Narcisse T. Tsona

Vapor-liquid nucleation of argon: Exploration of various intermolecular potentials

The homogeneous vapor-liquid nucleation of argon has been explored at T=70 and 90 K using classical nucleation theory, semiempirical density functional theory, and Monte Carlo simulations using the aggregation-volume-bias algorithm with umbrella sampling and histogram-reweighting. In contrast with previous simulation studies, which employed only the Lennard-Jones intermolecular potential, the current studies were carried out using various pair potentials including the Lennard-Jones potential, a modified Buckingham exponential-six potential, the Barker-Fisher-Watts pair potential, and a recent ab initio potential developed using the method of effective diameters. It was found that the differences in the free energy of formation of the critical nuclei between the potentials cannot be explained solely in terms of the difference in macroscopic properties of the potentials, which gives a possible reason for the failure of classical nucleation theory.

Matrix isolation FTIR study of hydrogen-bonded complexes of methanol with heterocyclic organic compounds

Acting as hydrogen bond acceptors, heterocyclic compounds could interact with a hydrogen bond donor, where either the heteroatom or the π system is the bonding site. The hydrogen bonded complexes of heterocyclic compounds with methanol (MeOH) were studied using matrix isolation FTIR spectroscopy and theoretical calculations based on density functional theory. Four heterocyclic compounds, furan (Fu), 2,5-dihydrofuran (DHF), pyrrole (Py) and thiophene (Th) were selected as representative examples for acceptors. For each of the MeOH–Fu, MeOH–DHF and MeOH–Th complexes, the O–H⋯π and O–H⋯Y (YO, S) hydrogen bonded structures were obtained, while an N–H⋯O instead of O–H⋯N hydrogen bonded conformer was found in the MeOH–Py complex. The measured OH-stretching transitions of the complexes in the IR spectra were assigned to the O–H⋯O bonded MeOH–Fu (b) and MeOH–DHF (b) conformers and the N–H⋯O bonded Py–MeOH (b) conformer, respectively. However, it was hard to assign the spectra to the exact MeOH–Th conformer, because all the hydrogen bond characteristic features obtained for different MeOH–Th conformers are too close. DHF forms a stronger O–H⋯O hydrogen bond than furan, and the O–H⋯O hydrogen bonded MeOH–Fu complex is more stable than the O–H⋯S bonded MeOH–Th complex. Atoms in molecules analysis was also performed to understand the nature of interaction in the MeOH complexes. This analysis allowed us to characterize the new bond critical point generated in the complexes. The present results help to evaluate the atmospheric behavior of some heterocyclic compounds, and indicate their importance in the pre-nucleation mechanism at the molecular level.

Structures, Hydration, and Electrical Mobilities of Bisulfate Ion-Sulfuric Acid-Ammonia/Dimethylamine Clusters: A Computational Study.

Despite the well-established role of small molecular clusters in the very first steps of atmospheric particle formation, their thermochemical data are still not completely available due to limitation of the experimental techniques to treat such small clusters. We have investigated the structures and the thermochemistry of stepwise hydration of clusters containing one bisulfate ion, sulfuric acid, base (ammonia or dimethylamine), and water molecules using quantum chemical methods. We found that water facilitates proton transfer from sulfuric acid or the bisulfate ion to the base or water molecules, and depending on the hydration level, the sulfate ion was formed in most of the base-containing clusters. The calculated hydration energies indicate that water binds more strongly to ammonia-containing clusters than to dimethylamine-containing and base-free clusters, which results in a wider hydrate distribution for ammonia-containing clusters. The electrical mobilities of all clusters were calculated using a particle dynamics model. The results indicate that the effect of humidity is negligible on the electrical mobilities of molecular clusters formed in the very first steps of atmospheric particle formation. The combination of the results of this study with those previously published on the hydration of neutral clusters by our group provides a comprehensive set of thermochemical data on neutral and negatively charged clusters containing sulfuric acid, ammonia, or dimethylamine.

Gas-Phase Reaction of Methyl n-Propyl Ether with OH, NO3, and Cl: Kinetics and Mechanism

Rate constants at room temperature (293 ± 2 K) and atmospheric pressure for the reaction of methyl n-propyl ether (MnPE), CH3OCH2CH2CH3, with OH and NO3 radicals and the Cl atom have been determined in a 100 L FEP-Teflon reaction chamber in conjunction with gas chromatography-flame ionization detector (GC-FID) as the detection technique. The obtained rate constants k (in units of cm3 molecule-1 s-1) are (9.91 ± 2.30) × 10-12, (1.67 ± 0.32) × 10-15, and (2.52 ± 0.14) × 10-10 for reactions with OH, NO3, and Cl, respectively. The products of these reactions were investigated by gas chromatography-mass spectrometry (GC-MS), and formation mechanisms are proposed for the observed reaction products. Atmospheric lifetimes of the studied ether, calculated from rate constants of the different reactions, reveal that the dominant loss process for MnPE is its reaction with OH, while in coastal areas and in the marine boundary layer, MnPE loss by Cl reaction is also important.

Gas-Phase Oxidation of Allyl Acetate by O3, OH, Cl, and NO3: Reaction Kinetics and Mechanism

Allyl acetate (AA) is widely used as monomer and intermediate in industrial chemicals synthesis. To evaluate the atmospheric outcome of AA, kinetics and mechanism of its gas-phase reaction with main atmospheric oxidants (O3, OH, Cl, and NO3) have been investigated in a Teflon reactor at 298 ± 3 K. Both absolute and relative rate methods were used to determine the rate constants for AA reactions with the four atmospheric oxidants. The obtained rate constants (in units of cm3 molecule-1 s-1) are (1.8 ± 0.3) × 10-18, (3.1 ± 0.7) × 10-11, (2.5 ± 0.5) × 10-10, and (1.1 ± 0.4) × 10-14, for reactions with O3, OH, Cl, and NO3, respectively. While results for reactions with O3, OH and Cl are in good agreement with previous studies, the kinetics for the reaction with NO3 is reported for the first time in this study. On the basis of determined rate constants, the tropospheric lifetimes of AA are τO3 = 9 days, τOH = 5 h, τCl = 5 days, τNO3 = 2 days. On the basis of the products study, reaction mechanisms for these oxidations have been proposed and the reaction products were detected using thermal desorption-gas chromatography-mass spectrometry (TD-GC-MS) and Fourier transform infrared spectroscopy (FTIR). Results show that the main products formed in these reactions are carbonyl compounds. In particular, 2-oxoethyl acetate was detected in all four AA oxidation reactions. Compared to previous studies, several new products were determined for reactions with OH and Cl. These results form a set of comprehensive kinetic data for AA reactions with main atmospheric oxidants and provide a better understanding of the degradation and atmospheric outcome of unsaturated acetate esters in the troposphere, during both daytime and nighttime.

The interaction of trace heavy metal with lipid monolayer in the sea surface microlayer

Lipid molecules and trace heavy metals are enriched in sea surface microlayer and can be transferred into the sea spray aerosol. To better understand their impact on marine aerosol generation and evolution, we investigated the interaction of trace heavy metals including Fe3+, Pb2+, Zn2+, Cu2+, Ni2+, Cr3+, Cd2+, and Co2+, with dipalmitoylphosphatidylcholine (DPPC) monolayers at the air-water interface. Phase behavior of the DPPC monolayer on heavy metal solutions was probed with surface pressure-area (π-A) isotherms. The conformation order and orientation of DPPC alkyl chains were characterized by infrared reflection-absorption spectroscopy (IRRAS). The π-A isotherms show that Zn2+ and Fe3+ strongly interact with DPPC molecules, and induce condensation of the monolayers in a concentration-dependent manner. IRRAS spectra show that the formation of cation-DPPC complex gives rise to conformational changes and immobilization of the headgroups. The current results suggest that the enrichment of Zn2+ in sea spray aerosols is due to strong binding to the DPPC film. The interaction of Fe3+ with DPPC monolayers can significantly influence their surface organizations through the formation of lipid-coated particles. These results suggest that the sea surface microlayer is capable of accumulating much higher amounts of these metals than the subsurface water. The organic and metal pollutants may transfer into the atmosphere by this interaction.

The Influence of the Position of the Double Bond and Ring Size on the Stability of Hydrogen Bonded Complexes

To study the influence of the position of the double bond and ring size on the stability of hydrogen bonded complexes, the 1:1 complexes formed between 2,2,2-trifluoroethanol (TFE) and three heterocyclic compounds including 2,3-dihydrofuran (2,3-DHF), 2,5-dihydrofuran (2,5-DHF) and 3,4-dihydropyran (3,4-DHP) were investigated systematically. The formation of hydrogen bonded TFE−2,3-DHF, TFE−2,5-DHF and TFE−3,4-DHP complexes were identified by gas phase FTIR spectroscopy at room temperature, and the OH-stretching fundamental transition of TFE was red shifted upon complexation. The competition between the O atom and π-electrons bonding sites within the complexes was studied, and the O−H···π type hydrogen bond was found to be less stable than the O−H···O in all three cases. The observed red shifts of the OH-stretching fundamental transitions in the complexes were attributed to the formation of O−H···O hydrogen bond. Equilibrium constants of the complexation reactions were determined from measured and calculated OH-stretching fundamental intensities. Both theoretical calculations and experimental results reveal that the hydrogen bond strengths in the complexes follow the sequence: TFE−2,5-DHF > TFE−2,3-DHF ≈ TFE−3,4-DHP, thus the position of the double bond exerts significantly larger influence than ring size on the stability of the selected hydrogen bonded complexes.

Role of water on the H-abstraction from methanol by ClO

The influence of a single water molecule on the reaction mechanism and kinetics of hydrogen abstraction from methanol (CH3OH) by the ClO radical has been investigated using ab initio calculations. The reaction proceeds through two channels: abstraction of the hydroxyl H-atom and methyl H-atom of CH3OH by ClO, leading to the formation of CH3O+HOCl (+H2O) and CH2OH+HOCl (+H2O), respectively. In both cases, pre- and post-reactive complexes were located at the entrance and exit channel on the potential energy surfaces. Results indicate that the formation of CH2OH+HOCl (+H2O) is predominant over the formation of CH3O+HOCl (+H2O), with ambient rate constants of 3.07×10-19 and 3.01×10-23cm3/(molecule·sec), respectively, for the reaction without water. Over the temperature range 216.7-298.2K, the presence of water is seen to effectively lower the rate constants for the most favorable pathways by 4-6 orders of magnitude in both cases. It is therefore concluded that water plays an inhibitive role on the CH3OH+ClO reaction under tropospheric conditions.

Matrix isolation study of the early intermediates in the ozonolysis of selected vinyl ethers

The matrix isolation technique combined with infrared spectroscopy has been used to characterize Criegee Intermediates (CI) and other products formed during the ozonolysis reactions of ethyl vinyl ether and n-butyl vinyl ether. Twin jet deposition at 14 K led to a number of new bands indicating the formation of products, with an intensity increase of ∼150% to 400% when annealing to 30 K. All the infrared absorptions could be assigned to different bands, which provided direct evidence for the formation of primary ozonides, CI and secondary ozonides in the systems investigated. Theoretical calculations at the B3LYP-D3/aug-cc-pVTZ level were carried out to complement the experimental observations. Experimental and theoretical results demonstrate that the studied ozonolysis reactions predominantly follow the Criegee mechanism. The current results will allow a better assessment of the potential environmental impacts of vinyl ethers.

Hydrogen bond docking preference in furans: OH ⋯ π vs. OH ⋯ O

The docking sites of hydrogen bonds in complexes formed between 2,2,2-trifluoroethanol (TFE), furan (Fu), and 2-methyl furan (MF) have been investigated. Using density functional theory (DFT) calculations, gas phase and matrix isolation FTIR spectroscopies, the strengths of OH⋯O and OH⋯π hydrogen bonds in the complexes were compared to find the docking preference. Calculations suggest that the hydrogen bond donor, TFE, is more likely to dock onto the oxygen atom of the aromatic furans ring, and consequently, the OH⋯O type hydrogen bond is relatively stronger than the OH⋯π type. The FTIR spectrum in the OH-stretching fundamental range obtained at room temperatures has been compared with that obtained at extremely low temperatures in the matrix. The fundamental and the red shifts of OH-stretching vibrations were observed in both FTIR spectra, confirming the formation of hydrogen bonded complexes. By assessing the ability of furan and MF to participate in the formation of OH⋯O hydrogen bond, the effect of ring methylation has been highlighted. From the calculated geometric and thermodynamic parameters as well as the frequency shift of the OH-stretching vibrations in complexes, TFE-MF is found to be more stable than TFE-Fu, which suggests that the strength of the OH⋯O hydrogen bond in TFE-MF originates from the high activity of the furan molecule caused by the methylation of the aromatic ring. The present study furthers the knowledge of docking preference in heteroaromatic molecules and is helpful to understand the nature of intermolecular interactions between hydrogen bond donors and acceptors, including both electron-deficient atoms and π cloud.