Thermodynamic and Kinetic Activity Descriptors for the Catalytic Hydrogenation of Ketones.

Activity descriptors are a powerful tool for the design of catalysts that can efficiently utilize H2 with minimal energy losses. In this study, we develop the use of hydricity and H- self-exchange rates as thermodynamic and kinetic descriptors for the hydrogenation of ketones by molecular catalysts. Two complexes with known hydricity, HRh(dmpe)2 and HCo(dmpe)2, were investigated for the catalytic hydrogenation of ketones under mild conditions (1.5 atm and 25 °C). The rhodium catalyst proved to be an efficient catalyst for a wide range of ketones, whereas the cobalt catalyst could only hydrogenate electron-deficient ketones. Using a combination of experiment and electronic structure theory, thermodynamic hydricity values were established for 46 alkoxide/ketone pairs in both acetonitrile and tetrahydrofuran solvents. Through comparison of the hydricities of the catalysts and substrates, it was determined that catalysis was observed only for catalyst/ketone pairs with an exergonic H- transfer step. Mechanistic studies revealed that H- transfer was the rate-limiting step for catalysis, allowing for the experimental and computation construction of linear free-energy relationships (LFERs) for H- transfer. Further analysis revealed that the LFERs could be reproduced using Marcus theory, in which the H- self-exchange rates for the HRh/Rh+ and ketone/alkoxide pairs were used to predict the experimentally measured catalytic barriers within 2 kcal mol-1. These studies significantly expand the scope of catalytic reactions that can be analyzed with a thermodynamic hydricity descriptor and firmly establish Marcus theory as a valid approach to develop kinetic descriptors for designing catalysts for H- transfer reactions.

Remotely Bonded Bridging Dioxygen Ligands Enhance Hydrogen Transfer in a Silica-Supported Tetrairidium Cluster Catalyst.

A longstanding challenge in catalysis by noble metals has been to understand the origin of enhancements of rates of hydrogen transfer that result from the bonding of oxygen near metal sites. We investigated structurally well-defined catalysts consisting of supported tetrairidium carbonyl clusters with single-atom (apical iridium) catalytic sites for ethylene hydrogenation. Reaction of the clusters with ethylene and H2 followed by O2 led to the onset of catalytic activity as a terminal CO ligand at each apical Ir atom was removed and bridging dioxygen ligands replaced CO ligands at neighboring (basal-plane) sites. The presence of the dioxygen ligands caused a 6-fold increase in the catalytic reaction rate, which is explained by the electron-withdrawing capability induced by the bridging dioxygen ligands, consistent with the inference that reductive elimination is rate-determining. Electronic-structure calculations demonstrate an additional role of the dioxygen ligands, changing the mechanism of hydrogen transfer from one involving equatorial hydride ligands to that involving bridging hydride ligands. This mechanism is made evident by an inverse kinetic isotope effect observed in ethylene hydrogenation reactions with H2 and, alternatively, with D2 on the cluster incorporating the dioxygen ligands and is a consequence of quasi-equilibrated hydrogen transfer in this catalyst. The same mechanism accounts for rate enhancements induced by the bridging dioxygen ligands for the catalytic reaction of H2 with D2 to give HD. We posit that the mechanism involving bridging hydride ligands facilitated by oxygen ligands remote from the catalytic site may have some generality in catalysis by oxide-supported noble metals.

Next-Generation 3,3'-AlkoxyBTPs as Complexants for Minor Actinide Separation from Lanthanides: A Comprehensive Separations, Spectroscopic, and DFT Study.

Progress toward the closure of the nuclear fuel cycle can be achieved if satisfactory separation strategies for the chemoselective speciation of the trivalent actinides from the lanthanides are realized in a nonproliferative manner. Since Kolarik's initial report on the utility of bis-1,2,4-triazinyl-2,6-pyridines (BTPs) in 1999, a perfect complexant-based, liquid-liquid separation system has yet to be realized. In this report, a comprehensive performance assessment for the separation of 241Am3+ from 154Eu3+ as a model system for spent nuclear fuel using hydrocarbon-actuated alkoxy-BTP complexants is described. These newly discovered complexants realize gains that contemporary aryl-substituted BTPs have yet to achieve, specifically: long-term stability in highly concentrated nitric acid solutions relevant to the low pH of unprocessed spent nuclear fuel, high DAm over DEu in the economical, nonpolar diluent Exxal-8, and the demonstrated capacity to complete the separation cycle with high efficiency by depositing the chelated An3+ to the aqueous layer via decomplexation of the metal-ligand complex. These soft-N-donor BTPs are hypothesized to function as bipolar complexants, effectively traversing the organic/aqueous interface for effective chelation and bound metal/ligand complex solubility. Complexant design, separation assays, spectroscopic analysis, single-crystal X-ray crystallographic data, and DFT calculations are reported.

The electronic structure of diatomic nickel oxide.

The nature of the Ni-O bond is relevant to catalytic and environmental applications. The vibrational frequency and electronic structure of NiO were calculated using CASSCF, icMRCI+Q, CCSD(T), and DFT. CASSCF predicted a quintet state (5Σ-) ground state for the equilibrium bond distance with a state crossing at 1.65 Å, where the triplet (3Σ-) state becomes of lower energy. These states arise from the 3d8(3F)4s2 (3F) and 3d9(2D)4s1 (3D) configurations of Ni. The icMRCI+Q method predicts a triplet (3Σ-) ground state and does not predict a state crossing with the quintet. This state has significant ionic character with the 2pz of O bonding with the 4s/3dz2 of the Ni to form a σ bond. The NiO frequency at the icMRCI+Q level of 835.0 cm-1 is in excellent agreement with experiment; the value of re is 1.5992 Å at this computational level. CCSD(T) predicts ωe = 888.80 cm-1 when extrapolated to the complete basis set limit. Frequencies predicted using CCSD(T) deviate from experiment consistent with the calculations showing large multireference character. A wide array of density functionals were benchmarked. Of the 43 functionals tested, the ones that gave the best prediction of the frequency are ωB97XD, CAM-B3LYP, and τ-HCTH with respective values of 831.8, 838.3, and 837.4 cm-1 respectively. The bond dissociation energy (BDE) of NiO is predicted to be 352.4 kJ mol-1 at the Feller-Peterson-Dixon (FPD) level in good agreement with one of the experimental values. The calculated BDEs at the DFT level are sensitive to the choice of functional and atomic asymptote. Sixteen functionals predicted the BDE within 20 kJ mol-1 of the FPD value.

Prediction of the pKa's of Hydrated Metal Carbonates and Bicarbonates for Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn Dications.

The gas- and aqueous-phase acidities of hydrated metal dication carbonates, bicarbonates, and hydroxide complexes M(CO3)(H2O)n for n = 1 to 3, M(HCO3)2, M(HCO3)2(H2O)2, M(HCO3)(OH), and M(HCO3)(H2O)2(OH) for M = Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn were calculated at the CCSD(T)/aug-cc-pwCVDZ/cc-pwCVDZ level in the gas phase and at the B3LYP/aug-cc-pVTZ/cc-pVTZ(-PP) level with the COSMO self-consistent reaction field (SCRF) method in the aqueous phase. The composite correlated molecular orbital theory G3(MP2) and G3(MP2)B3 methods were used to predict the pKa's of the Mg structures and cis-cis carbonic acid to provide additional benchmarks. Using values scaled to experiment for H2CO3, the pKa's of bicarbonate ligands in group 2 and transition-metal complexes were compared to carbonic acid to gauge the effect of the metal complex on the bicarbonate. The group 2 metal complexes M(HCO3)2 and M(HCO3)(OH) decreased the acidity of the bicarbonate ligands, whereas their dihydrates were even less acidic. The transition-metal di-bicarbonate and bicarbonate hydroxide complexes generally made the bicarbonate more acidic especially when reduction of the metal occurs consistent with electron donation from the ligands; this is accompanied by spin transfer which typically increases in the order Mn < Fe < Co < Ni < Cu. The transition-metal dihydrates were less acidic than carbonic acid. Using values scaled to experiment for hydrated metal dications, the pKa's of water coordinated to group 2 and transition-metal complexes were generally more acidic than the hydrated metal dications, with the exception of Ca bicarbonate dihydrate, Co carbonate, Ni di-bicarbonate dihydrate, and Cu bicarbonate hydroxide di-bicarbonate.

Prediction of Redox Potentials for U, Np, Pu, and Am in Aqueous Solution.

The redox properties of the actinides in aqueous solution are important for fuel production/reprocessing and understanding the environmental impact of nuclear waste. The redox potentials for U, Np, Pu, and Am in oxidation states from 0 up to VII (as appropriate) in aqueous solutions have been predicted at the density functional theory level with the B3LYP functional, Stuttgart small core pseudopotential basis sets for the actinides, and explicit (30H2O molecules)/implicit treatment of the aqueous solvent using the self-consistent reaction field COSMO and SMD approaches for the implicit solvation. The predictions of the structural parameters of clusters incorporating first and second solvation shells are consistent with the available experimental data. Our results are typically within 0.2 V of the available experimental data using two explicit solvation shells with an implicit solvent model. The use of the PW91 functional substantially improved the prediction of the Pu(VI/V) redox couple. The redox couples for An(VI/IV) and An(V/IV) which involve the addition of protons and removal of the actinyl oxygens led to slightly larger differences from an experiment. The An(IV/0) and An(III/0) couples were reliably predicted with our approach. Predictions of the unknown An(II/I) redox potentials were negative, consistent with expectations, and predictions for unknown An(VII/VI), An(III/II), and An(II/0) redox couples improve prior estimates.

Energetic and Electronic Properties of UO0/± and UF0/±.

High-level electronic structure calculations were conducted to examine the bonding and spectroscopic properties of the UO0/± and UF0/± diatomic molecules. The low-lying Ω states were described by using multireference SO-CASPT2 calculations. The adiabatic electronic affinity (AEA), adiabatic ionization energy (IE), and bond dissociation energy (BDE) were calculated at the Feller-Peterson-Dixon (FPD) level. The ground state of UO is predicted to be 5I4, and that of UF is 4I9/2. The calculated AEAs of UO and UF are 1.123 and 0.453 eV, respectively, and the corresponding IEs are 5.976 and 6.278 eV. The BDE of UO (749.5 kJ/mol) is predicted to be considerably higher than that of UF (627.2 kJ/mol), and both are higher than those predicted for UB, UC, and UN. NBO calculations show strong ionic character for the ground states of UO and UF and bond orders that range from 2 to 3 and from 1 to 2, respectively. Comparisons of the calculated properties to those of the series comprising UB, UC, and UN diatomic molecules are given.

Prediction of the structures and heats of formation of MO2, MO3, and M2O5 for M = V, Nb, Ta, Pa.

Structures for the mono-, di-, and tri-bridge isomers of M2O5 as well as those for the MO2 and MO3 fragments for M = V, Nb, Ta, and Pa were optimized at the density functional theory (DFT) level. Single point CCSD(T) calculations extrapolated to the complete basis set (CBS) limit at the DFT geometries were used to predict the energetics. The lowest energy dimer isomer was the di-bridge for M = V and Nb and the tri-bridge for M = Ta and Pa. The di-bridge isomers were predicted to be composed of MO2+ and MO3- fragments, whereas the mono- and tri-bridge are two MO2+ fragments linked by an O2-. The heats of formation of M2O5 dimers, as well as MO2 and MO3 neutral and ionic species were predicted using the Feller-Peterson-Dixon (FPD) approach. The heats of formation of the MF5 species were calculated to provide additional benchmarks. Dimerization energies to form the M2O5 dimers are predicted to become more negative going down group 5 and range from -29 to -45 kcal mol-1. The ionization energies (IEs) for VO2 and TaO2 are essentially the same at 8.75 eV whereas the IEs for NbO2 and PaO2 are 8.10 and 6.25 eV, respectively. The predicted adiabatic electron affinities (AEAs) range from 3.75 eV to 4.45 eV for the MO3 species and vertical detachment energies from 4.21 to 4.59 eV for MO3-. The calculated MO bond dissociation energies increase from 143 kcal mol-1 for M = V to ∼170 kcal mol-1 for M = Nb and Ta to ∼200 kcal mol-1 for M = Pa. The M-O bond dissociation energies are all similar ranging from 97 to 107 kcal mol-1. Natural bond analysis provided insights into the types of chemical bonds in terms of their ionic character. Pa2O5 is predicted to behave like an actinyl species dominated by the interactions of approximately linear PaO2+ groups.

Triphenylphosphine─Closed-Shell Metal Cation Interactions.

The interactions between group 1 and 11 monocations and group 2 dications with triphenylphosphine were studied by using a combination of correlated molecular orbital theory and density functional theory. Two binding modes were found: the front side (phosphorus lone pair) and back side (phenyl rings). Group 1 and 2 cations prefer binding to the π system rather than to the lone pair of the phosphorus atom, and their ligand binding energies (LBEs) correlate with the atomic ionic radii as well as the hardness of the atomic ion. Group 11 monocations prefer binding to the lone pair of the phosphorus atom, and their LBEs are correlated with the hardness of the cation but exhibit a different trend than for the groups 1 and 2 cations. The LBEs of the cations with C2H4, C6H6, and C6H5PH2 are also reported to aid in the analysis of the cation-π interactions and the influence of the PH2 substituent on the energy of this interaction. The LBEs for binding to C2H4 and C6H6 are the most complete and reliable set of values for these species.

Acid Gas Capture by Nitrogen Heterocycle Ring Expansion.

Acid gases including CO2, OCS, CS2, and SO2 are emitted by industrial processes such as natural gas production or power plants, leading to the formation of acid rain and contributing to global warming as greenhouse gases. An important technological challenge is to capture acid gases and transform them into useful products. The capture of CO2, CS2, SO2, and OCS by ring expansion of saturated and unsaturated substituted nitrogen-strained ring heterocycles was computationally investigated at the G3(MP2) level. The effects of fluorine, methyl, and phenyl substituents on N and/or C were explored. The reactions for the capture CO2, CS2, SO2, and OCS by 3- and 4-membered N-heterocycles are exothermic, whereas ring expansion reactions with 5-membered rings are thermodynamically unfavorable. Incorporation of an OCS into the ring leads to the amide product being thermodynamically favored over the thioamide. CS2 and OCS capture reactions are more exothermic and exergonic than the corresponding CO2 and SO2 capture reactions due to bond dissociation enthalpy differences. Selected reaction energy barriers were calculated and correlated with the reaction thermodynamics for a given acid gas. The barriers are highest for CO2 and OCS and lowest for CS2 and SO2. The ability of a ring to participate in acid gas capture via ring expansion is correlated to ring strain energy but is not wholly dependent upon it. The expanded N-heterocycles produced by acid gas capture should be polymerizable, allowing for upcycling of these materials.

Bonding, Thermodynamics, and Spectroscopy of the Metal Borides UB0/+/- and WB0/+/.

The bonding and spectroscopy of the UB0/+/- and WB0/+/- molecules were examined by performing high-level electronic structure calculation on their low-lying electronic states. The calculations were performed at the SO-CASPT2 level to obtain the low-lying excited states and at the FPD level to calculate the adiabatic electronic affinities (AEA), ionization energies (IE), and bond dissociation energies (BDE). Compared to UC and UN, UB has a much denser manifold of states below 1.7 eV. The ground state of UB is predicted to be 8I5/2, and that of WB is 6Π7/2. The calculated IEs of UB and WB are 6.241 and 7.314 eV, respectively, and the corresponding AEAs are 1.160 and 1.422 eV. The BDE of UB is predicted to be 223.1 kJ/mol, which is considerably lower than those predicted for UC and UN and ∼35 kJ/mol lower than the BDE of WB. NBO calculations show that the U and B are connected by two 1-electron π bonds and one 1-electron σ bond with substantial ionic character and a bond order of 1.5. There are three unpaired electrons in the 5f on U. WB has less ionic character than UB with a doubly occupied π bond and a singly occupied σ bond for a bond order of ∼1.5. The results show that the U in UB behaves more like an actinide and the W in WB more like a transition metal.

Hydrolysis Reactions of the High Oxidation State Dimers Th2O4, Pa2O5, U2O6, and Np2O6. A Computational Study.

The energetics of the hydrolysis reactions for high oxidation states of the dimeric actinide species Th2IVO4, Pa2VO5, and U2VIO6 were calculated at the CCSD(T) level and those for triplet Np2VIO6 at the B3LYP level. Hydrolysis is initiated by the formation of a Lewis acid/base adduct with H2O (physisorbed product), followed by a proton transfer to form a dihydroxide molecule (chemisorbed product); this process was repeated until the initial actinide oxide is fully hydrolyzed. For Th2O4, hydrolysis (chemisorption) by the initial and subsequent H2O molecules prefers proton transfer to terminal oxo groups before the bridge oxo groups. The overall Th2O4 hydration pathway is exothermic with chemisorbed products preferred over the physisorption products, and the fully hydrolyzed Th2(OH)8 can form exothermically. Hydrolysis of Pa2O5 forms isomers of similar energies with no initial preference for bridge or terminal hydroxy groups. The most exothermic hydrolysis product for Pa is Pa2O(OH)8 and the most stable species is Pa2O(OH)8(H2O). Hydrolysis of U2O6 and Np2O6 with strong [O═An═O]2+ actinyl groups occurs first at the bridging oxygens rather than at the terminal oxo groups. The U2O6 and Np2O6 pathways predict hydrated products to be more favored than hydrolyzed products, as more H2O molecules are added. The stability of the U and Np clusters is predicted to decrease with increasing number of hydroxyl groups. The most stable species on the hydration reaction coordinate for U and Np is An2O3(OH)6(H2O).

Energetic and Electronic Properties of NpH0/+/- and PuH0/+/.

High-level correlated molecular orbital theory calculations have been performed to predict the thermodynamic and electronic properties of diatomic NpH0/+/- and PuH0/+/-. The excited states up to ∼10,000 cm-1 were predicted for these molecules at the multireference SO-CASPT2 level. The inclusion of spin-orbit effects is fundamental to predict the low-lying state ordering. NpH is predicted to have a 5Π0 ground state, and PuH has a 6Π1/2 ground state at the SO-CASPT2 level. The adiabatic electron affinities (AEAs) and ionization energies (IEs) of NpH and PuH were calculated to be 0.389 and 6.156 and 0.396 and 6.296 eV, respectively, using the Feller-Peterson-Dixon approach. The AEA increases going from AcH (0.425 eV) to ThH (0.820 eV) and decreases from ThH to PuH. The IEs of Pa-Np hydrides are close to ∼6.2 eV followed by an increase of 0.14 eV to PuH (6.296 eV). The An-H bond dissociation energy (BDE) decreases from 276.4 (AcH) to 107.1 (PuH) kJ/mol; the BDE(NpH) is ∼80 kJ/mol higher than that of PuH. Natural bond orbital calculations show that the bond character for these molecules is mainly ionic, An+H-. The additional electron in NpH- and PuH- populates the 6d orbital, and NpH+ and PuH+ are formed by the removal of a 7s electron. The current work in conjunction with prior work on the AcH to UH in different charge states provides insights into how these properties change across the actinide series.

Assignment of Vibrational Bands of Critical Surface Species Containing Nitrogen in the Selective Catalytic Reduction of NO by NH3.

The selective catalytic reduction (SCR) of NO by NH3 on metal oxides plays a key role in minimizing NOx emissions. Electronic structure calculations at the density functional theory level have been performed to predict the vibrational modes of NH3/NH4+ bound to validated cluster models of vanadium oxide bound to a TiO2 surface. Excellent agreement of the scaled calculated values with the observed bands attributed to surface-bound species is found. The presence of NH3 bound to Lewis acid sites and NH4+ bound to Brønsted acid sites when VOH groups are present is supported by our predictions. NH4+ is expected to dominate the spectra even at low concentrations, with predicted intensities 5 to 30 times greater than those predicted for surface-bound NH3. This is particularly evident in the lowest-energy N-H stretches of surface NH4+ due to partial proton transfer interactions with the vanadium oxide surface model. The current work is consistent with experimental vibrational spectroscopy results and does not support the presence of a significant amount of NH2 on the catalyst surface for the SCR reaction on VOx/TiO2. The combined experimental and computational results support the presence of both NH3- and NH4+-type species bound to the surface.

Computational Study of Dehydration and Dehydrogenation of Ethanol on (TiO2)n (n = 2-4) Nanoclusters.

Dehydration and dehydrogenation of an ethanol molecule on (TiO2)n, n = 2-4, nanoclusters were studied at the correlated molecular orbital theory CCSD(T)/aug-cc-pVDZ(-PP(Ti)) level using density functional theory B3LYP/DZVP2-optimized geometries. Physisorption and chemisorption of ethanol at the bridge Ti site on the trimer and tetramer are thermodynamically preferred over these reactions at the Ti site with a terminal Ti═O. Two possible lowest energy reaction coordinates of dehydration were predicted for the dimer and trimer where the ß hydrogen on ethanol transfers to the adjacent terminal oxygen, or to the adjacent bidentate oxygen. Only the latter reaction coordinate was predicted to be the lowest energy one for the tetramer. Removal of ethylene from the (TiO2)nOH2-C2H4 complex for n = 2-4 at 0 K requires 2-7 kcal/mol. For dehydrogenation, transfer of the α hydrogen to the adjacent Ti atom results in the lowest energy reaction coordinate following a proton-coupled electron-transfer (PCET) process. Removal of the acetaldehyde molecule requires 14-26 kcal/mol from the (TiO2)nH2-C2H4O complex. Loss of H2 from the (TiO2)nH2 complex requires 5-8 kcal/mol. Dehydration and dehydrogenation of one ethanol molecule occur below the reactant asymptote for (TiO2)n, n = 2-4, whereas for (WO3)3 and (MoO3)3, two ethanol molecules are required for this process to be below the reactant asymptote. Dehydration of ethanol is thermodynamically preferred over dehydrogenation on (TiO2)n, n = 2-4. There is an approximate linear correlation of metal Lewis acidity with physisorption of ethanol. A quadratic correlation is predicted between the chemisorption barrier of ethanol and the corresponding proton affinity of oxygen to which the proton is being transferred. There are linear correlations between the basicity of the oxygen site and the acidity of the OH group versus the energy to remove C2H4 from that site. The results for the nanoclusters for n = 3 and 4 are consistent with the experimental results for the reactivity of ethanol on Ti5c4+ rutile TiO2 (110) surface sites.

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.

Prediction of Aqueous Reduction Potentials of X•, ChH•, and XO• Radicals with X = Halogen and Ch = Chalcogen.

The aqueous electron affinity and aqueous reduction potentials for F•, Cl•, Br•, I•, OH•, SH•, SeH•, TeH•, ClO•, BrO•, and IO• were calculated using electronic structure methods for explicit cluster models coupled with a self-consistent reaction field (SMD) to treat the aqueous solvent. Calculations were conducted using MP2 and correlated molecular orbital theory up to the CCSD(T)-F12b level for water tetramer clusters and MP2 for octamer cluster. Inclusion of explicit waters was found to be important for accurately predicting the redox potentials in a number of cases. The calculated reduction potentials for X• and ChH• were predicted to within ∼0.1 V of the reported literature values. Fluorine is anomalous due to abstraction of a hydrogen from one of the surrounding water molecules to form a hydroxyl radical and hydrogen fluoride, so its redox potential was calculated using only an implicit model. Larger deviations from experiment were predicted for ClO• and BrO•. These deviations are due to the free energy of solvation of the anion being too negative, as found in the pKa calculations, and that for the neutral being too positive with the current approach.

NHC Carbene-Metal Complex Ligand Binding Energies.

The ligand binding energies (LBEs) of N-heterocyclic carbenes (NHCs) and CH2 and CF2 adducts with group 1, 2, 10, and 11 metals and complexes with metals from these groups are predicted at the coupled cluster CCSD(T) level of theory by using density functional theory optimized geometries. The differences in LBEs as a function of the metal and the types of bonding interactions as well as the type of carbene are described. The bonding between the alkali cations and alkaline earth dications is predominantly ionic with a linear correlation between the LBEs and the cation hardness. In contrast, the bonding behaviors of the group 10 and 11 metals and metal complexes have only a weak, indirect correlation between the LBEs and the metal hardness. The difference in bonding behavior between the groups of metals arises due to the accessibility of electron donation between the ligand and the metal in the transition metal complexes, which results in more covalent-like bonding behavior. The presence of the methyl groups on the NHC nitrogen results in only slightly more delocalized charge from the metal onto the ring, but there is significant redistribution of the charge on the ring. Saturation of the NHC ring had a much smaller effect on how the charge was distributed on the ring. The analysis of the bonding behavior of NHCs with various metal groups enables improved understanding of carbene-metal interactions to inform rational design of NHC-based systems.

Supersonically expanded sodium metal-dilute halogen gas interactions. The importance of reaction populated and energy storing reservoir states and population inversion created amplification in Na2.

The reactions of Cl, Br, and I with Nan=2,3 produced in a supersonic expansion form Na2* and Na* excited states extending across the visible and ultraviolet regions. Emission in the region extending from 410 to 600 nm indicates selectively formed excited state Na2 emission features. Experimental evidence suggests that this emission is associated with Na3 + X reactions. Broadband (0.5 cm-1) laser measurements demonstrate gain (population inversion) for select features at∼524-528(1%), ∼492(0.3%), and ∼458.7-461(0.8%) nm. Single mode (0.007 cm-1) measurements extending from 528.03 to 527.63 nm demonstrate amplification involving five to six individual rovibronic levels with a maximum gain close to 3% recorded at 527.9 nm. The observed gain is associated with select transitions from levels of the Na2 11Πu state populated, via identified curve crossings, through collision induced transfer from long-lived Na2 21Σg+ and 11Πg reservoir states. Collision induced population buildup in the lowest vibrational levels of these reservoir states and collision induced transfer to the Na2 11Πu state create a population inversion in transitions to the X 1Σg+ state of Na2. The observed amplification is aided by rapid vibrational and rotational relaxation in both the Na2 ground and excited reservoir states producing amplifiers in the visible region like the HF amplifier in the infrared. This study suggests the importance of reaction populated and energy storing long-lived reservoir states in small sodium molecule combustion processes and indicates the potential for providing new short wavelength visible and ultraviolet amplifiers for future laser-based chemical propulsion concepts.

Crystallographic evidence of Watson-Crick connectivity in the base pair of anionic adenine with thymine.

Utilizing an ionic liquid strategy, we report crystal structures of salts of free anionic nucleobases and base pairs previously studied only computationally and in the gas phase. Reaction of tetrabutylammonium ([N4444]+) or tetrabutylphosphonium ([P4444]+) hydroxide with adenine (HAd) and thymine (HThy) led to hydrated salts of deprotonated adenine, [N4444][Ad]·2H2O, and thymine, [P4444][Thy]·2H2O, as well as the double salt cocrystal, [P4444]2[Ad][Thy]·3H2O·2HThy. The cocrystal includes the anionic [Ad-(HThy)] base pair which is a stable formation in the solid state that has previously not even been suggested. It exhibits Watson-Crick connectivity as found in DNA but which is unusual for the free neutral base pairs. The stability of the observed anionic bases and their supramolecular formations and hydrates has also been examined by electronic structure calculations, contributing to more insight into how base pairs can bind when a proton is removed and highlighting mechanisms of stabilization or chemical transformation in the DNA chains.