Binding of SO3 to Group 4 Transition Metal Oxide Nanoclusters.

Transition metal oxide (TMO) clusters are being studied for their ability to absorb acid gases generated by energy production processes. The interaction of SO3, a byproduct of common industrial processes, with group 4 metal (Ti, Zr, and Hf) oxide nanoclusters, has been predicted using electronic structure methods. The calculations were done at the density functional theory (DFT) and correlated molecular orbital coupled cluster singles and doubles CCSD(T) theory levels. There is a reasonable agreement between the DFT/ωB97x-D energies with the CCSD(T) results. SO3 is predicted to strongly chemisorb to these clusters, as do NO2 and CO2. For SO3, these chemisorption processes favor binding to TMO clusters as SO42- sulfate in both the terminal and bridging configurations. It is predicted that SO3 fully extracts the bridging oxygen from the TMO lattice to form bridging SO42-. This is favorable because of the lower S-O bond dissociation energy of SO3, whereas other acid gases add across the bridging oxygen because of their higher A-O bond dissociation energy. SO3 is capable of physisorption as long as an exposed metal center is present in the lattice. If a metal center has a terminal oxo-group, then SO3 will prefer the SO42- configuration. An approximately linear relationship exists between the physisorption energy and proton affinity for rows 2 and 3 elements.

Dinuclear Complexes of Uranyl, Neptunyl, and Plutonyl: Structures and Oxidation States Revealed by Experiment and Theory.

Dinuclear perchlorate complexes of uranium, neptunium, and plutonium were characterized by reactivity and DFT, with results revealing structures containing pentavalent, hexavalent, and heptavalent actinyls, and actinyl-actinyl interactions (AAIs). Electrospray ionization produced native complexes [(AnO2)2(ClO4)3]- for An:An = U:U, Np:Np, Pu:Pu, and Np:Pu, which are intuitively formulated as actinyl(V) perchlorates. However, DFT identified lower-energy structures [(AnO2)(AnO3)(ClO4)2(ClO3)]- comprising a perchlorate fragmented to ClO3, actinyl(VI) cation AnVIO22+, and neutral AnO3. For U:U and Np:Np, and Np in Np:Pu, the coordinated AnO3 is calculated as actinyl(VI) with an equatorial oxo, [Oyl═AnVI═Oyl][═Oeq], whereas for Pu:Pu, it is plutonyl(V) oxyl, [Oyl═PuV═Oyl][-Oeq•]. The implied lower stability of PuVI versus NpVI indicates weaker Pu═Oeq versus Np═Oeq bonding. Adsorption of O2 by the U:U complex suggests oxidation of UV to UVI, corroborating the assignment of perchlorate [(AnVO2)2(ClO4)3]-. DFT predicts the O2 adducts are [(AnVIO2)(O2)(AnVIO2)(ClO4)3]- with actinyls oxidized from +V to +VI by bridging peroxide, O22-. In accordance with reactivity, O2- addition is computed as substantially exothermic for U:U and least favorable for Pu:Pu. Collision-induced dissociation of native complexes eliminated ClO2 to yield [(AnO2)(O)2(AnO2)(ClO4)2]-, in which fragmented O atoms bridge as oxyl O-• and oxo O2- to yield uranyl(VI) and plutonyl(VI), or as oxos O2- to yield neptunyl(VII), [Oyl═NpVII═Oyl]3+.

Predicting the Mechanism and Products of CO2 Capture by Amines in the Presence of H2O.

An extensive correlated molecular orbital theory study of the reactions of CO2 with a range of substituted amines and H2O in the gas phase and aqueous solution was performed at the G3(MP2) level with a self-consistent reaction field approach. The G3(MP2) calculations were benchmarked at the CCSD(T)/CBS level for NH3 reactions. A catalytic NH3 reduces the energy barrier more than a catalytic H2O for the formation of H2NCOOH and H2CO3. In aqueous solution, the barriers to form both H2NCOOH and H2CO3 are reduced, with HCO3- formation possible with one amine present and H2NCOO- formation possible only with two amines. Further reactions of H2NCOOH to form HNCO and urea via the Bazarov reaction have high barriers and are unlikely in both the gas phase and aqueous solution. Reaction coordinates for CH3NH2, CH3CH2NH2, (CH3)2NH, CH3CH2CH2NH2, (CH3)3N, and DMAP were also calculated. The barrier for proton transfer correlates with amine basicity for alkylammonium carbamate (ΔG‡aq < 15 kcal/mol) and alkylammonium bicarbonate (ΔG‡aq < 30 kcal/mol) formation. In aqueous solution, carbamic acids, carbamates, and bicarbonates can all form in small amounts with ammonium carbamates dominating for primary and secondary alkylamines. These results have implications for CO2 capture by amines in both the gas phase and aqueous solution as well as in the solid state, if enough water is present.

Reaction of NO2 with Groups IV and VI Transition Metal Oxide Clusters.

The addition of NO2 to Group IV (MO2)n and Group VI (MO3)n (n = 1-3) nanoclusters was studied using both density functional theory (DFT) and coupled cluster theory (CCSD(T)). The structures and overall binding energetics were predicted for Lewis acid-base addition without transfer of spin (a physisorption-type process) and the formation of either cluster-ONO (HONO-like or bidentate bonding) or NO3- formation where for both the spin is transferred to the metal oxide clusters (a chemisorption-type process). Only chemisorption of NO2 is predicted to be thermodynamically allowed at temperatures ≥298 K for Group IV (MO2)n clusters with the formation of surface chemisorbed NO2 being by far the most energetically favorable. The ligand binding energies (LBEs) for physisorption and chemisorption on the TiO2 nanoclusters are consistent with computational studies of the bulk solids. Chemisorption is only predicted to occur for (CrO3)n clusters in the form of a terminal nitrate containing species whereas the larger chemisorbed nitrate structures for (MoO3)n and (WO3)n were found to be metastable and unlikely to form in any appreciable amount at temperatures of 298 K and higher. NO2 is predicted to only be capable of physisorbing to (MoO3)n and (WO3)n at lower temperatures and therefore unlikely to bind NO2 at temperatures ≥298 K. Correlations between the (MO3)nNO2 ligand bond energies and the chemical properties of the parent (MO3)n clusters (Lewis acidity, ionization potentials, excitation energies, and M = O/M-O bond strengths) are described.

Predicting the Formation of Sulfur-Based Brønsted Acids from the Reactions of SOx with H2O and H2S.

We have performed an extensive computational investigation of the potential energy surfaces for the reactions of SOx (x = 2 or 3) with H2S and H2O in the gas phase and in aqueous solution at the CCSD(T)/CBS level of theory plus a self-consistent reaction field approach. Formation of a gas-phase H2SO4 from the hydrolysis of SO3 at lower temperatures requires the presence of additional water molecules. When additional waters are introduced, the barrier for H2SO4 formation is significantly reduced, and a barrierless transition occurs with only three excess waters as well as in aqueous solution. In a mixture of H2O, H2S, and SO3, formation of H2S2O3 has a lower barrier in the gas phase than does the formation of H2SO4. In aqueous solution, no barrier is predicted, and both are likely to be formed. Introduction of an additional water to the SO3 + H2S reaction results in a decrease in barrier height, nearly identical to the ∼18 kcal/mol catalytic effect of the first additional water in SO3 hydrolysis, with the barrier for H2S2O3 formation disappearing altogether in aqueous solution. Formation of H2SO3 by the way of SO2 hydrolysis is unlikely. Excess waters reduce the barrier for SO2 hydrolysis; however, the overall endothermicity is increased as waters are added. The formation of H2S2O2 from SO2 and H2S via an isostructural pathway to SO2 hydrolysis is unlikely, with additional water molecules resulting in a small increase in the overall endothermicity and a catalytic effect smaller than that observed for the SO3 reactions. The results of this work have implications pertaining to the formation of H2SO4, H2S2O3, H2SO3, and H2S2O2 in the atmospheres of Earth and Venus. These results also question the existence of H2S2O2 as an intermediate in the Claus process.