Accurate intermolecular potentials are needed for quantitative molecular simulations, but their calculation from quantum mechanics can be very demanding. We have developed several variations of a procedure, which we collectively refer to as quantum mechanical Hybrid Methods for Interaction Energies (HM-IE), to accurately estimate interaction energies from CCSD(T) calculations with a large basis set (LBS). HM-IE was tested for interaction energies of Ne 2 , (C 2 H 2 ) 2 , and N 2 -benzene for many orientations sampling the entire potential energy surface and was found to be in excellent agreement with the CCSD(T)/LBS results while requiring considerably less computational time and resources. Furthermore, for neon, an intermolecular potential fit to interaction energies using HM-IE and a potential fit to CCSD(T)/LBS energies resulted in nearly identical predictions for densities and vapor pressures.

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Gas-phase nitrosation of amines implies a nonionic pathway different from the nitrosonium nitrosation via acidification of nitrite. Electronic structure calculations discussed in this work suggest a free radical mechanism, in which NO2 abstracts a hydrogen atom from the nitrogen in primary and secondary amines to form an intermediate complex of an aminyl radical and nitrous acid. The aminyl radical intermediate is then quenched by nitric oxide, leading to the formation of nitrosamine. High-level calculations (CBS-QB3) show that alkyl substitutions on amines can activate the H-abstraction reaction. Thus, while H-abstraction from NH3 was found to exhibit a reaction barrier (DeltaH) of 106 kJ/mol, similar calculations indicate that the corresponding barriers decrease to 72 and 45 kJ/mol for methylamine and dimethylamine, respectively. Heterocyclic secondary amines have also been investigated in a similar manner. The five-membered-ring (5-m-r) amine appears to be the most reactive: pyrrolidine (DeltaH=30 kJ/mol), azetidine (DeltaH=44 kJ/mol), piperidine (DeltaH=44 kJ/mol), and aziridine (DeltaH=74 kJ/mol). The reaction barrier for 1H-pyrrole, an aromatic 5-m-r secondary amine, was found to be 59 kJ/mol. The origin of the high activity for the 5-m-r alkylamine stems from a hydrogen-bond-like interaction between the aminyl radical and the nascent nitrous acid molecule. This theoretical study suggests that, in the presence of nitrogen oxides, the gas-phase nitrosation of secondary amines is feasible.

Two detailed, unit-cell models, a transverse fin design and a longitudinal fin design, of a combined hydride bed and heat exchanger are developed in COMSOL ® Multiphysics incorporating and accounting for heat transfer and reaction kinetic limitations. MatLab ® scripts for autonomous model generation are developed and incorporated into (1) a grid-based and (2) a systematic optimization routine based on the Nelder-Mead downhill simplex method to determine the geometrical parameters that lead to the optimal structure for each fin design that maximizes the hydrogen stored within the hydride.The optimal designs for both the transverse and longitudinal fin designs point toward closely-spaced, small cooling fluid tubes. Under the hydrogen feed conditions studied (50 bar), a 25 times improvement or better in the hydrogen storage kinetics will be required to simultaneously meet the Department of Energy technical targets for gravimetric capacity and fill time. These models and methodology can be rapidly applied to other hydrogen storage materials, such as other metal hydrides or to cryoadsorbents, in future work.

The design and evaluation of media based hydrogen storage systems requires the use of detailed numerical models and experimental studies, with significant amount of time and monetary investment. Thus a scoping tool, referred to as the Acceptability Envelope, was developed to screen preliminary candidate media and storage vessel designs, identifying the range of chemical, physical and geometrical parameters for the coupled media and storage vessel system that allow it to meet performance targets. The model which underpins the analysis allows simplifying the storage system, thus resulting in one input-one output scheme, by grouping of selected quantities. SRNL-TR-2011-002112 highlighting the potential of storage systems, utilizing them, to achieve 40% of the 2010 DOE technical target. Results show that systems based on Li-Mg media have the best potential to attain these performance targets.

ABSTRACT. Density Functional Theory and small-core, relativistic pseudopotentials were used to look for symmetric and asymmetric transitions states of the gas-phase hydrolysis reaction of uranium hexafluoride, UF 6 , with water. At the B3LYP/6-31G(d,p)/SDD level, an asymmetric transition state leading to the formation of a uranium hydroxyl fluoride, U(OH)F 5 , and hydrogen fluoride was found with an energy barrier of +77.3 kJ/mol and an enthalpy of reaction of +63.0 kJ/mol (both including zero-point energy corrections). Addition of diffuse functions to all atoms except uranium led to only minor changes in the structure and relative energies of the reacting complex and transition state. However, a significant change in the product complex structure was found, significantly reducing the enthalpy of reaction to +31.9 kJ/mol. Similar structures and values were found for PBE0 and MP2 calculations with this larger basis set, supporting the B3LYP results. No symmetric transition state leading to the direct formation of uranium oxide tetrafluoride, UOF 4 , was found, indicating that the reaction under ambient conditions likely includes several more steps than the mechanisms commonly mentioned. The transition state presented here appears to be the first published transition state for the important gas-phase reaction of UF 6 with water.

We have developed an ab initio potential for acetylene by computing interaction energies for a range of orientations and center-of-mass separation distances. These energies are initially fit with a simple weighting scheme to a pairwise-additive, site−site Morse-C6 intermolecular potential. Additional interaction energies were then calculated at separation distances determined to be important from the center-of-mass radial distribution function calculated from molecular simulation with use of the initial potential. The expanded set of interaction energies is then fit using Boltzmann-like weighting to obtain an improved intermolecular potential. The phase behavior calculated from Gibbs ensemble Monte Carlo simulations using this improved potential is in excellent agreement with experimental data. Also, the results of NVT ensemble Monte Carlo calculations show good agreement with experimental data at supercritical temperatures and pressures, and results are presented for conditions that would be hazardous experimentally. Additionally, the second virial coefficients calculated using this potential indicate that one set of experimental data reported in the literature is likely to be erroneous. The prescription described here for obtaining the optimum potential from quantum chemical methods should be applicable to other systems.

The hydrolysis reaction of uranium hexafluoride (UF 6 ) is a key step in the synthesis of uranium dioxide (UO 2 ) powder for nuclear fuels. Mechanisms for the hydrolysis reactions are studied here with density f unctional theory a nd t he Stuttgart s mall-core scalar relativistic pseudopotential and associated basis set for uranium. The reaction of a single UF 6 molecule with a water molecule in the gas phase has been previously predicted to proceed over a relatively sizeable barrier of 78.2 kJ·mol -1 , indicating this reaction is only feasible at elevated temperatures.Given the observed formation of a second morphology for the UO 2 product coupled with the observations of rapid, spontaneous hydrolysis at ambient conditions, an alternate reaction pathway must exist. In the present work, two trimolecular hydrolysis mechanisms are studied with density functional theory: (1) the reaction between two UF 6 molecules and one water molecule, and (2) the reaction of two water molecules with a single UF 6 molecule. The predicted reaction of two UF 6 molecules with one water molecule displays an interesting "fluorine-shuttle" mechanism, a significant energy barrier of 69.0 kJ·mol -1 to the formation of UF 5 OH, and an enthalpy of reaction (ΔH 298 ) of +17.9 kJ·mol -1 . The reaction of a single UF 6 molecule with two water molecules displays a "proton-shuttle" mechanism, and is more favorable, having a slightly lower computed energy barrier of 58.9 kJ·mol -1 and an exothermic enthalpy of reaction (ΔH 298 ) of -13.9 kJ·mol -1 . The exothermic nature of the overall UF 6 + 2·H 2 O trimolecular reaction and the lowering of the barrier height with respect to the bimolecular reaction are encouraging; however, the sizable energy barrier indicates further study of the UF 6 hydrolysis reaction mechanism is warranted to resolve the remaining discrepancies between the predicted mechanisms and experimental observations. Lind et al.,