J. Mark Parnis

Electron bombardment matrix isolation of Rg/Rg′/methanol mixtures (Rg= Ar, Kr, Xe): Fourier-transform infrared characterization of the proton-bound dimers Kr2H+, Xe2H+, (ArHKr)+ and (ArHXe)+ in Ar matrices and (KrHXe)+ and Xe2H+ in Kr matrices

Matrices formed by subjecting methanol vapor diluted in argon/krypton mixtures to electron bombardment and subsequent matrix isolation (EBMI) reveal a new feature at 885.3 cm−1 which is assigned to the v3 fundamental of the triatomic cation (ArHKr)+. In samples containing about 5% krypton, the v3 fundamental of Kr2H+ is also observed in a predominantly solid argon environment. These assignments are supported by annealing experiments and by density functional theory calculations reported in a separate paper. Similar experiments with xenon diluted in argon yield infrared spectra showing the v3 and v3+v1 bands of (ArHXe)+, and v3 of Xe2H+ in predominantly argon environments. EBMI of methanol diluted in krypton in the presence of xenon gives rise to infrared bands assigned to the v3 and v3+v1 fundamentals and combination bands of both (KrHXe)+ and Xe2H+ isolated in predominantly krypton environments. The energetics and plausible mechanisms to the formation of these homogenous and mixed proton-bound rare-gas d...

Density functional theory study of the proton-bound rare-gas dimers Rg2H+ and (RgHRg′)+ (Rg=Ar, Kr, Xe): Interpretation of experimental matrix isolation infrared data

Density functional theory calculations have been performed on the various proton-bound rare-gas dimers Rg2H+ and (RgHRg′)+ (Rg=Ar, Kr, or Xe, and Rg≠Rg′) employing the BP86 method coupled with either a Gaussian split valence basis set (DZVP) or a numerical split valence basis set (DN**). The calculations with the DN** basis represent the first calculations in which correct qualitative agreement is obtained with respect to the trend in experimental data for the antisymmetric stretching wavenumbers of the three Rg2H+ cations. Good qualitative agreement is also obtained for the antisymmetric stretching wavenumber of the mixed-rare-gas species (ArHKr)+. For the xenon-containing mixed-rare-gas cations, the agreement with experimental wavenumbers is not good as is the case for the DZVP basis set with any of the aforementioned cations. This is believed to be due to the inability of these basis sets to predict some physical and chemical properties for these species. Quantitative agreement between theory and exper...

Dry deposition of polycyclic aromatic compounds to various land covers in the Athabasca oil sands region

A framework was developed to estimate dry deposition of polycyclic aromatic compounds (PACs), including 17 polycyclic aromatic hydrocarbons (PAHs), 21 alkylated PAHs, and 5 parent and alkylated dibenzothiophenes (DBTs), to various land covers surrounding three monitoring sites in the Athabasca oil sands region. Modeled dry deposition velocities for various gaseous PACs and over various land covers were mostly in the range of 0.01–0.5 cm s−1 with median and annual mean values between 0.08 and 0.24 cm s−1, comparable with literature values obtained from field studies. Annual dry deposition of the sum of PAHs was estimated to range from 330 to 560 μg m−2 over forested canopies surrounding the three sites and from 270 to 490 μg m−2 over grass and shrubs. The corresponding values are 3920–5380 and 2850–4920 μg m−2 for the sum of 21 alkylated PAHs, and are 230–1120 and 450–930 μg m−2 for the sum of 5 DBTs. The three monitoring sites are situated nearby the Athabasca River, and the direct annual atmospheric dry deposition to water surface was estimated to range from 350 to 500, 3170 to 4530, and 170 to 840 μg m−2 for PAHs, alkylated PAHs, and DBTs, respectively. Alkylated PAHs contributed 80% of the total dry and 60% of the total wet deposition budget, suggesting the importance of including this group of PAHs in the atmospheric deposition budget estimation for subsequent ecosystem impact studies.

Gas‐phase niobium cluster reactivity with isobutane

A laser ablation fast‐flow reactor apparatus, equipped with a laser ionization time‐of‐flight mass spectrometer detector, has been used to investigate the reactivity of gas‐phase Nbn with isobutane. Absolute second‐order rate coefficients k(2) for the removal of Nbn by isobutane were measured at 279, 300, and 371 K in a continuous flow of He carrier gas held at 1 or 2 Torr total flow tube pressure. The rate coefficients for a given cluster species were found to be pressure independent, and became larger with increasing temperature. A gradual increase in k(2) with increasing cluster size has been observed. This increase is simply explained by a direct correlation of k(2) with the hard‐sphere collision cross section. Pseudo‐first‐order kinetic plots for all cluster reactions with isobutane were linear with no bimodal character evident, indicating an insensitivity to structural forms of Nbn. Activation energies have been estimated, and their values were found to be close to zero for all Nbn cluster reactions...

Photophysics of atomic magnesium isolated in solid methane and perdeuteromethane. III. Evidence for a kinetic isotope effect in the competitive process of atomic magnesium insertion into a C–H bond

The formation of the C–H bond insertion product methylmagnesium hydride (CH3MgH) and the simultaneous emission of atomic triplet magnesium are observed following photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in solid methane matrices at 12 K. Isotopic variation of the solid methane hosts produces observable differences in the relative branching ratios into the photophysical (atomic triplet emission) and photochemical (insertion product formation) channels. In solid perdeuteromethane (CD4), the intensity of the atomic emission is approximately five times that in solid methane (CH4) while the rate of formation of the insertion product shows the opposite behavior in the two solids. No singlet atomic magnesium emission is observed in the solid Mg/methane systems and the rise time of the atomic triplet emission is deduced to occur on a time scale of less than 10 ns. A simple model derived from spin and orbital correlations between reactants and products is prese...

A density functional theory study of the catalytic role of Ar, Kr, Xe, and N2 in the CH3OH·+ to CH2OH2·+ isomerization reaction

Density functional theory has been employed to investigate the influence of the neutral bases Ar, Kr, Xe and N 2 on the isomerization reaction of the methanol radical cation (CH3OH z1 ) to its distonic isomer (CH2OH21 ). Results of all electron, BP86/DN** calculations predict that these neutral bases do catalyze this isomerization reaction. It is expected that, in the presence of Xe or N2, this intramolecular H-atom transfer could be catalyzed efficiently, competing with the unimolecular dissociation channel which produces a hydrogen atom and the hydroxy methylene cation. Using theoretical data for four different isomerization reactions and numerous different catalysts, a linear relationship is found between the difference in proton affinities of the catalyst and the H-leaving site and the difference between the barrier heights of the uncatalyzed and catalyzed reactions, i.e. the lowering of the barrier. With this relationship, the decrease in the barrier height associated with a given base can be estimated, and the likely consequence of the coexistence of neutral base and hydrogen-containing cations in experiments can be assessed. It is further established that the density functional theory (DFT) methods and basis sets employed in this study do not predict very accurately the relative thermodynamic properties of the ionic species studied in this work. They do, however, give reasonable estimates of the decrease in energy because of association of a catalyst with the transition state. (Int J Mass Spectrom 190/191 (1999) 181‐194)

Characterizing PUF disk passive air samplers for alkyl-substituted PAHs: Measured and modelled PUF-AIR partition coefficients with COSMO-RS

Isomers of alkyl-substituted polycyclic aromatic hydrocarbons (PAHs) and dibenzothiophenes are modelled with COSMO-RS theory to determine the effectiveness and accuracy of this approach for estimation of isomer-specific partition coefficients between air and polyurethane foam (PUF), i.e., KPUF-AIR. Isomer-specific equilibrium partitioning coefficients for a series of 23 unsubstituted and isomeric alkyl-substituted PAHs and dibenzothiophenes were measured at 22 °C. This data was used to determine the accuracy of estimated values using COSMO-RS, which is isomer specific, and the Global Atmospheric Passive Sampling (GAPS) template approach, which treats all alkyl-substitutions as a single species of a given side-chain carbon number. A recently developed oligomer-based model for PUF was employed, which consisted of a 1:1 condensed pair of 2,4-toluene-diisocyanide and glycerol. The COSMO-RS approach resulted in a significant reduction in the RMS error associated with simple PAHs and dibenzothiophene compared with the GAPS template approach. When used with alkylated PAHs and dibenzothiophenes grouped into carbon-number categories, the GAPS template approach gave lower RMS error (0.72) compared to the COSMO-RS result (0.87) when the latter estimates were averaged within the carbon-number-based categories. When the isomer-specific experimental results were used, the COSMO-RS approach resulted in a 21% reduction in RMS error with respect to the GAPS template approach, with a 0.57 RMS error for all alkylated PAHs and dibenzothiophenes studied. The results demonstrate that COSMO-RS theory is effective in generating isomer-specific PUF-air partition coefficients, supporting the application of PUF-based passive samplers for monitoring and research studies of polycyclic aromatic compounds (PACs) in air.

Direct and indirect effects of different types of microplastics on freshwater prey (Corbicula fluminea) and their predator (Acipenser transmontanus)

We examined whether environmentally relevant concentrations of different types of microplastics, with or without PCBs, directly affect freshwater prey and indirectly affect their predators. Asian clams (Corbicula fluminea) were exposed to environmentally relevant concentrations of polyethylene terephthalate (PET), polyethylene, polyvinylchloride (PVC) or polystyrene with and without polychlorinated biphenyls (PCBs) for 28 days. Their predators, white sturgeon (Acipenser transmontanus), were exposed to clams from each treatment for 28 days. In both species, we examined bioaccumulation of PCBs and effects (i.e., immunohistochemistry, histology, behavior, condition, mortality) across several levels of biological organization. PCBs were not detected in prey or predator, and thus differences in bioaccumulation of PCBs among polymers and biomagnification in predators could not be measured. One of the main objectives of this study was to test the hypothesis that bioaccumulation of PCBs would differ among polymer types. Because we could not answer this question experimentally, a bioaccumulation model was run and predicted that concentrations of PCBs in clams exposed to polyethylene and polystyrene would be greater than PET and PVC. Observed effects, although subtle, seemed to be due to microplastics rather than PCBs alone. For example, histopathology showed tubular dilation in clams exposed to microplastics with PCBs, with only mild effects in clams exposed to PCBs alone.

The chemistry of ionized Acetone/Ar mixtures under varying gas flow and mole ratio conditions: A matrix‐isolation study

The chemistry of ionized acetone:Ar mixtures under varying total gas flow rate and acetone:Ar mole ratio conditions has been studied using matrix-isolation techniques. Gaseous acetone diluted in excess argon gas was subjected to electron bombardment with 300-eV electrons. The products of subsequent reaction processes were matrix isolated and analyzed by Fourier transform infrared (FTIR) absorption spectroscopy. Products included 1-propen-2-ol (the enol isomer of acetone), methane, ketene, carbon monoxide, ethane, ethene, acetylene and tricarbon monoxide. Variations in the total flow rate of gas resulted in changes in the efficiency of product formation without significant changes in the relative amounts of the major species formed. Variations in the acetone:Ar mole ratio at fixed total gas flow resulted in striking variations in the products formed, demonstrating a shift from single acetone molecule-derived charge-transfer ionization chemistry at low acetone mole ratios, to processes consistent with the participation of two or more acetone molecules at intermediate mole ratios. These results are interpreted in the context of ion-molecule reaction processes, the onset of which occurs at intermediate acetone mole ratios. Ethane dehydrogenation products are proposed to result from product secondary ionization, a process that is prevalent at high ionizing electron fluxes.

FTIR Analysis of Dimethyl Ether Decomposition Products Following Charge Transfer Ionization with Ar + under Matrix Isolation Conditions

Fourier transform infrared spectroscopic analysis has been performed on argon matrices formed following electron bombardment of argon/dimethyl ether mixtures. Products consistent with the ionization and subsequent fragmentation of dimethyl ether cation have been observed. Following ionization of dimethyl ether, fragmentation occurs that is consistent with ionization energy greater than 15 eV due to efficient charge transfer from dimethyl ether to Ar(+) as the major ionization process. Major products observed in the infrared spectra are methane, formaldehyde, HCO(*), CO, and Ar(2)H(+). These products are consistent with the known fragmentation of photoionized dimethyl ether in a 15-16 eV ionization energy range. However, the observation of dehydrogenated products is consistent with additional abstraction of hydrogen from proximally located species isolated within the matrix. Analogous experiments employing CD(3)OCH(3) give similar results, and the observed isotopically substituted products are consistent with the proposed fragmentation pathways.