C₇₄ endohedral metallofullerenes violating the isolated pentagon rule: a density functional theory study

Precise studies on M(2)@C(74) (M = Sc, La) series by means of DFT methods have disclosed that certain non-IPR isomers are more stable than the IPR structure. M(2)@C(2)(13295)-C(74) and M(2)@C(2)(13333)-C(74), both of which have two pentagon adjacencies (PA), present excellent thermodynamic stability with very small energy differences. Statistical mechanics calculations on the M(2)@C(74) series reveal that M(2)@C(2)(13295)-C(74) and M(2)@C(2)(13333)-C(74) are quite favoured by entropy effects below 3000 K. Sc(2)@C(74) and La(2)@C(74) series are found to have similar electronic transfer but different electronic structures due to the distinct properties of scandium and lanthanum elements according to Natural Bond Orbital (NBO) analysis in conjunction with orbital interaction diagrams. Investigations of bonding energies reflect quite different influences of the two types of metal atoms to C(74) metallo-fullerenes. Further examinations on C(74) metallo-fullerenes uncover significant stabilization effects of metal atoms acting on PA fragments. Geometrical structures of certain non-IPR cages (from C(72) to C(76)), which exhibit splendid stabilities when encapsulating metallo-clusters, are found to be related by Stone-Wales transformation and C(2) addition. Furthermore, IR spectra and (13)C NMR spectra of M(2)@C(2)(13295)-C(74) and M(2)@C(2)(13333)-C(74) have been simulated to assist further experimental characterization.     

Mechanistic study on the cellulose dissolution in ionic liquids by density functional theory☆

Ionic liquids (ILs) have attracted many attentions in the dissolution of cellulose due to their unique physicochem-ical properties as green solvents. However, the mechanism of dissolution is stil under debate. In this work, com-putational investigation for the mechanisms of dissolution of cellulose in [Bmim]Cl, [Emim]Cl and [Emim]OAc ILs was performed, and it was focused on the process of breakage of cel ulose chain and ring opening using cel obiose as a model molecule. The detailed mechanism and reaction energy barriers were computed for various possible pathways by density functional theoretical method. The key finding was that ILs catalyze the dissolution process by synergistic effect of anion and cation, which led to the cleavage of cellulose chain and formation of derivatives of cel ulose. The investigation on ring opening process of cellobiose suggested that carbene formed in ILs played an important role in the side reaction of cellulose, and it facilitated the formation of a covalent bond between cel-lulose and imidazolium core. These computation results may provide new perspective to understand and apply ILs for pretreatment of cellulose.     

Theoretical study of the enhanced Brønsted acidity of Zn2+‐exchanged zeolites

The effect of Zn²⁺ exchange on the Brønsted acidity of a protonic zeolite has been studied by the ab initio DFT (density functional theory) approach using the BLYP generalized gradient approximation. Three different zeolite cluster models have been compared: two 6“T” models (two 4“T” rings with an oxygen atom bridge) with Si/Al=1 and Si/Al=2 and a 4“T” model (ring form) with Si/Al=1. The Brønsted acidity has been probed by computation of the acetonitrile adsorption and the cluster deprotonation energy. The presence of Zn²⁺ does not affect the cluster Brønsted acidity but it creates a very strong Lewis site (Zn²⁺) in all models studied. On the other hand, the presence of ZnOH⁺ enhanced the Brønsted site acidity in the case of the 6T model with Si/Al=1. This enhancement is due to a change in cluster geometry and position of OH group in ZnOH⁺ upon acetonitrile adsorption.     

Catalytic reduction of N2O by NH3 in presence of oxygen using Fe‐exchanged zeolites

The influence of ammonia on the reduction of N₂O in presence of oxygen over Fe‐zeolite has been studied. It is found that BEA zeolite is the most efficient host structure for iron ions to catalyse the reduction of N₂O with NH₃. This revised version was published online in July 2006 with corrections to the Cover Date.     

Hydrogen adsorption on M-ZSM-12 zeolite clusters (M = K,Na and Li): a density functional theory study

The molecular adsorption of hydrogen has been studied theoretically via DFT on additional framework with alkali metal atoms (K, Na and Li) in ZSM-12 zeolite. A 14T channel zeolite cluster model was used. Lewis acidity of alkali metals decreases with increasing atomic radius of alkali metal and H₂ adsorption. Adsorption enthalpy values were computed to be ?7.4 and ?5.1 kJ/mol on Li- and Na-ZSM-12 clusters, respectively. Hydrogen adsorption enthalpy values for Li- and Na-cases are meaningfully larger than the liquefaction enthalpy of hydrogen molecule. This designates that Li- and Na-ZSM-12 zeolites are potential cryoadsorbent materials for hydrogen storage.     

Water‐promoted ammonia oxidation by a platinum amine complex in zeolite HZSM‐5 catalyst

In this study it was found that [Pt(NH₃)₄]HZSM‐5 is an active catalyst for the oxidation of ammonia at low temperature that, in contrast with other catalysts, becomes more active in the presence of water. Furthermore, the selectivity to nitrogen was found to increase when water is present. This revised version was published online in July 2006 with corrections to the Cover Date.     

Dechlorination of 1,2‐dichloroethane catalyzed by Pt–Cu/C: unraveling the role of each metal

The effect of Pt–Cu/C catalyst composition on the activity and selectivity in the reaction of 1,2‐dichloroethane dechlorination in a H₂‐containing atmosphere has been investigated. For monometallic Pt catalysts and those with Cu/Pt atomic ratio ⩽1, the reaction products are almost entirely ethane and monochloroethane. However, increasing the Cu/Pt ratio increases the selectivity towards ethylene. Approximately 90% selectivity towards ethylene is obtained for catalysts with Cu/Pt ratio ⩾9. Monometallic Cu/C produce only ethylene, but the activity is two orders of magnitude lower than that of other catalysts studied. With time on stream during the initial 2–30 h there is a continuous increase in selectivity towards ethylene at the expense of ethane. This behavior was rationalized in terms of equilibration of bimetallic particle surface composition exposed to the reaction mixture. This revised version was published online in July 2006 with corrections to the Cover Date.     

A density functional theory insight into the extraction mechanism of lithium recovery from alkaline brine by β-diketones

In this study, the mechanism of lithium extraction with β-diketones was investigated by resolving the reaction mechanism, thermodynamic property, and molecular interaction through density functional theory approach. Before the selective extraction of lithium, the β-diketones had tautomerism with the product of enol. The enol structure was unsteady in alkaline environment so that Li⁺ can be selectively extracted by taking the place of hydroxyl hydrogen. Meanwhile, the synergistic effect of TOPO intensified the extraction ability by the generation of strong electrostatic. Based on the calculated Gibbs energy changes, the parameters of extraction efficiency and separation factor were analyzed and they had qualitative consistence with the experimental results. Meanwhile, it was determined that the electron-withdrawing effect of the subsistent group in the β-diketones had a critical influence on the extraction separation performance. Besides, the theoretical model predicted successfully the transition of extraction complex from hydrophilicity to lipophilicity after the combination with synergistic reagent.     

A density functional theory study of the effects of metal cations on the Brøsted acidity of H-ZSM-5

Density functional theory calculations have been carried out to establish the influence of mono- and polyvalent cations on the Brønsted acidity of H-ZSM-5. The zeolite was modeled as a cluster containing 41-45 atoms, in the center of which is an Al⁽¹⁾(OH)SiOAl⁽²⁾(OM)unit, where M⁺ = H⁺, Li⁺, Na⁺, K⁺, Ca(OH)⁺, AlO⁺, Al(OH)⁺ ₂. The local geometry of the Brønsted acid site is affected by the nature of M⁺ and this in turn causes a change in the value of the proton affinity (PA) for the site. The highest value of PA is 330 kcal/mol for M⁺ = H⁺ and the lowest value of PA is 305 kcal/mol for M⁺ = AlO⁺. No correlation was found between the value of PA and the Mulliken charge on Al⁽¹⁾. With the exception of the case where M⁺ = AlO⁺ the binding energy of CO with the Brøsted acid proton is approximately 8.8 kcal/mol, independent of the nature of M⁺. When M⁺ = AlO⁺, the binding energy for CO is 11.1 kcal/mol. The present calculations suggest that different factors affect proton affinity and the binding energy for CO adsorption.     

Understanding the C?H activation of methane over single-atom alloy catalysts by density functional theory calculations

As an important rate-determining step in the oxidation of methane, the catalytic behaviors of single-atom alloys (SAAs) on the C H activation reactions have not been fully understood. In this work, density functional theory calculations are performed to investigate the C H activation of methane on a series of SAAs with Cu(111/100) and Ag(111/100) as hosts. The reaction energies and the barriers for the C H dissociation reaction follow the traditional Brønsted–Evans–Polanyi (BEP) relationship, but the deviation from this linearity fitting is relatively large. To better describe the catalytic activity, a new nonlinear binary function is proposed to correlate the reaction barrier with the adsorption energies of *H and *CH₃ products, which has higher correlation than the BEP relationship, and showed more accurate predictions of the C H activation reaction for unknown SAAs catalysis. Such mode might be able to extend to other catalyst systems to achieve large-scale catalyst screening.     

Redox and photo‐redox properties of isolated Mo5+ ions in MoH‐ZSM‐5 and MoH‐beta zeolites: in situ ESR study

Redox and photo‐redox properties of isolated Mo⁵⁺ ions stabilized in H‐ZSM‐5 and H‐beta zeolites are studied by in situ ESR in flowing O₂, NO, H₂, and C₃H₆. Upon oxidation of pre‐reduced samples at 20 °C, NO demonstrates a higher oxidative ability, as compared with O₂. Interaction of Mo⁵⁺ ions with propene at 20 °C results in formation of a chemisorption complex with enhanced reactivity of Mo(V) toward NO. Illumination of the Mo⁵⁺/HZSM‐5 sample with UV‐visible light causes measurable acceleration of Mo(V) oxidation by NO at 20 °C. Therefore, photochemical activation of the oxidation step could be realized, in principle, for Mo/zeolite catalysts. At 500 °C in the reaction mixture NO + H₂, the step of the catalytic site reduction is fast, and the dynamic equilibrium of the redox reaction Mo(VI) ↔ Mo(V) for MoH‐ZSM‐5 and MoH‐beta seems to be strongly shifted to Mo⁵⁺. This revised version was published online in July 2006 with corrections to the Cover Date.     

Electrochemical and density functional theory study of bis(cyclopentadienyl) mono(β-diketonato) titanium(IV) cationic complexes

The electrochemical behaviour of fluorinated bis(cyclopentadienyl) mono(β-diketonato) titanium(IV) complexes, of general formula [Cp₂Ti(R′COCHCOR)]⁺ClO₄⁻ with Cp = cyclopentadienyl and R′, R = CF₃, C₄H₃S; CF₃, C₄H₃O; CF₃, Ph (C₆H₅); CF₃, CH₃; CH₃, CH₃; Ph, Ph and Ph, CH₃ is described. Both metal and ligand based redox processes are observed. The chemically and electrochemically reversible Ti^IV/Ti^III couple is followed by an irreversible ligand reduction at a considerably more negative (cathodic) potential. A comparison of the ligand reduction in its free and chelated state indicates that the β-diketonato ligand (R′COCHCOR)⁻ in [Cp₂Ti(R′COCHCOR)]⁺ClO₄⁻ is electroactive at more negative potentials. A theoretical density functional theory (DFT) study shows that a highly localized metal centred frontier orbital dominates the Ti^IV/Ti^III redox chemistry resulting in a non-linear relationship between the formal redox potential (E°′) and the sum of the group electronegativities of the R and R′ groups, χ_R + χ_R′, of the ligand. Linear relationships, however, are obtained between the DFT calculated electron affinity (EA) of the complexes and χ_R + χ_R′, the pK_a of the free β-diketones R′COCH₂COR and the carbonyl stretching frequency, v_CO, of the complexes. The DFT calculated electronic structure of the second reduced species [Cp₂Ti(β-diketonato)]⁻ shows that it is best described as Ti(III) coupled to a β-diketonato radical.     

The kinetics of CO oxidation by adsorbed oxygen on well‐defined gold particles on TiO2(110)

Very tiny Au particles on TiO₂ show excellent activity and selectivity in a number of oxidation reactions. We have studied the vapor deposition of Au onto a TiO₂(110) surface using XPS, LEIS, LEED and TPD and found that we can prepare Au islands with controlled thicknesses from one to several monolayers. In order to understand at the atomic level the unusual catalytic activity in oxidation reactions of this system, we have studied oxygen adsorption on Au/TiO₂(110) as a function of Au island thickness, and have measured the titration of this adsorbed oxygen with CO gas to yield CO₂, as function of Au island thickness, CO pressure and temperature. A hot filament was used to dose gaseous oxygen atoms. TPD results show higher O₂ desorption temperatures (741 K) from ultrathin gold particles on TiO₂(110) than from thicker particles (545 K). This implies that O_a bonds much more strongly to ultrathin islands of Au. Thus from Brønsted relations, ultrathin gold particles should be able to dissociatively adsorb O₂ more readily than thick gold particles. Our studies of the titration reaction of oxygen adatoms with CO (to produce CO₂) show that this reaction is extremely rapid at room temperature, but its rate is slightly slower for the thinnest Au islands. Thus the association reaction (CO_g + O_a → CO_2,g) gets faster as the oxygen adsorption strength decreases, again as expected from Brønsted relations. For islands of about two atomic layers thickness, the rate increases slowly with temperature, with an apparent activation energy of 11.4 ± 2.8 kJ/mol, and shows a first‐order rate in CO pressure and oxygen coverage, similar to bulk Au(110).     

Aerobic hydrocarbon oxidation catalyzed by the vanadomolybdophosphate polyoxometalate, H5PV2Mo10O40, supported on mesoporous MCM‐41

The impregnation of H₅PV₂Mo₁₀O₄₀ polyoxometalate onto MCM‐41 and amino‐modified MCM‐41 materials provided mesoporous active catalysts with large surface areas for aerobic hydrocarbon oxidation using isobutyraldehyde as a reducing agent. The results of the oxidation of alkenes and alkanes gave product selectivities similar to those observed in the corresponding homogeneous reaction although catalytic activity was somewhat reduced. Under appropriate experimental conditions there was no leaching and the solid catalyst could be recovered and reused without loss in activity. This revised version was published online in July 2006 with corrections to the Cover Date.     

TiC–NiAl composites obtained by SHS: a time-resolved XRD study

Self-propagating high-temperature synthesis (SHS) has been performed in the quaternary Al–Ni–Ti–C system in order to obtain intermetallic–ceramic composites. These kind of reaction synthesis are very fast (linear velocity of the synthesis front is in the range 1–100 mm/s) and the productivity would be very high from the industrial point of view. Nevertheless, this characteristic is a negative point when synthesis mechanism and kinetics must be studied. There are only a few tools with enough time resolution in order to study these kinds of fast reactions. Synchrotron radiation (ESRF, Grenoble) has been employed to follow the reactions in situ on a time-scale of ∼100 ms. Powder diffraction patterns were recorded in this time-interval using a high-speed CCD camera coupled to an image intensifier X-ray sensitive detector with 1024×1024 pixels frames. As the reactions proceed patterns from the pre-heated, reaction front, post-heated and cooling zones of the reaction were sampled. The phases occurring during the reactions were identified and information of the reaction mechanism and the nucleation kinetics were obtained. SEM was used to characterize the final microstructure.     

High and low surface area NiO–MgO and CoO–MgO solid solutions: a study of XPS surface composition and CO oxidation activity

Oxide solid solutions NiO–MgO of high surface area were studied by XPS. The surface Ni²⁺ concentration was found to be equal, within experimental errors, to the bulk concentration. The result is analogous to that found previously for the low surface area NiO–MgO system and for both the high and low surface area systems of CoO–MgO. The catalytic oxidation of CO by O₂, on high and low surface area NiO–MgO and CoO–MgO materials, was investigated with the aim of relating the catalytic activity with transition metal ion nature and concentration. Turnover frequency data (CO₂ molecules produced per second per surface atom) show that the activity is due primarily to the transition metal ions and is not subject to the ions being in special configurations (dimers or trimers) or in special positions (edges, corners). The activity of CoO–MgO is higher than that of NiO–MgO solid solution. This revised version was published online in June 2006 with corrections to the Cover Date.     

Methanol synthesis pathway over Cu/ZrO2 catalysts: a time‐resolved DRIFT 13C‐labelling experiment

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been used to characterize a series of Cu/Ce/Al₂O₃ catalysts. Catalysts were prepared by incipient wetness impregnation using metal nitrate and alkoxide precursors. Catalyst loadings were held constant at 12 wt% CuO and 5.1 wt% CeO₂. Mixed oxide catalysts were prepared by impregnation of cerium first, followed by copper. The information obtained from surface and bulk characterization has been correlated with CO and CH₄ oxidation activity of the catalysts. Cu/Al₂O₃ catalysts prepared using Cu(II) nitrate (CuN) and Cu(II) ethoxide (CuA) precursors consist of a mixture of copper surface phase and crystalline CuO. The CuA catalyst shows higher dispersion, less crystalline CuO phase, and lower oxidation activity for CO and CH₄ than the CuN catalyst. For Cu/Ce/Al₂O₃ catalysts, Ce has little effect on the dispersion and crystallinity of the copper species. However, Cu impregnation decreases the Ce dispersion and increases the amount of crystalline CeO₂ present in the catalysts, particularly in Ce modified alumina prepared using cerium alkoxide precursor (CeA). Cerium addition dramatically increases the CO oxidation activity, however, it has little effect on CH₄ oxidation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Identification of important chemical features of 11β-hydroxysteroid dehydrogenase type1 inhibitors: application of ligand based virtual screening and density functional theory

11β-Hydroxysteroid dehydrogenase type1 (11βHSD1) regulates the conversion from inactive cortisone to active cortisol. Increased cortisol results in diabetes, hence quelling the activity of 11βHSD1 has been thought of as an effective approach for the treatment of diabetes. Quantitative hypotheses were developed and validated to identify the critical chemical features with reliable geometric constraints that contribute to the inhibition of 11βHSD1 function. The best hypothesis, Hypo1, which contains one-HBA; one-Hy-Ali, and two-RA features, was validated using Fischer's randomization method, a test and a decoy set. The well validated, Hypo1, was used as 3D query to perform a virtual screening of three different chemical databases. Compounds selected by Hypo1 in the virtual screening were filtered by applying Lipinski's rule of five, ADMET, and molecular docking. Finally, five hit compounds were selected as virtual novel hit molecules for 11βHSD1 based on their electronic properties calculated by Density functional theory.     

Nitrogen donor-controlled chemoselectivity of reaction in oxidation of sulfides with tetra-n-butylammonium hydrogen monopersulfate catalyzed by a partially β-brominated meso-tetraphenylporphyrinatomanganese(III) acetate: a clue to the nature of active oxidant

Highly efficient and rapid oxidation of different sulfides to the corresponding sulfones with tetra-n- butylammonium hydrogen monopersulfate (TBAO) in the presence of catalytic amounts of Mn(TPPBr₂)OAc at room temperature is reported. Contrary to other nitrogen donors, using 4-cyanopyridine as co-catalyst leads to an increase in the ratio of sulfoxide to sulfone in the products. Comparison of the chemoselectivity of reaction in the presence of different nitrogen donors as co-catalyst shows the involvement of a high-valent Mn-oxo species as well as the six-coordinate Mn(TPPBr₂)(HSO₅)(B) (B = nitrogen donors) complex in sulfide oxidation reactions with TBAO.     

In-situ FT-IRAS study of the CO oxidation reaction over Ru(001): III. Observation of a 2140 cm−1 C-O stretching vibration

Utilizing Fourier Transform Reflection Absorption-Infrared Spectroscopy (FT-IRAS), we have investigated the CO oxidation reaction in-situ on a Ru(001) surface at high ( 10 Torr) pressures. Under certain temperature and reactant (CO and O₂) partial pressure conditions, we observe for the first time on unsupported Ru a weakly adsorbed CO species which is characterized by an unusually high C-O stretching frequency of 2140 cm^–1. A similar feature has been identified previously on small Ru particles in supported catalysts and attributed by some to a multicarbonyl species (–Ru(CO) _n ,n > 1). By following the intensity of this feature on Ru(001) relative to other peaks in the spectra, we believe that the 2140 cm^–1 peak observed here is most likely due to a highly perturbed linearly adsorbed monocarbonyl on partially oxidized Ru sites generated by locally high concentrations of coadsorbed oxygen.Sandia National Laboratories is supported by the United States Department of Energy under contract number DE-AC04-76DP00789.