Computational Studies of Uranium Hexafluoride Interacting with Functionalized Organics. I. Screening Intermolecular Potentials between UF6 and Small Organic Functional Groups.

We report the intermolecular binding energies (IBEs) between UF6 and over 50 different functionalized small organic molecules as predicted by electronic structure calculations. Optimized geometries of UF6-molecule dimers were found at the MP2/aug-cc-pwCVDZ (non-U), cc-pVDZ-PP (U) level. IBEs were calculated using MP2 and dispersion-corrected DFT theory. We characterize the various functional groups based on the inclusion of specific heteroatoms. Those functional groups containing "nitrogen only" heteroatoms result in larger IBEs than groups containing both nitrogen and oxygen or oxygen alone. Halogen-containing and regular hydrocarbon molecules show the lowest IBEs with UF6. Nonorganic phosphoryl species are also shown to display large IBEs with UF6. These interactions are characterized in part by how much the impinging functionalized molecule distorts the UF6 from its optimal octahedral geometry. Of all the investigated groups, the amine group displayed the largest IBE values (IBE ∼ >12-14 kcal/mol for methyl amine), while hydrocarbons and perfluorocarbons both showed the weakest interactions (IBE ∼ 0.5-1.5 and 0.1-0.8 kcal/mol for methane and perfluoromethane, respectively). The study examines how the strength of the IBE is contingent on a combination of conformational deformation, stabilizing nonbonding interactions, and sterics.

Methyl-Cyclohexane Methanol (MCHM) Isomer-Dependent Binding on Amorphous Carbon Surfaces.

In January 2014, over 10,000 gallons of methyl-cyclohexane methanol (MCHM) leaked into the Elk River in West Virginia, in a chemical spill incident that contaminated a large portion of the state's water supply and left over 300,000 residents without clean water for many days and weeks. Initial efforts to remove MCHM at the treatment plant centered on the use of granulated activated carbon (GAC), which removed some of the chemical from the water, but MCHM levels were not lowered to a "non-detect" status until well after the chemical plume had moved downstream of the intake. Months later, MCHM was again detected at the outflow (but not the inflow) at the water treatment facility, necessitating the full and costly replacement of all GAC in the facility. The purpose of this study is to investigate the hypothesis that preferential absorbance of one of the two MCHM isomers, coupled with seasonal variations in water temperature, explain this contrary observation. Calculated intermolecular potentials between ovalene (a large planar polycyclic aromatic hydrocarbon) and the MCHM isomers were compared to physisorption potentials of MCHM onto an amorphous carbon model. While a molecular mechanics (MM) force field predicts no difference in the average interaction potentials between the cis- and trans-MCHM with the planar ovalene structure, MM predicts that the trans isomer binds stronger than the cis isomer to the amorphous carbon surface. Semi-empirical and density functional theory also predict stronger binding of trans-MCHM on both the planar and amorphous surfaces. The differences in the isomer binding strengths on amorphous carbon imply preferential absorbance of the trans isomer onto activated charcoal filter media. Considering seasonal water temperatures, simple Arrhenius kinetics arguments based on these predicted binding energies help explain the environmental observations of MCHM leeching from the GAC filters months after the spill. Overall, this work shows the important implications that can arise from detailed interfacial chemistry investigations.

Enabling Science Support for Better Decision-Making when Responding to Chemical Spills.

Chemical spills and accidents contaminate the environment and disrupt societies and economies around the globe. In the United States there were approximately 172,000 chemical spills that affected US waterbodies from 2004 to 2014. More than 8000 of these spills involved non-petroleum-related chemicals. Traditional emergency responses or incident command structures (ICSs) that respond to chemical spills require coordinated efforts by predominantly government personnel from multiple disciplines, including disaster management, public health, and environmental protection. However, the requirements of emergency response teams for science support might not be met within the traditional ICS. We describe the US ICS as an example of emergency-response approaches to chemical spills and provide examples in which external scientific support from research personnel benefitted the ICS emergency response, focusing primarily on nonpetroleum chemical spills. We then propose immediate, near-term, and long-term activities to support the response to chemical spills, focusing on nonpetroleum chemical spills. Further, we call for science support for spill prevention and near-term spill-incident response and identify longer-term research needs. The development of a formal mechanism for external science support of ICS from governmental and nongovernmental scientists would benefit rapid responders, advance incident- and crisis-response science, and aid society in coping with and recovering from chemical spills.

Dipole moments of trans- and cis-(4-methylcyclohexyl)methanol (4-MCHM): obtaining the right conformer for the right reason.

Accurate computational estimates of fundamental physical properties can be used as inputs in the myriad of extant models employed to predict toxicity, transport, and fate of contaminants. However, as molecular complexity of contaminants increases, it becomes increasingly difficult to determine the magnitude of the errors introduced by ignoring the 3D conformational space averaging within group-additivity and semi-empirical approaches. The importance of considering 3D molecular structure is exemplified for the dipole moments of cis and trans isomers of (4-methylcyclohexyl)methanol (4-MCHM). When 10 000 gallons of 4-MCHM was spilled into the Elk River in January 2014, a lack of toxicological data and environmental partitioning coefficients hindered the immediate protection of human health and the local water supply in West Virginia, USA. Post-spill analysis of the contaminants suggested that the cis and trans isomers had observably different partitioning coefficients and solubility, and thus differing environmental fates. Obtaining high-quality dipole moments using ab initio quantum chemical methods for the isomeric pair was crucial in validating their experimental differences in solubility [Environ. Sci. Technol. Lett., 2015, 2, 127]. The use of first principles electronic structure theory is further explored here to obtain accurate conformer relative energies and dipole moments of cis- and trans-4-MCHM. Overall, the MP2 aug-cc-pVDZ level of theory affords the best balance between accuracy and computational cost.

Collisions of sodium atoms with liquid glycerol: insights into solvation and ionization.

The reactive uptake and ionization of sodium atoms in glycerol were investigated by gas-liquid scattering experiments and ab initio molecular dynamics (AIMD) simulations. A nearly effusive beam of Na atoms at 670 K was directed at liquid glycerol in vacuum, and the scattered Na atoms were detected by a rotatable mass spectrometer. The Na velocity and angular distributions imply that all impinging Na atoms that thermally equilibrate on the surface remain behind, likely ionizing to e(-) and Na(+). The reactive uptake of Na atoms into glycerol was determined to be greater than 75%. Complementary AIMD simulations of Na striking a 17-molecule glycerol cluster indicate that the glycerol hydroxyl groups reorient around the Na atom as it makes contact with the cluster and begins to ionize. Although complete ionization did not occur during the 10 ps simulation, distinct correlations among the extent of ionization, separation between Na(+) and e(-), solvent coordination, and binding energies of the Na atom and electron were observed. The combination of experiments and simulations indicates that Na-atom deposition provides a low-energy pathway for generating solvated electrons in the near-interfacial region of protic liquids.

Kinematics and dynamics of atomic-beam scattering on liquid and self-assembled monolayer surfaces.

We have conducted investigations of the energy transfer dynamics of atomic oxygen and argon scattering from hydrocarbon and fluorocarbon surfaces. In light of these results, we appraise the applicability and value of a kinematic scattering model, which views a gas-surface interaction as a gas-phase-like collision between an incident atom or molecule and a localized region of the surface with an effective mass. We have applied this model to interpret the effective surface mass and energy transfer when atoms strike two different surfaces under identical bombardment conditions. To this end, we have collected new data, and we have re-examined existing data sets from both molecular-beam experiments and molecular dynamics simulations. We seek to identify trends that could lead to a robust general understanding of energy transfer processes induced by collisions of gas-phase species with liquid and semi-solid surfaces.

Reactions of solvated electrons initiated by sodium atom ionization at the vacuum-liquid interface.

Solvated electrons are powerful reagents in the liquid phase that break chemical bonds and thereby create additional reactive species, including hydrogen atoms. We explored the distinct chemistry that ensues when electrons are liberated near the liquid surface rather than within the bulk. Specifically, we detected the products resulting from exposure of liquid glycerol to a beam of sodium atoms. The Na atoms ionized in the surface region, generating electrons that reacted with deuterated glycerol, C(3)D(5)(OD)(3), to produce D atoms, D(2), D(2)O, and glycerol fragments. Surprisingly, 43 ± 4% of the D atoms traversed the interfacial region and desorbed into vacuum before attacking C-D bonds to produce D(2).

Gas-surface energy exchange and thermal accommodation of CO2 and Ar in collisions with methyl, hydroxyl, and perfluorinated self-assembled monolayers.

Molecular beams of CO(2) and Ar were scattered from long-chain methyl (CH(3)-), hydroxyl (OH-), and perfluoro ((CF(2))(7)CF(3)-) functionalized alkanethiol self-assembled monolayers (SAMs) on gold to study the dynamics of energy exchange and thermal accommodation on model organic surfaces. Ar collisions, for incident energies ranging from 25 to 150 kJ mol(-1), exhibit final energy distributions that depend significantly on the terminal functional group of the SAM. The long-chain CH(3)-terminated monolayers serve as an excellent energy sink for dissipating the incident translational energy. For example, at 150 kJ mol(-1), greater than 90% of the collision energy is transferred to the CH(3)-SAM surface for specularly-scattered atoms (θ(i) = θ(f) = 30° from normal). However, the OH-SAM is a more rigid collision partner due to the formation of an intra-monolayer hydrogen bonding network and the (CF(2))(7)CF(3)-SAM (F-SAM) provides a high degree of rigidity due to the massive CF(3) groups. The final energies for the triatomic, CO(2), scattering from the three surfaces are remarkably similar to the results for Ar scattering. The only significant difference in the translational energy transfer dynamics for these two gases appears in collisions with the OH-SAM. Strong gas-surface attractive forces between CO(2) and the OH-SAM surface appear to counter the rigidity of the hydrogen-bonding network to help bring the majority of the molecules to thermal equilibrium at all incident energies up to 150 kJ mol(-1), resulting in increased energy transfer in comparison to Ar. The similarities in energy transfer for Ar and CO(2) final energy distributions in scattering from the CH(3)- and F-SAMs suggest that the internal degrees of freedom in the triatomic play only a small role in determining the outcome of the gas-surface collision under the scattering conditions employed in this work.

Experimental and theoretical study of CO collisions with CH3- and CF3-terminated self-assembled monolayers.

We present an experimental and theoretical study of the dynamics of collisions of the CO molecule with organic surfaces. Experimentally, we scatter CO at 60 kJ mol(-1) and 30 degrees incident angle from regular (CH(3)-terminated) and omega-fluorinated (CF(3)-terminated) alkanethiol self-assembled monolayers (SAMs) and measure the time-of-flight distributions at the specular angle after collision. At a theoretical level, we carry out classical-trajectory simulations of the same scattering process using CO/SAM potential-energy surfaces derived from ab initio calculations. Agreement between measured and calculated final translational energy distributions justifies use of the calculations to examine dynamical behavior of the gas/surface system not available directly from the experiment. Calculated state-to-state energy-transfer properties indicate that the collisions are notably vibrationally adiabatic. Similarly, translational energy transfer from and to CO rotation is relatively weak. These trends are examined as a function of collision energy and incident angle to provide a deeper understanding of the factors governing state-to-state energy transfer in gas/organic-surface collisions.

Theoretical study of the stereodynamics of CO collisions with CH3- and CF3-terminated alkanethiolate self-assembled monolayers.

We present a classical-trajectory study of CO collisions with regular (CH3-terminated) and omega-fluorinated (CF3-terminated) alkanethiol self-assembled monolayers (SAMs) with a focus on analyzing the stereodynamics properties of the collision. The CO molecule is scattered with incident angles of either 30 degrees or 60 degrees with respect to the surface normal and with 60 kJ x mol(-1) collision energy, and we analyze final translational and rotational energy, mechanism of the collisions, and orientation and alignment of the rotational angular momentum. Analysis of the alignment of the final rotational angular momentum in collisions involving initially rotationally cold CO indicates a slight preference for "cartwheel" and "corkscrew" rotational motions. In contrast, collisions of initially excited CO slightly favor "helicopter" motion of the recoiling molecule. Moreover, studies of final orientation reveal that, while cartwheel "topspin" motion is favored for collisions in which initially cold CO becomes rotationally excited, no preferred handedness is observed when CO leaves the surfaces with "helicopter" motion. Analysis of trajectories involving initially rotationally excited CO in which the initial rotational angular momentum is aligned and/or oriented shows a non-negligible effect of the initial rotational motion on the dynamics of energy transfer. For instance, CO approaching the SAMs with helicopter motion retains a larger fraction of its initial rotation than molecules colliding with cartwheel-type motions. Conservation of the alignment and orientation of the initial rotational angular momentum vector is also enhanced with helicopter motion relative to cartwheel or random motions. The calculated trends in the stereodynamic properties for the two SAMs indicate that the CH3-SAM is effectively more corrugated than the CF3-SAM.

Experimental and theoretical studies of the effect of mass on the dynamics of gas/organic-surface energy transfer.

The effect of mass on gas/organic-surface energy transfer is explored via investigation of the scattering dynamics of rare gases (Ne, Ar, and Kr) from regular (CH3-terminated) and omega-fluorinated (CF3-terminated) alkanethiol self-assembled monolayers (SAMs) at 60 kJmol collision energy. Molecular-beam scattering experiments carried out in ultrahigh vacuum and molecular-dynamics simulations based on high-accuracy potentials are used to obtain the rare-gases' translational-energy distributions after collision with the SAMs. Simulations indicate that mass is the most important factor in determining the changes in the energy exchange dynamics for Ne, Ar, and Kr collisions on CH3- and CF3-terminated SAMs at 60 kJmol collision energy. Other factors, such as changes in the gas-surface potential and intrasurface interactions, play only a minor role in determining the differential dynamics behavior for the systems studied.

Theoretical study of the Ar-, Kr-, and Xe-CH4, -CF4 intermolecular potential-energy surfaces.

We present a theoretical study of the intermolecular potentials for the Ar, Kr, and Xe-CH4, -CF4 systems. The potential-energy surfaces of these systems have been calculated utilizing second-order Möller-Plesset perturbation theory and coupled-cluster theory in combination with correlation-consistent basis sets (aug-cc-pvnz; n = d, t, q). The calculations show that the stabilizing interactions between the rare gases and the molecules are slightly larger for CF4 than for CH4. Moreover, the rare-gas-CX4 (X = H, F) potentials are more attractive for Xe than for Kr and Ar. Our highest quality ab initio data (focal-point-CCSD(T) extrapolated to the complete basis set limit) have been used to develop pairwise analytical potentials for rare-gas-hydrocarbon (-fluorocarbon) systems. These potentials can be applied in classical-trajectory studies of rare gases interacting with hydrocarbon surfaces.

Theoretical study of the effect of surface density on the dynamics of Ar + alkanethiolate self-assembled monolayer collisions.

We present a classical-trajectory study of energy transfer in collisions of Ar atoms with alkanethiolate self-assembled monolayers (SAMs) of different densities. The density of the SAMs is varied by changing the distance between the alkanethiolate chains in the organic monolayers. Our calculations indicate that SAMs with smaller packing densities absorb more energy from the impinging Ar atoms, in agreement with recent molecular-beam scattering experiments. We find that energy transfer is enhanced by a decrease in the SAM density because (1) less dense SAMs increase the probability of multiple encounters between Ar and the SAM, (2) the vibrational frequencies of large-amplitude motions of the SAM chains decrease for less dense SAMs, which makes energy transfer more efficient in single-encounter collisions, and (3) increases in the distance between chains promote surface penetration of the Ar atom. Analysis of angular distributions reveals that the polar-angle distributions do not have a cosine shape in trapping-desorption processes involving penetration of the Ar atom into the alkanethiolate self-assembled monolayers. Instead, there is a preference for Ar atoms that penetrate the surface to desorb along the chain-tilt direction.