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Polycyclic aromatic hydrocarbons (PAHs) are intermediates in the formation of soot particles and interstellar grains. However, their formation mechanisms in combustion and interstellar environments are not fully understood. The production of tricyclic PAHs and, in particular, the conversion of a PAH containing a five-membered ring to one with a six-membered ring are of interest to explain PAH abundances in combustion processes. In the present work, resonant ionization mass spectrometry in conjunction with isotopic labeling is used to investigate the formation of the phenalenyl radical from acenaphthylene and methane in an electrical discharge. We show that in this environment the CH cycloaddition mechanism converts a five-membered ring to a six-membered ring. This mechanism can occur in tandem with other PAH formation mechanisms such as hydrogen abstraction/acetylene addition (HACA) to produce larger PAHs in flames and the interstellar medium.

Chemical processes to destroy per- and polyfluoroalkyl substances (PFAS) are being actively developed to deal with contaminated soil and water, legacy waste streams, and unwanted stockpiles of these hazardous compounds. Common to these processes are complex networks of chemical reactions, including thermal decomposition, oxidation and/or hydrolysis steps, and the development and optimization of destruction technology requires predictive chemical kinetic modelling tools. Here we report the first detailed chemical kinetic model targeted at PFAS destruction, capable of describing the pyrolysis and incineration of perfluorinated carboxylic acids up to C3 and perfluorinated sulfonic acids up to C2. Simulations of the pyrolysis and incineration of these PFAS indicate that reaction is initiated via degradation of the acid headgroup, followed by chain-shortening via carbonyl (acyl fluoride) intermediates. Although applied to gas-phase degradation of short-chain perfluorinated acids, the model provided here should prove useful as a basis for modelling the destruction of a diverse range of PFAS both in condensed and supercritical phases.

Quantum chemistry and statistical reaction rate theory calculations have been performed to investigate the products and kinetics of indenyl radical decomposition. Three competitive product sets are identified, including formation of a cyclopentadienyl radical (c-C5H5) and diacetylene (C4H2), which has not been included in prior theoretical kinetics investigations. Rate coefficients for indenyl decomposition are determined from master equation simulations at 1800-2400 K and 0.01-100 atm, and temperature- and pressure-dependent rate coefficient expressions are incorporated into a detailed chemical kinetic model for indene pyrolysis. Indenyl is found to predominantly decompose to o-benzyne (o-C6H4) + propargyl (C3H3), with lesser amounts of fulvenallenyl (C7H5) + C2H2 and c-C5H5 + C4H2.

A key step in gas-phase polycyclic aromatic hydrocarbon (PAH) formation involves the addition of acetylene (or other alkyne) to σ-type aromatic radicals, with successive additions yielding more complex PAHs. A similar process can happen for N-containing aromatics. In cold diffuse environments, such as the interstellar medium, rates of radical addition may be enhanced when the σ-type radical is charged. This paper investigates the gas-phase ion-molecule reactions of acetylene with nine aromatic distonic σ-type radical cations derived from pyridinium (Pyr), anilinium (Anl), and benzonitrilium (Bzn) ions. Three isomers are studied in each case (radical sites at the ortho, meta, and para positions). Using a room temperature ion trap, second-order rate coefficients, product branching ratios, and reaction efficiencies are measured. The rate coefficients increase from para to ortho positions. The second-order rate coefficients can be sorted into three groups: low, between 1 and 3 × 10-12 cm3 molecule-1 s-1 (3Anl and 4Anl); intermediate, between 5 and 15 × 10-12 cm3 molecule-1 s-1 (2Bzn, 3Bzn, and 4Bzn); and high, between 8 and 31 × 10-11 cm3 molecule-1 s-1 (2Anl, 2Pyr, 3Pyr, and 4Pyr); and 2Anl is the only radical cation with a rate coefficient distinctly different from its isomers. Quantum chemical calculations, using M06-2X-D3(0)/6-31++G(2df,p) geometries and DSD-PBEP86-NL/aug-cc-pVQZ energies, are deployed to rationalize reactivity trends based on the stability of prereactive complexes. The G3X-K method guides the assignment of product ions following adduct formation. The rate coefficient trend can be rationalized by a simple model based on the prereactive complex forward barrier height.

The B1A' ← X1A' electronic spectra of the 1-butyn-3-yl cation (H3CCHCCH+) and the H3CCHCCH+-Ne and H3CCHCCH+-Ar complexes are measured using resonance enhanced photodissociation over the 245-285 nm range, with origin transitions occurring at 35936, 35930, and 35928 cm-1, respectively. Vibronic bands are assigned based on quantum chemical calculations and comparison of the spectra with those of the related linear methyl propargyl (H3C4H2+) and propargyl (H2C3H+) cations. The photofragment ions are C2H3+ (major) and C4H3+ (minor), with the preference for C2H3+ consistent with master equation simulations for a mechanism that involves rapid electronic deactivation and dissociation on the ground state potential energy surface.

Polycyclic aromatic hydrocarbons (PAHs) are intermediates in the formation of soot particles and interstellar grains. However, their formation mechanisms in combustion and interstellar environments are not fully understood. The production of tricyclic PAHs and, in particular, the conversion of a PAH containing a five-membered ring to one with a six-membered ring is of interest to explain PAH abundances in combustion processes. In the present work, resonant ionization mass spectrometry in conjunction with isotopic labelling is used to investigate the formation of the phenalenyl radical from acenaphthylene and methane in an electrical discharge. We show that in this environment, the CH cycloaddition mechanism converts a five-membered ring to a six-membered ring. This mechanism can occur in tandem with other PAH formation mechanisms such as hydrogen abstraction/ acetylene addition (HACA) to produce larger PAHs in flames and the interstellar medium.

Using VUV synchrotron photoionization mass spectrometry, quantum chemistry and RRKM kinetic modeling, our paper identifies the products formed when a substituted aromatic free radical (ortho- methylphenyl) reacts with two C3H4 isomers found in flames – propyne and allene.Both reactions form polycyclic aromatic hydrocarbons but profound structural differences result from the nature of the C3H4 isomer. The allene case favors the formation of six-six bicyclic rings (graphitic), while the formation of new five-six membered bicylics dominate for the propyne case. These conclusions are rigorously supported by experiments, calculations and kinetic modelling.

Using VUV synchrotron photoionization mass spectrometry, quantum chemistry and RRKM kinetic modeling, our paper identifies the products formed when a substituted aromatic free radical (ortho- methylphenyl) reacts with two C3H4 isomers found in flames – propyne and allene.Both reactions form polycyclic aromatic hydrocarbons but profound structural differences result from the nature of the C3H4 isomer. The allene case favors the formation of six-six bicyclic rings (graphitic), while the formation of new five-six membered bicylics dominate for the propyne case. These conclusions are rigorously supported by experiments, calculations and kinetic modelling.

The gas-phase ion–molecule identity exchange reactions of phenyl chalcogen iranium ions with alkenes have been examined experimentally in a linear ion trap mass spectrometer by isotope labeling experiments. The nature of both the alkene and the chalcogen play crucial roles, with the bimolecular rates for π-ligand exchange following the order: [PhTe(c-C6H10)]+ + c-C6D10 > [PhTe(C2D4)]+ + C2H4 > [PhSe(c-C6H10)]+ + c-C6D10, with no reaction being observed for [PhSe(C2D4)]+ + C2H4, [PhS(C2D4)]+ + C2H4, and [PhS(c-C6H10)]+ + c-C6D10. The experimental results correlate with RRKM modeling and density functional theory (DFT) calculations, which also demonstrates that these reactions proceed via associative mechanisms. Natural bond orbital (NBO) analysis reveals a shift in the association complexes from a σ-hole interaction to ones mirroring the π–p+ and n−π* at the transition state in accordance with the rates of reaction.

The unimolecular reactions of C7H8+ radical cations are among those most studied by mass spectrometry, especially the rearrangement of toluene and cycloheptatriene molecular ions, which are directly connected to the formation of benzylium and tropylium cations. This study reveals important new isomers and isomerization pathways on the C7H8+ potential energy surface, through the application of gas-phase electronic photodissociation spectroscopy in conjunction with ab initio calculations. Presented are the first gas-phase vibrationally resolved electronic spectra of the o-isotoluene, norcaradiene, bicyclo[3.2.0]hepta-2,6-diene radical cations, and ring-opened products from cyclic C7H8+ species. The isomerization route from the norbornadiene radical cation to the toluene radical cation, which competes with isomerization to the bicyclo[2.2.1]hepta-2-ene-5-yl-7-ylium radical cation, is identified. Further, this work expands understanding of the C7H8+ potential energy surface by connecting spiro[2.4]hepta-4,6-diene and acyclic 1,2,4,6-heptatetraene radical cations, and confirms the important role of the o-isotoluene radical cation in the interconversion pathways of C7H8+ species.

A key step in gas-phase polycyclic aromatic hydrocarbon (PAH) formation involves the addition of acetylene (or other alkyne) to σ-type aromatic radicals, with successive additions yielding more complex PAHs. A similar process can happen for N- containing aromatics. In cold diffuse environments, such as the interstellar medium, rates of radical addition may be enhanced when the σ-type radical is charged. This paper investigates the gas-phase ion-molecule reactions of acetylene with nine aromatic distonic σ-type radical cations derived from pyridinium (Pyr), anilinium (Anl) and benzonitrilium (Bzn) ions. Three isomers are studied in each case (radical sites at the ortho, meta and para positions). Using a room temperature ion trap, second-order rate coefficients, product branching ratios and reaction efficiencies are reported.

A key step in gas-phase polycyclic aromatic hydrocarbon (PAH) formation involves the addition of acetylene (or other alkyne) to σ-type aromatic radicals, with successive additions yielding more complex PAHs. A similar process can happen for N- containing aromatics. In cold diffuse environments, such as the interstellar medium, rates of radical addition may be enhanced when the σ-type radical is charged. This paper investigates the gas-phase ion-molecule reactions of acetylene with nine aromatic distonic σ-type radical cations derived from pyridinium (Pyr), anilinium (Anl) and benzonitrilium (Bzn) ions. Three isomers are studied in each case (radical sites at the ortho, meta and para positions). Using a room temperature ion trap, second-order rate coefficients, product branching ratios and reaction efficiencies are reported.

Favipiravir (T-705) is an antiviral medication used to treat influenza. T-705 is also currently being trialled as a repurposed COVID-19 treatment. To help accelerate these efforts, this study provides important solution-phase properties of T-705 determined via computational chemistry. Density functional theory (DFT) calculations combined with the SMD continuum solvation model demonstrate that T-705 prefers the aromatic enol form in solution over the ketone tautomer. Deprotonation constants for the conjugate acids of T-705 (pKas) are then evaluated, by combining the DFT/SMD calculations with accurate G4 gas-phase basicities. These calculations indicate that T-705 is a weak base that should not significantly protonate at physiological pH. The preferential site for protonation is at the ring nitrogen ortho to the alcohol functional group (pKa ~ 7.4), followed by protonation of the oxygen on the amide side-chain at more acidic conditions (pKa ~ -9.8). Significantly, protonation of the ring nitrogen produces an acid that can deprotonate to the enol form (pKa ~ -5.1), providing a pathway for their interconversion. Finally, base-pairing of the active ribose-bound form of T-705 to cytidine and uridine is also examined. These calculations indicate that both base pairs have large binding free energies of around 4 – 5 kcal/mol, supporting previous findings that T-705 can bind with both nucleobases, leading to mis-incorporation of these pairs into viral RNA.

Volatile silicon compounds (VOSiCs) are air pollutants present in both indoor and outdoor environments. Here, tetramethylsilane (TMS) is selected as a model to study the photochemical oxidation mechanisms for VOSiCs using ab initio and RRKM theory / master equation kinetic modelling. Under tropospheric conditions the TMS radical (CH3)3SiCH2• reacts with O2 to produce a stabilized peroxyl radical which is expected to ultimately yield the alkoxyl radical (CH3)3SiCH2O•. At combustion-relevant temperatures, however, a well-skipping reaction to (CH3)3SiO• + HCHO dominates. Importantly, the (CH3)3SiCH2O• radical is predicted to rearrange to (CH3)3SiOCH2• with a very low reaction barrier, enabling an auto-oxidation process involving addition of a second O2. Subsequent oxidation reaction mechanisms of (CH3)3SiOCH2• have been developed, with the major product predicted to be the ester (CH3)3SiOCHO, an experimentally observed TMS oxidation product. The production of substantially oxygenated compounds following a single radical initiation reaction has implications for the ability of VOSiCs to contribute to ozone and particle formation in both outdoor and indoor environments.

The ground-state rearrangement and decomposition of methyl vinyl ketone (MVK) and methacrolein (MACR) has been investigated using quantum chemical calculations and RRKM theory/master equation simul...

Pt/IrO2 bifunctional catalysts synthesized with varying Pt:Ir ratios and characterized using several techniques, including energy dispersive X-ray spectroscopy, transmission electrons microscopy, X...

Donor-acceptor Stenhouse adducts (DASAs) are a new class of photoswitching molecules with excellent fatigue resistance and synthetic tunability. Here, tandem ion mobility mass spectrometry coupled with laser excitation is used to characterize the photocyclization reaction of isolated, charge-tagged DASA molecules over the 450-580 nm range. The experimental maximum response at 530 nm agrees with multireference perturbation theory calculations for the S1 ← S0 transition maximum at 533 nm. Photocyclization in the gas phase involves absorption of at least two photons; the first photon induces Z-E isomerization from the linear isomer to metastable intermediate isomers, while the second photon drives another E-Z isomerization and 4π-electrocyclization reaction. Cyclization is thermally reversible in the gas phase with collisional excitation.

Glycerol is a massive byproduct of biodiesel fabrication, which decreases its price and increases the risks of inadequate disposal. In this sense, more environmentally friendly instruments and processes using glycerol are required to make this matrix more valuable. Here, a 3D-printed electrolyzer was developed and tested for long-period glycerol electrolysis in an alkaline medium. The new electrolyzer contains only three mobile parts and can be manufactured in less than 4 h using ∼30 g of polylactic acid filament, with a total cost of less than US $5. This easily built and inexpensive reduced-scale electrolyzer has the advantage of using only a few milliliters of solution to perform tests for electrosynthesis. We synthesized Pd nanocubes to modify a glassy carbon working electrode, which was used for glycerol electrolysis. We found a remarkable selectivity of 99% toward tartronate production, which was induced by the extended (100) surface of Pd in the alkaline medium. Hence, we report a new 3D-printed pl...

Collision-induced dissociation mass spectrometry of the ammonium ions 4a and 4b results in the formation of the seleniranium ion 5, the structure and purity of which were verified using gas-phase infrared spectroscopy coupled to mass spectrometry and gas-phase ion-mobility measurements. Ion-molecule reactions between the ion 5 (m/z = 261) and cyclopentene, cyclohexene, cycloheptene, and cyclooctene resulted in the formation of the seleniranium ions 7 (m/z = 225), 6 (m/z = 239), 8 (m/z = 253), and 9 (m/z = 267), respectively. Further reaction of seleniranium 6 with cyclopentene resulted in further π-ligand exchange giving seleniranium ion 7, confirming that direct π-ligand exchange between seleniranium ion 5 and cycloalkenes occurs in the gas phase. Pseudo-first-order kinetics established relative reaction efficiencies for π-ligand exchange for cyclopentene, cyclohexene, cycloheptene. and cyclooctene as 0.20, 0.07, 0.43, and 4.32. respectively. DFT calculations at the M06/6-31+G(d) level of theory provide the following insights into the mechanism of the π-ligand exchange reactions; the cycloalkene forms a complex with the seleniranium ion 5 with binding energies of 57 and 62 kJ/mol for cyclopentene and cyclohexene, respectively, with transition states for π-ligand exchange having barriers of 17.8 and 19.3 kJ/mol for cyclopentene and cyclohexene, respectively.

This paper presents an experimental and numerical investigation of the autoignition of ethanol blended with several primary reference fuels (PRFs) and toluene reference fuels (TRFs) in a Cooperative Fuel Research (CFR) engine. Autoigniting, in-cylinder pressure traces are acquired under standard research octane number (RON) conditions. Equivalent, non-autoignitiing traces are obtained by adding a small amount of tetraethyl lead (TEL) to each fuel to suppress knock, and these are used to calibrate a two-zone engine model of autoignition. The simulated autoignition timing of the PRFs without ethanol, TRFs without ethanol, and ethanol/PRF and ethanol/TRF blends are then compared to those measured in the engine. These results suggest that the incorporation of residual NO significantly improves the agreement between simulations and experiment for all cases with no or low ethanol content. Ethanol also appears to suppress the low-temperature activity of n-heptane. Both of these results are consistent with previo...

Glyphosate (GP) is a widely used herbicide worldwide, yet accumulation of GP and its main byproduct, aminomethylphosphonic acid (AMPA), in soil and water has raised concerns about its potential effects on human health. Thermal treatment, in which contaminants are vaporised and decomposed in the gas-phase, is one option for decontaminating material containing GP and AMPA, yet the thermal decomposition chemistry of these compounds remains poorly understood. Here, we have revealed the thermal decomposition mechanism of GP and AMPA in the gas phase by applying computational chemistry and reaction rate theory methods. The preferred decomposition channel for both substances involves the elimination of P(OH)3 to yield the imine N-methylene-glycine (from GP) or methanimine (from AMPA), with relatively low barrier heights (ca. 45 kcal mol-1). The half-life of GP and AMPA at 1000 K are predicted to be 0.1 and 4 ms respectively, and they should be readily destroyed via conventional incineration processes. The further decomposition of N-methylene-glycine is expected to also take place at similar temperatures, leading to N-methyl-methanimine + CO2, with a barrier height of ca. 48 kcal mol-1. The imine decomposition products of GP and AMPA are expected to react with water vapour to form simple amines and carbonyl compounds.

Piperazine (Pz) mixed with other amines is a proposed carbon capture solvent, but there is concern about the impact of Pz on air quality, including the potential to produce toxic products. Here, the •OH initiated oxidation of Pz has been studied by ab initio modelling and RRKM / master equation kinetic simulations. The Pz + •OH reaction is found to proceed at around the capture rate, consistent with experiment, with abstraction predominantly from C—H sites. The subsequent reaction kinetics of carbon centred Pz radicals with O2 are also studied, so as to determine the first-generation oxidation products. We find that the Pz radical predominantly reacts with O2 to produce a cyclic imine product + HO2• under tropospheric conditions, with the stabilized peroxyl radical formed as a minor product. Subsequent reaction of the peroxyl radical with NO produces an alkoxyl radical that can react with O2 to yield a cyclic amide or undergo unimolecular ring opening followed by a second O2 addition / HO2• elimination step to produce CH2=NCH2CH2NHCHO.

Atmospheric oxidation of isoprene produces significant yields of eight unique nitrate 11 compounds, each with a β- or δ-hydroxy group. These isoprene hydroxy nitrates (ISOPNs) 12 significantly impact upon global NOx budgets, O3 levels, and aerosol formation. 13 Uncertainties exist, however, in our understanding of ISOPN chemistry, particularly in their 14 yields from the reaction of isoprene peroxyl radicals with NO. This study describes novel 15 isomerization reactions of the ISOPNs, identified through the application of computational 16 chemistry techniques. These reactions produce saturated polycyclic orthonitrite compounds 17 via attack of the R–NO2 group on the vinyl moiety. For the δ-hydroxy nitrates, low-energy 18 isomerization pathways exist to six-membered ring compounds that are around 5 kcal mol-1 19 exothermic. These reactions proceed with barriers around 15 kcal mol-1 below the 20 respective peroxyl radical + NO reactants and yield orthonitrites that can further isomerize 21 to β-hydroxy ISOPNs. Moreover, the δ-hydroxy nitrates can directly interconvert with their β 22 substituted counterparts via NO3 group migration, with barriers that are lower yet. It follows 23 that β-hydroxy nitrates may be stabilized in the δ-hydroxy form, and vice versa. Moreover, 24 the lowest-energy pathway for dissociation of the δ-hydroxy ISOPNs is for the formation of 25 β-hydroxy alkoxyl radicals, and because of this established branching fractions between the 26 various isoprene peroxyl radicals may require re-evaluation. The results presented here also 27 suggest that ISOPNs may be stabilized to some extent in their saturated orthonitrite forms, 28 which has implications for both the total nitrate yield and for their subsequent removal by 29 OH, O3, and photolysis.

Perfluorooctanesulfonic acid (PFOS) is a widespread and persistent pollutant of concern to human health and the environment. Although incineration is often used to treat material contaminated with PFOS and related per- and polyfluoroalkyl substances (PFAS), little is known about the precise chemical mechanism for the thermal decomposition of these substances of concern. Here, we present the first study of the thermal decomposition kinetics of PFOS and related perfluorinated acids, using computational chemistry and reaction rate theory methods. We discover that the preferred channel for PFOS decomposition is via an α-sultone that spontaneously decomposes to form perfluorooctanal and SO2. At 1000 K the halflife for PFOS is predicted to be 0.2 s, decreasing sharply as temperature increases further. These results show that the acid headgroup in PFOS can be efficiently destroyed in incinerators operating at relatively modest temperatures. The new insights provided into the exact decomposition mechanism and kinetics of PFOS will help to improve remediation technologies actively under development.

Cigarette smoke is recognized as having harmful health effects for the smoker and for people breathing second-hand smoke. In an atmospheric chemistry context, however, little is known about the fate of organic nitrogen compounds present in cigarette smoke. Indeed, the atmospheric oxidation of nicotine, a major nitrogen-containing component of cigarette smoke, by OH radicals has yet to be investigated. We measured the first rate coefficient between OH and nicotine to be (8.38 ± 0.28) × 10–11 cm3 molecule–1 s–1 at 298 ± 3 K. We use an online proton-transfer-reaction mass spectrometer (PTR-MS) to quantify nicotine’s oxidation products, including formamide and isocyanic acid (HNCO). We present the first evidence that HNCO is formed from nicotine’s gas phase oxidation, and we highlight the potential for this toxic molecule to be an indoor air pollutant after smoking has ended. Mechanistic pathways for the oxidation of nicotine by OH radicals were investigated by theoretical calculations at the M06-2X level of ...

The gas-phase protodecarboxylation of acetic acid catalyzed by group 10 metal complexes was examined using a combination of multistage mass spectrometry experiments in an ion trap mass spectrometer, DFT calculations, and theoretical kinetic modeling. Two related catalytic cycles sharing two common intermediates were examined. The entry points to both cycles are the metal acetate complexes [(phen)M(O2CCH3)]+ (where phen = 1,10-phenanthroline), which were formed via direct electrospray ionization of solutions of the complexes [(phen)M(O2CCH3)2] in water. Step 1 of both cycles involves decarboxylation of [(phen)M(O2CCH3)]+ under collision-induced dissociation (CID) conditions to form the organometallic species [(phen)M(CH3)]+. The ease of decarboxylation follows the order Pd > Pt > Ni as determined via energy-resolved CID experiments, which is in agreement with the activation energies for decarboxylation estimated from DFT calculations. Step 2 of cycle 1 involves an ion–molecule reaction between [(phen)M(CH3...

Xylyl radicals are intermediates in combustion processes since their parent molecules, xylenes, are present as fuel additives. In this study we report on the photoelectron spectra of the three isomeric xylyl radicals and the subsequent decomposition reactions of the o-xylyl radical, generated in a tubular reactor and probed by mass selected threshold photoelectron spectroscopy and VUV synchrotron radiation. Franck-Condon simulations are applied to augment the assignment of elusive species. Below 1000 K, o-xylyl radicals decompose by hydrogen atom loss to form closed-shell o-xylylene, which equilibrates with benzocyclobutene. At higher temperatures relevant to combustion engines, o-xylylene generates styrene in a multistep rearrangement, whereas the p-xylylene isomer is thermally stable, a key point of difference in the combustion of these two isomeric fuels. Another striking result is that all three xylyl isomers can generate p-xylylene upon decomposition. In addition to C8H8 isomers, phenylacetylene and traces of benzocyclobutadiene are observed and identified as further reaction products of o-xylylene, while there is also some preliminary evidence for benzene and benzyne formation. The experimental results reported here are complemented by a comprehensive theoretical C8H8 potential energy surface, which together with the spectroscopic assignments can explain the complex high-temperature chemistry of o-xylyl radicals.

The kinetics of CO2 absorption into 30% w/w K2CO3 solutions containing 0.1–0.5 M K4V2O7 was investigated at temperatures of 40, 60, and 75 °C using a wetted wall column. Vanadium(V) speciation diagrams were developed under these conditions as a function of CO2 loading using 51V NMR spectroscopy. From these studies it was determined that there are two oxyvanadate ions that promote the absorption of CO2, HVO42–, and HV2O73–. The Arrhenius expressions for the rate constants of these two species were found to be kHVO4 = 2 × 1011 exp(−4992/T) and kHV2O7 = 5 × 1018 exp(−10218/T), respectively. Comparison of the observed rate constants with other promoters revealed that both active vanadium species showed performances comparable with that of MEA and vastly superior performances over those of other inorganic promoters. Due to speciation changes as the vanadium concentration is increased, the relative performance of vanadium diminished with increasing total vanadium concentration. As such, vanadium may be more sui...

The reactions of water with the coordinatively unsaturated group 10 organometallic cations [(L)M(R)]+ (4; where L = 1,10-phenanthroline (phen), neocuproine (neo); M = nickel, palladium, platinum; R = CH3, C6H5, CH2C6H5), formed via decarboxylation of the carboxylate complexes [(L)M(O2CR)]+, were examined in the gas phase using a combination of multistage mass spectrometry experiments and DFT calculations at the M06/SDD6-31+G(d) level of theory. Two main types of primary product ions were observed: the aqua adduct [(L)M(R)(H2O)]+ (5) and the hydroxide [(L)M(OH)]+ (7), formed via a hydrolysis reaction. A secondary product ion, arising from formation of the adduct [(L)M(OH)(H2O)]+, was also observed when L = phen, R = CH3, and M = Pt. The rates of reaction of 4 and the product branching ratios for 5 and 7 were dependent upon the nature of M, L, and R. When L = phen and R = CH3, the hydroxide 7 dominates for Ni, with the adduct 5 as the major product for both Pd and Pt. For R = C6H5 the rate of the reaction i...

Molecular photoisomerization plays a crucial role in diverse biological and technological contexts. Here, we combine ion mobility spectrometry and laser spectroscopy to characterize the photoisomerization of molecular cations in the gas phase. The target molecular ions, polymethine dye cations 3,3'-diethylthiacarbocyanine (DTC(+)), are propelled through helium buffer gas by an electric field and are photoisomerized by light from a tunable laser. Photoexcitation over the 450-570 nm range converts trans-DTC(+) to cis-DTC(+), noticeably modifying the ions' arrival time distribution. The photoisomerization action spectrum, which has a maximum at 535 nm, resembles the absorption spectrum of DTC(+) in solution but is shifted 25 nm to shorter wavelength. Comparisons between measured and calculated mobilities suggest that the photoisomer involves a twist about the second C-C bond in the methine chain (8,9-cis isomer) rather than a twist about the first methine C-C bond (2,8-cis isomer). It is postulated that the excited gas-phase ions internally convert from the S1 Franck-Condon region to the S0 manifold and explore the conformational landscape as they cool through He buffer gas collisions. Master equation simulations of the relaxation process in the S0 manifold suggest that the 8,9-cis isomer is preferred over the 2,8-cis isomer because it lies lower in energy and because it is separated from the trans isomer by a substantially higher barrier. The study demonstrates that the photoisomerization of molecular ions can be probed selectively in the gas phase, providing insights into photoisomerization mechanisms and information on the solvent-free absorption spectrum.

Xylyl (methylbenzyl) radicals are important combustion intermediates, formed in the pyrolysis and oxidation of xylenes and other substituted aromatic fuel additives. We have used VUV synchrotron radiation and imaging photoelectron photoion coincidence (iPEPICO) spectroscopy techniques to identify para-xylylene as the dominant stable C8H8 product arising from thermal decomposition of the meta-xylyl radical. A complex rearrangement from a meta- to a para-substituted aromatic, supported by quantum chemical calculations, can rationalize the observed reaction products. This work provides the first experimental evidence for the pyrolysis products of the meta-xylyl radical and can explain why the decomposition of this radical is considerably slower than that of the ortho and para isomers. This study emphasizes the utility of VUV synchrotron radiation and iPEPICO spectroscopy to tackle the reaction mechanism of combustion-relevant processes.

Aminyl free radicals of the form RN(•)H are formed in the photochemical oxidation of primary amines, and their reaction with (•)NO is an important tropospheric sink. Reaction of the parent methylamidogen radical (CH3N(•)H) with (•)NO in the gas phase has been studied using quantum chemical techniques and RRKM theory/master equation based kinetic modeling. Calculations with the G3X-K composite theoretical method indicate that reaction proceeds via exothermic formation of a primary nitrosamine intermediate, CH3NHNO, which can isomerize to an alkyldiazohydroxide, CH3NNOH, and further eliminate water to form diazomethane, CH2NN. Master equation simulations conducted at tropospheric conditions identify that the collisionally stabilized CH3NHNO and CH3NNOH isomers are the major reaction products, with smaller yields of CH2NN + H2O. A previously proposed mechanism in which the primary nitrosamine is destroyed via isomerization to CH2NHNOH, followed by reaction with O2 to produce CH2NH + HO2(•) + (•)NO, is disproved. In the atmosphere, CH2NN may be formed with sufficient vibrational energy to directly dissociate to singlet methylene ((1)CH2) and N2, whereas under combustion conditions this is expected to be the dominant pathway. This study suggests that stabilized primary nitrosamines can indeed form in the photochemical oxidation of amines, along with alkyldiazohydroxides and diazoalkanes. Both classes of compound are potent alkylating agents that may need to be considered in future atmospheric studies.

Amino acids are potential solvents for carbon dioxide separation processes, but the kinetics and mechanism of amino acid–CO2 reactions are not well-described. In this paper, we present a study of the reaction of glycine with CO2 in aqueous media using stopped-flow ultraviolet/visible spectrophotometry as well as gas/liquid absorption into a wetted-wall column. With the combination of these two techniques, we have observed the direct reaction of dissolved CO2 with glycine under dilute, idealized conditions, as well as the reactive absorption of gaseous CO2 into alkaline glycinate solvents under industrially relevant temperatures and concentrations. From stopped-flow experiments between 25 and 40 °C, we find that the glycine anion NH2CH2CO2– reacts with CO2(aq) with k (M–1 s–1) = 1.24 × 1012 exp[−5459/T (K)], with an activation energy of 45.4 ± 2.2 kJ mol–1. Rate constants derived from wetted-wall column measurements between 50 and 60 °C are in good agreement with an extrapolation of this Arrhenius expressi...

Methacrolein (MACR) plays an important role in atmospheric chemistry within the planetary boundary layer, as it is one of the major oxidation products of isoprene and has a short lifetime toward the hydroxyl radical (OH). In this study, quantum chemical techniques and statistical reaction rate theory have been used to simulate the addition of OH to MACR at conditions representative of the troposphere. In this chemically activated reaction, the time scales for product formation versus collisional deactivation of the vibrationally excited adduct are explicitly considered. Furthermore, the subsequent addition of O(2) is also incorporated within a single master equation, so as to investigate doubly activated peroxyl radical formation. The major reaction product of OH addition to MACR is the HOCH(2)C(•)(CH(3))CHO radical formed via addition to the outer (β) carbon. This radical is predominantly in the Z isomer although around a third of the population is quenched as the higher-energy E isomer. Calculated rate constants agree well with experiment when using M06-2X/aug-cc-pVTZ barrier heights, but are somewhat overpredicted using G3SX energies. The overall rate constant is controlled by competition between dissociation of the MACR···OH van der Waals complex back to reactants and isomerization on to MACR-OH adducts, which takes place on a time scale of several nanoseconds, but collisional deactivation of the MACR-OH adducts occurs on a time scale that is around an order of magnitude longer. When O(2) addition is included in the master equation, we observe that the MACR-OH adducts are removed by reaction with O(2) on a similar time scale to collisional deactivation. Around 50% of the subsequent peroxyl radical population is formed with some identifiable excess vibrational energy above singly activated [MACR-OH-O(2)]*, with around 20% provided with an additional 20 kcal mol(-1) (>40 kcal mol(-1) relative to quenched MACR-OH-O(2)) that can go into further unimolecular reaction. This double activation process is expected to lead to some prompt unimolecular decomposition of excited [MACR-OH-O(2)]** peroxyl radicals to yield products including hydroxyacetone and methylglyoxal, regenerating the initiating OH radical in the process.

Gas-phase decarboxylation of the acetate ligand in the magnesium acetate crown complex cations [CH3CO2Mg(3X-crown-X)]+ (where X = 4–6) has been explored as a means of synthesizing the corresponding organomagnesium cations as models for the organomagnesium core CH3Mg+, which is solvated by ether ligands. Low-energy collision-induced dissociation (CID) of these complexes in ion trap mass spectrometers gives rise to a range of product ions. High-resolution mass measurements reveal the formation of isobaric product ions arising from decarboxylation, [CH3Mg(3X-crown-X)]+, and from loss of C2H4O from the crown ether ligand. The use of low-energy CID of the isotopically labeled complexes [CH313CO2Mg(3X-crown-X)]+ allowed for (i) energy-resolved CID studies, which demonstrated that [CH313CO2Mg(12-crown-4)]+ is more easily decarboxylated than [CH313CO2Mg(15-crown-5)]+ and [CH313CO2Mg(18-crown-6)]+ and (ii) the separation and isolation of the methyl magnesium crown ether cations [CH3Mg(3X-crown-X)]+ for subsequent ...

The hydroxyl radical (OH) controls the removal of organic compounds from the troposphere. Atmospheric chemistry models significantly under-predict OH levels in unpolluted environments, implying that they are regenerated via some unknown mechanism(s). This work uses computational chemistry to demonstrate that the photochemical oxidation of alkyl carboxylic acids can efficiently regenerate the hydroxyl radical via unimolecular decomposition of alpha-carboxyalkylperoxy radicals. For acetic acid and propanoic acid the proposed mechanism is predicted to dominate in the unpolluted lower troposphere, and it may also operate to some extent in the mid to upper troposphere. Alkyl carboxylic acids are also predicted to act as a new source of nighttime OH throughout the planetary boundary layer, where OH levels are also under-predicted. The thermodynamic requirements for reactions of this class are discussed, and some candidate OH-reforming molecules particularly relevant to aromatic photooxidation are identified. Adopting a broader perspective, the alpha-carboxyalkyl radical precursors that react with O(2) to form the unstable alpha-carboxyalkylperoxy type radicals are also expected to form during combustion, in the interstellar medium, and from the gamma-irradiation of glycine and related amino acids, and the potential importance of this new chemistry in these environments is discussed. Master equation simulations suggest that alpha-carboxyalkyl + O(2) reactions provide a prompt OH source during the autoignition and combustion of biodiesel and other oxygenated biofuels, where carboxylic acids are formed as early stage oxidation products. Ketene combustion is also thought to proceed via these OH-reforming alpha-carboxyalkyl radicals. The in vivo formation of alpha-carboxyalkylperoxy radicals followed by oxidation to the highly reactive OH radical may induce oxidative stress in the human body, in a process initiated by gamma-rays. Finally, the reaction of ketenes with OH to form alpha-carboxyalkyl radicals, followed by addition of NH(2) or related species, is suggested as a new extraterrestrial pathway to amino acids.

This study proposes the existence of a new, highly resonantly stabilized free radical, fulvenallenyl (C7H5). Fulvenallenyl forms from dissociation or abstraction of the weak H atoms in the C7H6 compounds fulvenallene and 1-ethynylcyclopentadiene, which were recently identified as intermediates in the combustion of aromatic fuels and in sooting flames. The fulvenallenyl radical shares properties of the propargyl and cyclopentadienyl radicals, and like these species, we propose that fulvenallenyl has a significant resonance stabilization energy. This resonance energy is lost upon further reaction, making the fulvenallenyl radical resistant to oxidation and thermal decomposition. As with the resonantly stabilized radicals propargyl and cyclopentadienyl, fulvenallenyl is expected to react with other radicals in molecular weight growth reactions. Several pathways are proposed for self-reaction and cross-reactions of fulvenallenyl that directly produce two- and three-ring PAHs like naphthalene, diphenyl, and phenanthrene. Rate constants are calculated for H atom abstraction from fulvenallene and 1-ethynylcyclopentadiene by the common radicals H, OH, and CH3, to facilitate the inclusion of fulvenallenyl in kinetic models.

Bioethanol is currently a significant gasoline additive and the major blend component of flex-fuel formulations. Ethanol is a high-octane fuel component, and vehicles designed to take advantage of higher octane fuel blends could operate at higher compression ratios than traditional gasoline engines, leading to improved performance and tank-to-wheel efficiency. There are significant uncertainties, however, regarding the mechanism for ethanol autoignition, especially at lower temperatures such as in the negative temperature coefficient (NTC) regime. We have studied an important chemical process in the autoignition and oxidation of ethanol, reaction of the alpha-hydroxyethyl radical with O2(3P), using first principles computational chemistry, variational transition state theory, and Rice-Ramsperger-Kassel-Marcus (RRKM)/master equation simulations. The alpha-hydroxyethyl + O2 association reaction is found to produce an activated alpha-hydroxy-ethylperoxy adduct with ca. 37 kcal mol(-1) of excess vibrational energy. This activated adduct predominantly proceeds to acetaldehyde + HO(2), with smaller quantities of the enol vinyl alcohol (ethenol), particularly at higher temperatures. The reaction to acetaldehyde + HO2 proceeds with such a low barrier that collision stabilization of C2O3H5 isomers is unimportant, even for high-pressure/low-temperature conditions. The short lifetimes of these radicals precludes the chain-branching addition of a second O2 molecule, responsible for NTC behavior in alkane autoignition. This result helps to explain why ignition delays for ethanol are longer than those for ethane, despite ethanol having a weaker C-C bond energy. Given its relative instability, it is also unlikely that the alpha-hydroxy-ethylperoxy radical acts as a major acetaldehyde sink in the atmosphere, as has been suggested.

Reaction with the hydroxyl radical (HO(*)) is the primary removal mechanism for organic compounds in the atmosphere, and an important process in combustion. Molecules with unsaturated carbon sites are thought to react with HO(*) via a rapid addition mechanism, with little or no barrier; this results in short lifetimes relative to the saturated alkanes, which undergo slower abstraction reactions. Computational chemistry and reaction rate theory are used in this study to investigate the s-triazine + HO(*) reaction. We report that HO(*) addition at a carbon ring site proceeds with the largest known barrier for addition to an unsaturated carbon (9.8 kcal mol(-1) at the G3X level of theory). Abstraction of a hydrogen atom in s-triazine by HO(*), forming the s-triazinyl radical + H(2)O, proceeds with a barrier of only 3.3 kcal mol(-1), and this process dominates over HO(*) addition. Our results are in contrast to those for the analogous reactions in benzene, where the abstraction reaction to phenyl + H(2)O is slower than the HO(*) addition, which forms a radical adduct that can further react with O(2) or dissociate to phenol + H(*). The lifetime of s-triazine toward the hydroxyl radical in the troposphere is estimated at 6.4 years, potentially making it a long-lived pollutant. The aromatic s-triazine (1,3,5-triazine) molecule is a structural feature in herbicides such as atrazine and is a decomposition product of the common energetic material cyclotrimethylenetrinitramine (RDX). While the abstraction reaction dominates for the parent s-triazine, the addition mechanism may be of importance in the atmospheric degradation of substituted triazines, like atrazine, where ring H atoms are not available for abstraction. The high-barrier addition mechanism forms an activated hydroxy-triazinyl adduct which predominantly dissociates to 2-hydroxy-1,3,5-triazine (OST) + H(*). This OST species is a known intermediate of RDX decomposition. Results are also presented for isomerization of the less-stable 1,3,5-triazine-N-oxide OST species (which may form via unimolecular pathways in the liquid-phase decomposition of RDX) to 2-hydroxy-1,3,5-triazine. A reaction mechanism is proposed for further oxidation of the s-triazinyl radical, where an OST isomer is also a potential product.

The kinetics of benzoxyl radical decomposition was studied using ab initio computational chemistry and RRKM rate theory. The benzoxyl radical is an important but short-lived intermediate in the combustion of toluene and other alkylated aromatic hydrocarbons. A theoretical study of the thermochemistry and kinetics to products over a range of temperatures and pressures for benzoxyl decomposition is reported. Ab initio calculations with the G3X theoretical method reveal low-energy pathways from the benzoxyl radical to benzaldehyde + H and the phenyl radical + formaldehyde (CH(2)O), as well as a novel mechanism to benzene + the formyl radical (HC(*)O). RRKM simulations were performed for benzoxyl decomposition as a function of temperature and pressure. Benzaldehyde formation constitutes more than 80% of the total reaction products at temperatures below 1000 K, decreasing to around 50% at 2000 K. Formation of benzene + HC(*)O and phenyl + CH(2)O is of similar importance, each accounting for 5-10% of the decomposition products at around 1000 K, increasing to 20-30% at 2000 K. The results presented here should lead to improved kinetic models for the oxidation of alkylated aromatic hydrocarbons, particularly for the formation of benzene as a direct oxidation product of toluene. Re-evaluation of the phenyl radical heat of formation leads us to suggest a benzene C-H bond dissociation energy in the range of 113.5-114.5 kcal mol(-1).

Nitrogen-containing heterocycles form the basis for a new generation of high-energy density materials, and they serve as model compounds for nitrogen-containing fuels, such as coal and biomass, and they form the backbone of ionic liquids. A novel retro-[3 + 2]-cycloaddition to a three-membered diene and a two-membered dienophile, analogous to a retro-Diels-Alder reaction, may constitute an important initial reaction step in the thermal decomposition of these heterocyclic compounds. We investigate the kinetics and thermodynamics of these reactions for the heterocycles pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, 1,2,5-triazole, 1,3,4,-triazole, 1H-tetrazole, and 2H-tetrazole, using theoretical computational chemistry. The retro-cycloadditions are shown to form one of the three-membered products: hydrazoic acid (NH=N=N), nitrilimine (NH=N=CH), carbodiimide (NH=C=NH), or ketenimine (NH=C=CH2) plus one of the two-membered products acetylene, hydrogen cyanide, or N2. Accurate enthalpies of formation are calculated for the reaction products using the high-level W1 computational protocol, providing the previously undetermined enthalpy values of 70.09, 88.75, 35.03, and 44.28 kcal mol(-1) for hydrazoic acid, nitrilimine, carbodiimide, and ketenimine, respectively. We apply a variable-order form of the Marcus equation to the dissociation reactions in correlating the enthalpy of reaction with the activation enthalpy. Typical molecular elimination reactions from the heterocycles proceed with an intrinsic activation enthalpy of 36.8 kcal mol(-1) and intrinsic activation free energy of 42.1 kcal mol(-1). However, dissociation reactions resulting in the formation of either NH=C=NH or NH=C=CH2 demonstrate intrinsic barriers ca. 30 kcal mol(-1) higher, as a result of a concerted intramolecular hydrogen shift. Rate constants calculated between 300 and 3000 K indicate that the proposed dissociation reactions should be important in the decomposition of tetrazole and 1,2,3-triazole. This is confirmed by comparison with available experimental data. Decomposition of 1,2,4-triazole to HCN + nitrilimine may also be important at high temperatures. From extrapolation of our Marcus equation relationship, we predict pentazole to decompose to N2 + NHNN with an activation enthalpy of 19.5 kcal mol-1 and a half-life of only 14 s at 298 K.

o-Quinone methide (o-QM), or 6-methylene-2,4-cyclohexadiene-1-one, has been identified as an important intermediate in lignin and alkyl benzene combustion, and the thermal decomposition of o-QM is therefore relevant to the combustion of transportation fuels (which contain toluene) and of biomass and low-rank coals (which contain lignin). We present a comprehensive reaction mechanism for the unimolecular conversion of o-QM to the reaction intermediates tropone and fulvene, calculated using theoretical quantum chemical techniques. Enthalpies of formation for all reactants, products, and intermediates are calculated using the CBS-QB3 theoretical method. Transition states are determined with the CBS-QB3 method, which we use to obtain rate constants as a function of temperature from transition-state theory, with Wigner tunneling corrections applied to hydrogen-shift reactions. Barrier heights are also calculated with the BB1K density functional theory (DFT) method for thermochemical kinetics. Reaction pathways are identified leading to tropone (which rapidly decomposes to benzene + CO) and to fulvene + CO, via initial hydrogen transfer to 2-hydroxyphenylcarbene and via ring opening to 1,3,5,6-heptatetraen-1-one, respectively. Quantum Rice-Ramsperger-Kassel (QRRK) theory analysis of the reaction kinetics indicates that the dominant reaction pathway is formation of tropone via 2-hydroxyphenylcarbene; the formation of fulvene + CO via initial ring opening constitutes a secondary pathway, which becomes more important with increasing temperature. Our calculations, using BB1K barrier heights, yield the rate equation k(T) [s(-1)] = 2.64 x 10(14) exp(-35.9/T [K]) for o-QM decomposition, which is in relatively good agreement with the experimental rate equation. Calculations provide an apparent activation energy of 71.3 kcal mol(-1), versus 67.2 kcal mol(-1) from experiment.

N-Hydroxyphthalimide (NHPI) has received significant attention in recent years as a catalyst of oxidation synthesis reactions. In this study, we use quantum chemical methods to calculate the molecular geometries, vibrational frequencies, and thermochemical properties of NHPI and the species corresponding to OH hydrogen loss in NHPI, the phthalimide N-oxyl radical (PINO). We also study an analogue of NHPI, N-hydroxymaleimide (NHMI), and we demonstrate that NHPI and NHMI have similar thermochemical properties. G3B3 and CBS-APNO calculations with bond-isodesmic work reactions provide O−H BDEs of 83.3 and 83.5 kcal mol-1 (348.5 and 349.4 kJ mol-1) for NHPI and NHMI, respectively, in the gas phase. Liquid-phase bond energy calculations are performed using the solvents benzene, carbon tetrachloride, acetone, ethanol, DMSO, and acetonitrile with the PCM solvent model. From these liquid-phase G3B3 calculations, we calculate a NHPI O−H BDE of between 83.3 and 83.7 kcal mol-1 (348.5 and 350.2 kJ mol-1) in a variety...

The bonding between anionic nucleophiles and the nitroso group has been studied in the common nitrosating agents nitroso chloride (ONCl), nitroso bromide (ONBr), nitroso thiocyanate (ONSCN), and dinitrogen trioxide (N2O3) in aqueous solution. A variety of theoretical methods were employed, including ab initio, density functional theory (DFT), and composite theoretical techniques, with solvent effects described using the polarizable continuum model (PCM). Experimental nitroso bond heterolytic dissociation free energies were accurately reproduced with a number of composite theoretical methods, the most successful being CBS-Q and G2MP2, with average errors of 3.1 and 3.4 kJ mol-1, respectively. Using the MP2 and B3LYP methods, calculations were made with correlation consistent basis sets up to quadruple-ζ, extrapolated to the complete basis set (CBS) limit. The MP2/CBS calculations were accurate to around 10 kJ mol-1, while the B3LYP/CBS calculations routinely overpredicted experimental bond free energies by...

The nitrogen-containing heterocycles are of interest as high-energy-density materials for use as propellants and explosives, while the pyrolysis of these compounds is also important in understanding the evolution of unwanted NO and NO2 (NOx) from organic fuels such as coal and biomass. We have used ab initio and density functional methods to study the molecular structures and thermochemical properties of the five-membered nitrogen-containing heterocycles and their anions and radicals corresponding to respective heterolytic and homolytic loss of a hydrogen atom from either a nitrogen or carbon site. Many of these thermochemical properties have not previously been measured, especially for the heterocycles containing three and four nitrogen atoms. Using the theoretical methods CBS-APNO, G3, and G3B3, we calculate enthalpies of formation of 26.5, 42.4, 31.9, 63.7, 46.8, 81.0, and 79.0 kcal mol-1 for pyrrole, pyrazole, imidazole, 1,2,3-triazole, 1,2,4-triazole, 1H-tetrazole, and 2H-tetrazole. A correlation is developed between the number of nitrogen atoms in a heterocycle and its enthalpy, and we extrapolate this relationship to predict the enthalpy of formation of pentazole. N-H BDEs in the heterocycles typically increase with the number of nitrogen atoms in the molecule, while C-H BDEs are similar in all of the studied heterocycles, at around 120 kcal mol-1. In all cases the N-H BDEs are weaker than the C-H BDEs, suggesting abstraction of the N-H hydrogen atom is more likely. Deprotonation enthalpies and free energies reveal that the N-H protons become more acidic with increasing number of nitrogen atoms in the heterocycle. C-H protons are less acidic than N-H protons by ca. 49 kcal mol-1, or ca. 35 kcal mol-1 when adjacent to the NH group. Trends in N-H and C-H acidities can be qualitatively explained by electrostatic effects and electron affinities. From its use as a reference species in our calculations, we identify that the experimental enthalpy of pyrimidine (1,3-diazine) may be in error by ca. 1-3 kcal mol-1, and we recommend an enthalpy of formation of 44.8 +/- 1.0 kcal mol-1.

We present an ab initio procedure for accurately calculating aqueous-phase pKa values and apply it to study the acidity of nitrous acid (HNO2, or HONO). The aqueous-phase pK(a) of nitrous acid was obtained from calculated gas-phase acidities and solvation free energies via a thermodynamic cycle and the solvation model chemistry of Barone et al. (J. Chem. Phys. 1997, 107, 3210). Solvation free energies were calculated at the HF/6-31G(d) level using the dielectric-polarizable continuum and the integral equation formalism-polarizable continuum solvent models (D-PCM and IEF-PCM, respectively), with the D-PCM model yielding the most accurate pKa values. For HF free energies of solvation, significant improvements in accuracy could be made by moving to the larger 6-311++G(3df,3pd) and aug-cc-pVQZ basis sets. Solvation free energies were also calculated using the density functional theory (DFT) methods B3LYP, TPSS, PBE0, B1B95, VSXC, B98 and O3LYP, with the most accurate methods being TPSS and VSXC, which provided average errors of less than 0.11 pKa units. Solvation free energies calculated with the different DFT methods were relatively insensitive to the basis set used. Our theoretical calculations are compared with experimental results obtained using stopped flow spectrophotometry. The pKa of nitrous acid was measured as 3.16 at 25 degrees C, and the enthalpy and entropy of nitrous acid dissociation were calculated from measurements as 6.7 kJ mol(-1) and -38.4 J mol(-1) K(-1), respectively, between 25 and 45 degrees C. The UV/visible absorption spectra of the nitrite ion and nitrous acid were also examined, and molar extinction coefficients were obtained for each.

Vinyl alcohols (enols) have been discovered as important intermediates and products in the oxidation and combustion of hydrocarbons, while methyl vinyl ethers are also thought to occur as important combustion intermediates. Vinyl alcohol has been detected in interstellar media, while poly(vinyl alcohol) and poly(methyl vinyl ether) are common polymers. The thermochemical property data on these vinyl alcohols and methyl vinyl ethers is important for understanding their stability, reaction paths, and kinetics in atmospheric and thermal hydrocarbon-oxygen systems. Enthalpies , entropies , and heat capacities (C(p)()(T)) are determined for CH(2)=CHOH, C(*)H=CHOH, CH(2)=C(*)OH, CH(2)=CHOCH(3), C(*)H=CHOCH(3), CH(2)=C(*)OCH(3), and CH(2)=CHOC(*)H(2). Molecular structures, vibrational frequencies, , and C(p)(T) are calculated at the B3LYP/6-31G(d,p) density functional calculation level. Enthalpies are also determined using the composite CBS-Q, CBS-APNO, and G3 methods using isodesmic work reactions to minimize calculation errors. Potential barriers for internal rotors are calculated at the B3LYP/6-31G(d,p) level and used to determine the hindered internal rotational contributions to entropy and heat capacity. The recommended ideal gas phase values calculated in this study are the following (in kcal mol(-1)): -30.0, -28.9 (syn, anti) for CH(2)=CHOH; -25.6, -23.9 for CH(2)=CHOCH(3); 31.3, 33.5 for C(*)H=CHOH; 27.1 for anti-CH(2)=C(*)OH; 35.6, 39.3 for C(*)H=CHOCH(3); 33.5, 32.2 for CH(2)=C(*)OCH(3); 21.3, 22.0 for CH(2)=CHOC(*)H(2). Bond dissociation energies (BDEs) and group additivity contributions are also determined. The BDEs reveal that the O-H, O-CH(3), C-OH, and C-OCH(3) bonds in vinyl alcohol and methyl vinyl ether are similar in energy to those in the aromatic molecules phenol and methyl phenyl ether, being on average around 3 kcal mol(-1) weaker in the vinyl systems. The keto-enol tautomerization enthalpy for the interconversion of vinyl alcohol to acetaldehyde is determined to be -9.7 kcal mol(-1), while the activation energy for this reaction is calculated as 55.9 kcal mol(-1); this is the simplest keto-enol tautomerization and is thought to be important in the reactions of vinyl alcohol. Formation of the formyl methyl radical (vinoxy radical/vinyloxy radical) from both vinyl alcohol and methyl vinyl ether is also shown to be important, and its reactions are discussed briefly.

The rate of primary amino acid nitrosation, both with and without the addition of nucleophilic species, has been studied using stopped-flow spectrophotometry. The rate of nitrosation in the presence of strong nucleophilic species such as thiocyanate and thiourea was shown to be much faster than nitrosation without the addition of a nucleophile. Rate constants were determined at 25 degrees C for reaction of the amino acids alanine, glycine, and valine with five common nitrosating agents. For the nitrosating agents nitrosyl chloride, nitrosyl bromide, and dinitrogen trioxide the rate of reaction was observed to approach the predicted encounter-controlled limit. However, for nitrosyl thiocyanate and S-nitrosothiourea nitrosation was found to be reaction-controlled. In the reaction-controlled regime, rate constants were found to increase with increasing electrophilic strength of the nitrosating agent, as measured by the parameter En, with a slope indicative of a product-like transition state. Activation energies were also measured, being around 10-30 kJ mol-1 for encounter-controlled rate constants, and 30-50 kJ mol-1 for reaction-controlled rate constants. Our results are discussed in the context of in vivo amino acid nitrosation, where it is proposed that the rate of nitrosation may be considerably greater than currently thought, due to the presence of nucleophilic species.