N2O decomposition on TiO2 (110) from dynamic first-principles calculations

We have carried out a systematic study of N(2)O dissociation on a TiO(2) (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have made use of both static and dynamic calculations in order to elucidate N(2)O decomposition mechanisms. We find that dissociation is not favorable on the stoichiometric surface. On the other hand, the presence of oxygen bridging vacancies make the N(2)O decomposition possible. The role of the defective surface is to provide electrons to the adsorbed molecule. We find two channels for decomposition, depending on whether the molecule is adsorbed with the O or the N end of the molecule on a vacancy. The first case is energetically downhill and proceeds spontaneously, leading to N(2) ejection from the surface and vacancy oxidation. The second case relies on the formation of an intermediate bridging configuration of the adsorbed molecule and is hindered by a small energy barrier. In this case, molecule breaking produces N(2) in the gas phase and leaves oxygen adatoms on the surface. We relate our results to recent experimental findings.     

Studies of iron(II) and iron(III) complexes with fac-N2O, cis-N2O2 and N2O3 donor ligands: models for the 2-His 1-carboxylate motif of non-heme iron monooxygenases

Enzymes in the oxygen-activating class of mononuclear non-heme iron oxygenases (MNOs) contain a highly conserved iron center facially ligated by two histidine nitrogen atoms and one carboxylate oxygen atom that leave one face of the metal center (three binding sites) open for coordination to cofactor, substrate, and/or dioxygen. A comparative family of [Fe(II/III)(N(2)O(n))(L)(4-n))](±x), n = 1-3, L = solvent or Cl(-), model complexes, based on a ligand series that supports a facially ligated N,N,O core that is then modified to contain either one or two additional carboxylate chelate arms, has been structurally and spectroscopically characterized. EPR studies demonstrate that the high-spin d(5) Fe(III)g = 4.3 signal becomes more symmetrical as the number of carboxylate ligands decreases across the series Fe(N(2)O(3)), Fe(N(2)O(2)), and Fe(N(2)O(1)), reflecting an increase in the E/D strain of these complexes as the number of exchangeable/solvent coordination sites increases, paralleling the enhanced distribution of electronic structures that contribute to the spectral line shape. The observed systematic variations in the Fe(II)-Fe(III) oxidation-reduction potentials illustrate the fundamental influence of differential carboxylate ligation. The trend towards lower reduction potential for the iron center across the [Fe(III)(N(2)O(1))Cl(3)](-), [Fe(III)(N(2)O(2))Cl(2)](-) and [Fe(III)(N(2)O(3))Cl](-) series is consistent with replacement of the chloride anions with the more strongly donating anionic O-donor carboxylate ligands that are expected to stabilize the oxidized ferric state. This electrochemical trend parallels the observed dioxygen sensitivity of the three ferrous complexes (Fe(II)(N(2)O(1)) < Fe(II)(N(2)O(2)) < Fe(II)(N(2)O(3))), which form μ-oxo bridged ferric species upon exposure to air or oxygen atom donor (OAD) molecules. The observed oxygen sensitivity is particularly interesting and discussed in the context of α-ketoglutarate-dependent MNO enzyme mechanisms.     

Adsorption of NH(3) and H(2)O in acidic chabazite. Comparison of ONIOM approach with periodic calculations

The adsorption of NH(3) and H(2)O in acidic chabazite has been studied with the B3LYP method within the cluster approach (5T, 48T clusters) and the periodic approach adopting a Si/Al = 11/1 chabazite and a basis set of polarized double-zeta quality. The 5T cluster has been treated fully ab initio at the B3LYP level whereas the 48T cluster has been treated with the ONIOM2 scheme using B3LYP as the high level of theory and the MNDO, AM1, and HF/3-21G methods as low levels of theory. Periodic calculations show that the adsorption of NH(3) in acidic chabazite takes place through an ion pair (NH(4)(+)-CHA(-)) structure, the computed adsorption energy being -32 kcal/mol. The adsorption of H(2)O leads to a hydrogen bonded (H(2)O-HCHA) complex with the computed adsorption energy of -20 kcal/mol. All ONIOM combinations provide similar structures to those obtained with periodic calculations. Adsorption energies, however, are sensitive to the low level used, especially for NH(3). The ONIOM B3LYP:HF/3-21G method is the one that provides more satisfactory results. Present results show that, for larger zeolites, the ONIOM scheme can be successfully applied while drastically reducing the cost of a fully ab initio treatment.     

Selective binding of O2 over N2 in a redox-active metal-organic framework with open iron(II) coordination sites

The air-free reaction between FeCl(2) and H(4)dobdc (dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) in a mixture of N,N-dimethylformamide (DMF) and methanol affords Fe(2)(dobdc)·4DMF, a metal-organic framework adopting the MOF-74 (or CPO-27) structure type. The desolvated form of this material displays a Brunauer-Emmett-Teller (BET) surface area of 1360 m(2)/g and features a hexagonal array of one-dimensional channels lined with coordinatively unsaturated Fe(II) centers. Gas adsorption isotherms at 298 K indicate that Fe(2)(dobdc) binds O(2) preferentially over N(2), with an irreversible capacity of 9.3 wt %, corresponding to the adsorption of one O(2) molecule per two iron centers. Remarkably, at 211 K, O(2) uptake is fully reversible and the capacity increases to 18.2 wt %, corresponding to the adsorption of one O(2) molecule per iron center. Mo?ssbauer and infrared spectra are consistent with partial charge transfer from iron(II) to O(2) at low temperature and complete charge transfer to form iron(III) and O(2)(2-) at room temperature. The results of Rietveld analyses of powder neutron diffraction data (4 K) confirm this interpretation, revealing O(2) bound to iron in a symmetric side-on mode with d(O-O) = 1.25(1) ? at low temperature and in a slipped side-on mode with d(O-O) = 1.6(1) ? when oxidized at room temperature. Application of ideal adsorbed solution theory in simulating breakthrough curves shows Fe(2)(dobdc) to be a promising material for the separation of O(2) from air at temperatures well above those currently employed in industrial settings.     

O2 and CO binding to tetraaza-tripodal-capped iron(II) porphyrins

A series of tris(2-aminoethylamine) (tren) capped iron(II) porphyrins has been synthesized and characterized and their affinities for dioxygen and carbon monoxide measured. The X-ray structure of the basic scaffold with nickel inserted in the porphyrin is also reported. All the ligands differ by the nature of the group(s) attached to the secondary amine functions of the cap. These various substitutions were introduced to probe if a hydrogen bond with these secondary amine groups acting as the donor could rationalize the high affinity of these myoglobin models. This work clearly indicates that the cage structure of the tren predominates over all the other appended groups with the exception of p-nitrophenol.     

Optimal binding site of a methane molecule on the silanol covered (010) surface of silicalite-1: ONIOM calculations

The binding energies and the corresponding structures of a methane molecule on the silanol covered (010) surface of silicalite-1 have been investigated using ab initio methods. Different levels of calculations, HF/6-31G(d), MP2/6-31G(d) and ONIOM (MP2/6-31G(d):HF/6-31G(d)) including the correction of an error due to an unbalance of the basis set, known as basis set super position error (BSSE), as well as the size of the cluster representing the silicalite-1 surface, were systematically examined to validate the model used. The ONIOM method with the BSSE correction was found to be a compromise between accuracy and computer time required. The optimal binding site on the silicalite-1 surface was observed at the configuration where the methane molecule points one H atom toward the O atom of the silanol group. The corresponding binding energy is -1.71 kJ/mol. This value is significantly higher than that of -5.65 kJ/mol when the methane molecule approaches the center of the straight channel. At this configuration, the C atom of methane was observed to locate exactly at the center of the channel. This leads to the conclusion that the methane molecule will relatively seldom be adsorbed on the silanol covered (010) surface of silicalite-1. Instead, the adsorption process will take place directly at the center of the straight channel.     

Formation of iron(III) cluster complexes with a new (N2O2) Schiff base

Two new series of Fe(III) Schiff-base complexes have been prepared and characterized by elemental and thermogravimetric analyses, IR, electronic, ESR and Mössbauer spectra. The Fe(III) complexes possess octahedral, pseudo-octahedral or pseudo-tetrahedral geometries around Fe(III), depending on the nature of the Schiff-base ligand used.     

Solvation-induced cluster anion core switching from NNO2(-)(N2O)n-1 to O(-)(N2O)n

We report a photoelectron imaging study of the [O(N(2)O)(n)](-), 0or=4 (and up to at least n=9) signatures of an O(-) core are predominantly observed. Photofragmentation studies at 355 nm support these results.     

Photodissociative production of O(1S) and N(2D) from N2O in an argon matrix at 4 K

Vacuum ultraviolet photolysis of N₂O isolated in Ar matrices at 4 K gives direct luminescent evidence for the photodissociative production of both O(¹S) and N(²D). The matrix results are compared to relative atomic quantum yields measured in the gas phase.     

水环境对CO在Cu/N-TiO_2(001)表面吸附影响的理论研究

采用基于周期性密度泛函理论的平面波超软赝势的PBE+U方法,计算了CO吸附于Cu/N-TiO2(001)表面在无水及有水预吸附两种条件下不同位置的吸附能、最优化结构及态密度的变化.通过吸附能的比较,得出了CO在上述2个表面的最佳吸附位置、吸附结构及成键状态.通过态密度的变化分析了H2O对CO在表面吸附的影响.     

Deviation from tetrahedral geometry in Me(2)GeCl(2): crystal structure of a model compound and insight from ab initio calculations

The molecular structure of Me(2)GeCl(2), and the value of the C-Ge-C angle in particular, was studied by ab initio quantum calculations to examine the deviation of this molecule from ideal geometry in the gas phase and in the crystalline state. The results show that, in the crystal, intermolecular interactions do have a large influence on the geometry of the molecule. An experimental value of 121.2 +/- 0.2 degrees is found for the C-Ge-C angle in the crystal structure of dichlorodi(2-phenethyl)germane, and it provides the first crystallographic evidence for the deviation from tetrahedral geometry. This molecule crystallizes in the monoclinic space group P2(1)/c, with a = 9.2079(2) A, b = 19.5396(4) A, c = 9.7845(2) A, beta = 114.217(1) degrees, and Z = 4. Calculations show that the conformation of the organic substituents has a sizable effect on the local geometry of the Ge-atom. Analysis of the distribution of the electron density suggests that the larger value of C-Ge-C in Me(2)GeCl(2) compared to the equivalent but smaller angle in Me(2)CCl(2) is indirectly the result of the higher ionic character of the bonds in the former molecule.     

An ONIOM and DFT study of water adsorption on rutile TiO2 (110) cluster

Density functional theory (DFT) calculations performed at ONIOM DFT B3LYP/6‐31G**‐MD/UFF level are employed to study molecular and dissociative water adsorption on rutile TiO₂ (110) surface represented by partially relaxed Ti₂₅O₃₇ ONIOM cluster. DFT calculations indicate that dissociative water adsorption is not favorable because of high activation barrier (23.2 kcal/mol). The adsorption energy and vibration frequency of both molecularly and dissociatively adsorbed water molecule on rutile TiO₂ (110) surface compare well with the values reported in the literature. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

N2 emission in NO and N2O reduction on Rh(100) and Rh(110)

The angular distribution of desorbing N(2) was studied in both the thermal decomposition of N(2)O(a) on Rh(100) at 60-140 K and the steady-state NO (or N(2)O) + D(2) reaction on Rh(100) and Rh(110) at 280-900 K. In the former, N(2) desorption shows two peaks at around 85 and 110 K. At low N(2)O coverage, the desorption at 85 K collimates at about 66 degrees off normal towards the [001] direction, whereas at high coverage, it sharply collimates along the surface normal. In the NO reduction on Rh(100), the N(2) desorption preferentially collimates at around 71 degrees off normal towards the [001] direction below about 700 K, whereas it collimates predominantly along the surface normal at higher temperatures. At lower temperatures, the surface nitrogen removal in the NO reduction is due to the process of NO(a) + N(a) --> N(2)O(a) --> N(2)(g) + O(a). On the other hand, in the steady-state N(2)O + D(2) reaction on Rh(110), the N(2) desorption collimates closely along the [001] direction (close to the surface parallel) below 340 K and shifts to ca. 65 degrees off normal at higher temperatures. In the reduction with CO, the N(2) desorption collimates along around 65 degrees off normal towards the [001] direction above 520 K, and shifts to 45 degrees at 445 K with decreasing surface temperature. It is proposed that N(2)O is oriented along the [001] direction on both surfaces before dissociation and the emitted N(2) is not scattered by adsorbed hydrogen.     

The effect of varying carboxylate ligation on the electronic environment of N2O(x) (x = 1-3) nonheme iron: a DFT analysis

Mononuclear nonheme iron oxygenase (MNO) enzymes contain a subclass of metalloproteins capable of catalyzing the O(2)-dependent hydroxylation of unactivated substrates at a ferrous ion center coordinated to a highly conserved His-His-Glu/Asp motif. These enzymes, which utilize additional reducing equivalents obtained from the decarboxylation of a coordinated α-ketoglutarate (αKG) cofactor, do not readily interact with O(2) in the absence of αKG binding. Density functional theory calculations with the B3LYP functional were performed to gain insight into the electrochemical behavior of three sets of Fe(II/III) complexes containing a core N, N, O facial binding motif in which the number of carboxylate ligands was systematically altered, to provide one, two (cis) or three (fac) labile sites. The calculated trend in Fe(II/III) reduction potentials was observed to parallel that observed in cyclic voltammetry experiments, showing a decrease in potential (stabilized oxidized state) with increasing carboxylate ligation. This trend does not appear to be the result of differential charge on the metal complex. Changes in the redox-active molecular orbital (RAMO) energy due to covalent effects dominate across the series of complexes when chloride is modeled as the labile ligand, with the π anti-bonding nature of the RAMO being an important factor. With water molecules as the labile ligands, however, a much steeper redox dependence on the number of carboxylate ligands is observed and this effect seems to be largely electrostatic in origin. Differential relaxation of the occupied molecular orbitals in the ferric complexes appears to contribute to the redox trend as well. Finally, these observations are placed in the context of MNO enzyme mechanisms.     

From N(2) and O(2) to N(4) and O(4): pneumatic chemistry in the 21st century

A brief account is given of the theoretical and experimental endeavors that span the last decade and eventually led to the discovery of N(4) and O(4), which have been actively sought for their fundamental interest and their relevance to atmospheric sciences and the development of high energy density materials (HEDM). The discovery and characterization of N(4) and O(4) as metastable species with lifetimes exceeding 1 micros in the isolated gas state have finally been achieved utilizing neutralization-reionization mass spectrometry (NRMS), a powerful technique based on collisional redox processes. The principles of NRMS, experimental outline, main features, and limitations are succinctly examined in comparison with other current approaches to the detection of metastable, short-lived neutral species.     

CO2 adsorption on TiO2(110) rutile: insight from dispersion-corrected density functional theory calculations and scanning tunneling microscopy experiments

Adsorption of CO(2) on the rutile(110) surface was investigated using dispersion-corrected density functional theory and scanning tunneling microscopy (STM). On the oxidized surface the CO(2) molecules are found to bind most strongly at the five-fold coordinated Ti sites adopting tilted or flat configurations. The presence of bridging oxygen defects introduces two new adsorption structures, the most stable of which involves CO(2) molecules bound in tilted configurations at the defect sites. Inclusion of dispersion corrections in the density functional theory calculations leads to large increases in the calculated adsorption energies bringing these quantities into good agreement with experimental data. The STM measurements confirm two of the calculated adsorption configurations.