367.9 ± 0.9 kcal mol^(-1), and Δ_(acid)G_(298)(CH_3CH_2OOH) = 363.9 ± 2.0 kcal mol^(-1). From these acidities we have derived the enthalpies of deprotonation: Δ_(acid)H_(298)(CH_3OOH) = 374.6 ± 1.0 kcal mol^(-1), Δ_(acid)H_(298)(CD_3OOH) = 374.9 ± 1.1 kcal mol^(-1), and Δ_(acid)H_(298)(CH_3CH_2OOH) = 371.0 ± 2.2 kcal mol^(-1). Use of the negative-ion acidity/EA cycle provides the ROO−H bond enthalpies: DH_(298)(CH_3OO−H) = 87.8 ± 1.0 kcal mol^(-1), DH_(298)(CD_3OO−H) = 87.9 ± 1.1 kcal mol^(-1), and DH_(298)(CH_3CH_2OO−H) = 84.8 ± 2.2 kcal mol^(-1). We review the thermochemistry of the peroxyl radicals, CH_3OO and CH_3CH_2OO. Using experimental bond enthalpies, DH_(298)(ROO−H), and CBS/APNO ab initio electronic structure calculations for the energies of the corresponding hydroperoxides, we derive the heats of formation of the peroxyl radicals. The “electron affinity/acidity/CBS” cycle yields Δ_fH_(298)[CH_3OO] = 4.8 ± 1.2 kcal mol^(-1) and Δ_fH_(298)[CH_3CH_2OO] = −6.8 ± 2.3 kcal mol^(-1)

The products of the thermal decomposition of CH 3

We have used a heated 2 cm x 1 mm SiC microtubular (mu tubular) reactor to decompose acetaldehyde: CH3CHO + DELTA --&gt; products. Thermal decomposition is followed at pressures of 75 - 150 Torr and at temperatures up to 1700 K, conditions that correspond to residence times of roughly 50 - 100 mu sec in the mu tubular reactor. The acetaldehyde decomposition products are identified by two independent techniques: VUV photoionization mass spectroscopy (PIMS) and infrared (IR) absorption spectroscopy after isolation in a cryogenic matrix. Besides CH3CHO, we have studied three isotopologues, CH3CDO, CD3CHO, and CD3CDO. We have identified the thermal decomposition products CH3(PIMS), CO (IR, PIMS), H (PIMS), H2 (PIMS), CH2CO (IR, PIMS), CH2=CHOH (IR, PIMS), H2O (IR, PIMS), and HC=CH (IR, PIMS). Plausible evidence has been found to support the idea that there are at least three different thermal decomposition pathways for CH3CHO: Radical decomposition: CH3CHO + DELTA --&gt; CH3 + [HCO] --&gt; CH3 + H + CO Elimination: CH3CHO + DELTA --&gt; H2 + CH2=C=O. Isomerization/elimination: CH3CHO + DELTA --&gt; [CH2=CH-OH] --&gt; HC=CH + H2O. Both PIMS and IR spectroscopy show compelling evidence for the participation of vinylidene, CH2=C:, as an intermediate in the decomposition of vinyl alchohol: CH2=CH-OH + DELTA --&gt; [CH2=C:] + H2O --&gt; HC=CH + H2O

Photocatalytic oxidation of gas-phase BTEX-contaminated waste streams

Researchers at the National Renewable Energy Laboratory (NREL) have been exploring heterogeneous photocatalytic oxidation (PCO) as a remediation technology for air streams contaminated with benzene, toluene, ethyl-benzene, and xylenes (BTEX). This research is a continuation of work performed on chlorinated organics. The photocatalytic oxidation of BTEX has been studied in the aqueous phase, however, a study by Turchi et al. showed a more economical system would involve stripping organic contaminants from the aqueous phase and treating the resulting gas stream. Another recent study by Turchi et al. indicated that PCO is cost competitive with such remediation technologies as activated carbon adsorption and catalytic incineration for some types of contaminated air streams. In this work we have examined the photocatalytic oxidation of benzene using ozone (0{sub 3}) as an additional oxidant. We varied the residence time in the PCO reactor, the initial concentration of the organic pollutant, and the initial ozone concentration in a single-pass reactor. Because aromatic hydrocarbons represent only a small fraction of the total hydrocarbons present in gasoline and other fuels, we also added octane to the reaction mixture to simulate the composition of air streams produced from soil-vapor-extraction or groundwater-stripping of sites contaminated with gasoline

Comparison of Unimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels

Quantum mechanical molecular modeling is used [M06-2X/6-311++G­(2df,p)] to compare activation energies and rate constants for unimolecular decomposition pathways of saturated and unsaturated carboxylic acids that are important in the production of biofuels and that are models for plant and algae-derived intermediates. Dehydration and decarboxylation reactions are considered. The barrier heights to decarboxylation and dehydration are similar in magnitude for saturated acids (∼71 kcal mol^–1), with an approximate 1:1 [H₂O]/[CO₂] branching ratio over the temperature range studied (500–2000 K). α,β-Unsaturation lowers the barrier to decarboxylation between 2.2 and 12.2 kcal mol^–1 while increasing the barriers to dehydration by ∼3 kcal mol^–1. The branching ratio, as a result, is an order of magnitude smaller, [H₂O]/[CO₂] = 0.07. For some α,β-unsaturated acids, six-center transition states are available for dehydration, with barrier heights of ∼35.0 kcal mol^–1. The branching ratio for these acids can be as high as 370:1. β,γ-Unsaturation results in a small lowering in the barrier height to decarboxylation (∼70.0 kcal mol^–1). β,γ-Unsaturation also leads to a lowering in the dehydration pathway from 1.7 to 5.1 kcal mol^–1. These results are discussed with respect to predicted kinetic values for acids of importance in biofuels production

Bimolecular Decomposition Pathways for Carboxylic Acids of Relevance to Biofuels

The bimolecular thermal reactions of carboxylic acids were studied using quantum mechanical molecular modeling. Previous work investigated the unimolecular decomposition of a variety of organic acids, including saturated, α,β-unsaturated, and β,γ-unsaturated acids, and showed that the type and position of the unsaturation resulted in unique branching ratios between dehydration and decarboxylation, [H₂O]/[CO₂]. In this work, the effect of bimolecular chemistry (water–acid and acid–acid) is considered with a representative of each acid class. In both cases, the strained 4-centered, unimolecular transition state, typical of most organic acids, is opened up to 6- or 8-centered bimolecular geometries. These larger structures lead to a reduction in the barrier heights (20–45%) of the thermal decomposition pathways for organic acids and an increase in the decomposition kinetics. In some cases, they even cause a shift in the branching ratio of the corresponding product slates