Density Functional Theory Geometry Research Articles

Computational Study of Ethanol Conversion on Al8O12 as a Model for γ-Al2O3

Correlated molecular orbital theory at the coupled cluster CCSD(T) level with density functional theory geometries is used to study ethanol dehydration, dehydrogenation, and condensation reactions on an the Al8O12 cluster which is a model for γ-Al2O3. The Al in the active site on the cluster is a strong Lewis acid. The reactions begin with formation of a very stable Lewis acid–base ethanol–cluster adduct. Dehydration proceeds by β-H transfer to a bicoordinate oxygen leading to the direct formation of ethylene and two OH groups following an E2 mechanism. Dehydrogenation proceeds directly by α-H transfer to the active metal center and a proton transfer to a bicoordinate bridge O to form acetaldehyde plus a metal hydride and a hydroxyl, again an E2 mechanism. After addition of a second ethanol, diethyl ether is generated by an α-C transfer from the first to the second ethanol, an acid-driven SN2 mechanism. Condensation and dehydration with two alcohols have comparable energy barriers. The addition of a secon...

Structural Evolution of Tcn (n = 4-20) Clusters from First-Principles Global Minimization.

We explore the structural evolution of Tcn (n = 4-20) clusters using a first-principles global minimization technique, namely, basin-hopping from density functional theory geometry optimization (BH-DFT). Significantly more stable structures have been found in comparison with previous models, indicating the power of DFT-based basin hopping in finding new structures for clusters. The growth sequence and pattern for n from 4 to 20 are analyzed from the perspective of geometric shell formation. The binding energy per atom, relative stability, and magnetic moments are examined as a function of the cluster size. Several magic sizes of higher stability and symmetry are discovered. In particular, we find that Tc19 prefers an Oh symmetry structure, resembling a piece of a face-centered-cubic metal, and its electrostatic potential map shows interesting features that indicate special reactivity of the corner atoms.

Density functional theory screening of gas-treatment strategies for stabilization of high energy-density lithium metal anodes

To explore the potential of molecular gas treatment of freshly cut lithium foils in non-electrolyte-based passivation of high-energy-density Li anodes, density functional theory (DFT) has been used to study the decomposition of molecular gases on metallic lithium surfaces. By combining DFT geometry optimization and Molecular Dynamics, the effects of atmospheric (N2, O2, CO2) and hazardous (F2, SO2) gas decomposition on Li(bcc) (100), (110), and (111) surfaces on relative surface energies, work functions, and emerging electronic and elastic properties are investigated.The simulations suggest that exposure to different molecular gases can be used to induce and control reconstructions of the metal Li surface and substantial changes (up to over 1 eV) in the work function of the passivated system. Contrary to the other considered gases, which form metallic adlayers, SO2 treatment emerges as the most effective in creating an insulating passivation layer for dosages ≤1 mono-layer. The substantial Li → adsorbate charge transfer and adlayer relaxation produce marked elastic stiffening of the interface, with the smallest change shown by nitrogen-treated adlayers.

Computational Study of the Clustering of a Cyclohexene Autoxidation Product C6H8O7 with Itself and Sulfuric Acid.

We investigate the molecular interactions between sulfuric acid and a recently reported C6H8O7 ketodiperoxy acid formed through autoxidation from cyclohexene and ozone. Structurally similar but larger ELVOC (extremely low volatility organic compound) products formed from autoxidation of monoterpenes are believed to play a major role in the formation and early growth of atmospheric aerosol particles. Utilizing density functional theory geometries, with a DLPNO-CCSD(T)/def2-QZVPP single point energy correction, the stepwise Gibbs free energies of formation have been calculated, and the stabilities of the molecular clusters have been evaluated. C6H8O7 interacts weakly with both itself and sulfuric acid, with standard free energies of formation (ΔG at 298 K and 1 atm) around or above 0 kcal/mol. This is due to the presence of strong intramolecular hydrogen bonds in the peroxyacid groups of C6H8O7. These stabilize the isolated molecule with respect to its clusters, and lead to unfavorable interaction energies. The addition of sulfuric acid to clusters containing C6H8O7 is somewhat more favorable, but the formed clusters are still far more likely to evaporate than to grow further in atmospheric conditions. These findings indicate that the O/C ratio cannot exclusively be used as a proxy for volatility in atmospheric new particle formation involving organic compounds. The specific molecular structure, and especially the number of strong hydrogen binding moieties, are equally important. The interaction between the C6H8O7 compound and aqueous phase sulfate ions indicates that ELVOC-type compounds can contribute to aerosol mass by effectively partitioning into the condensed phase.

First-Principles Study on Structural, Electronic, and Spectroscopic Properties of γ-Ca2SiO4:Ce(3+) Phosphors.

In the present work, geometric structures, electronic properties, and 4f → 5d transitions of γ-Ca2SiO4:Ce(3+) phosphors have been investigated by using first-principles calculations. Four categories of typical substitutions (i.e., the doping of the Ce(3+) without the neighboring dopants/defects and with the neighboring VO(••), AlSi', and VCa″) are taken into account to simulate local environments of the Ce(3+) located at two crystallographically different calcium sites in the γ-Ca2SiO4. Density functional theory (DFT) geometry optimization calculations are first performed on the constructed supercells to obtain the information about the local structures and preferred sites for the Ce(3+). On the basis of the optimized crystal structures, the electronic properties of γ-Ca2SiO4:Ce(3+) phosphors are calculated with the Heyd-Scuseria-Ernzerhof screened hybrid functional, and the energies and relative oscillator strengths of the 4f → 5d transitions of the Ce(3+) are derived from the ab initio embedded cluster calculations at the CASSCF/CASPT2/RASSI-SO level. A satisfactory agreement with the available experimental results is thus achieved. Moreover, the relationships between the dopants/defects and the electronic as well as spectroscopic properties of γ-Ca2SiO4:Ce(3+) phosphors have been explored. Such information is vital, not least for the design of Ce(3+)-based phosphors for the white light-emitting diodes (w-LEDs) with excellent performance.

Azo dicarboxylates are not conjugated: X-ray crystal structure and theoretical calculations on di-t-butylazodicarboxylate

The X-ray crystal structure of trans-di-t-butyl azodicarboxylate (DTBAD, 2) was determined and this revealed that the torsion angle between the NN and CO double bonds is 84.0(2)°, and that between the anti-disposed CO vectors is 180°. This is the first report of the solid state structure of an azodicarboxylate ester. The molecule was subjected to Density Functional Theory geometry optimization at the B3LYP/6-31G(d) level in cyclohexane medium, and the global minimum structure agreed in principle with that determined in the solid state by crystallography. The N–C(O) torsion angle in the optimized structure is 107.7°, and the CO vectors lie in an anti relationship. Similar calculations on the unknown cis-NN isomer revealed an optimum geometry whose energy is predicted to lie only 11.9 kJ/mol higher than that of the trans isomer. M062X/6-311+G(d) model chemistry was used to determine relative electronic energies and to conduct Natural Bond Orbital (NBO) calculations. Exploration of the energetics of rotations about the N–C(O) bonds revealed a clear preference for near-orthogonality in azodicarboxylates, and suggests almost complete absence of classical conjugation between the neighbouring π bonds. Electronic transitions were simulated using the time-dependent DFT (TD-DFT) approach at the B3LYP/6-311+G(d) level, and the weak band in the near-UV for 2 in cyclohexane was reproduced in the calculations. The electronic isolation of the NN bond may be important in the numerous applications of azodicarboxylates in organic synthesis, and the small energy difference between the trans and cis isomers implies the likely involvement of the latter in the successful photochemical diaza-Diels–Alder reaction of diethyl azodicarboxylate with 1,3-cyclohexadiene.

Synthesis and spectroscopic characterization of a new (aryl-SCN)n polymer: Polythiocyanatohydroquinone

In the present work we have demonstrated the first synthesis of the polythiocyanogen-like (aryl-SCN)n compound (polythiocyanatohydroquinone) from the initial 1,4-benzoquinone and NH4SCN reagents under the normal conditions in the glacial acetic acid medium. The synthesized amorphous polymer was characterized experimentally by the FT-IR and UV–vis spectroscopic methods accompanied with theoretical assignments by the density functional theory (DFT) and time-dependent DFT calculations. The transmission electron microscopy and the XRD pattern analysis were used to indicate the amorphous structure of the synthesized polymer. The DFT geometry optimization of a number of oligomers (n=4–8) permit us to predict the possible structure of polythiocyanatohydroquinone and to assign the observed bands in IR and UV–vis absorption spectra.It was found that the synthesized polythiocyanatohydroquinone powder has a complicated structure which can be represented as a branched polymer constructed from the mono- and doubly-SCN-substituted benzene-1,4-diol moieties. This new material demonstrates a good stabilizing effect in respect to colloidal solutions of Ag and Au nanoparticles. Additionally, polythiocyanatohydroquinone is predicted to be a promising candidate for creation of metal-containing composite materials. Its application as a framework for the Pt electrode closing is found very useful.

Synthesis, Crystal Structures and DFT Calculations of Two Schiff Base Derivatives of (2-Amino-5-ethyl-thiophen-3-yl)-(2-chloro-phenyl)-methanone

Two new Schiff base derivatives of (2-amino-5-ethyl-thiophen-3-yl)-(2-chloro-phenyl)-methanone, namely C22H19N2O5SCl (I) and C20H15N2O3SCl (II), have been synthesized and characterized by means of NMR, single-crystal X-ray diffraction and density functional theory geometry optimization and molecular orbital calculations. Compound (I) crystallizes in the triclinic space group P-1, with a = 7.8235(6) A, b = 10.3256(5) A, c = 13.6678(9) A, α = 82.597(5)°, β = 74.759(6)°, γ = 89.968(5)°, V = 1055.96(12) A3 and Z = 2. Compound (II) also crystallizes in the triclinic space group P-1, with a = 8.0644(7) A, b = 10.7984(9) A, c = 10.8191(9) A, α = 104.142(7)°, β = 92.660(7)°, γ = 93.292(7)°, V = 910.31(13) A3 and Z = 2. Geometry optimization calculations for each compound support these observations. Both compounds display R 2 2 (10) ring motifs formed by weak C···H···O intermolecular interactions and contribute to crystal packing. Additionally, the frontier molecular orbitals of each complex are displayed and correlation between the calculated molecular orbital energies (eV) for the surfaces of the frontier molecular orbitals to the electronic excitation transitions from the absorption spectra has been proposed. Synthesis, crystal structures, density functional theory (DFT) geometry optimization and molecular orbital surface calculations of two new Schiff base derivatives of (2-amino-5-ethyl-thiophen-3-yl)-(2-chloro-phenyl)-methanone: (2-chloro-phenyl)-{2-[(4,5-dimethoxy-2-nitro-benzylidene)-amino]-5-ethyl-thiophen-3-yl}-methanone, (I): C22H19N2O5SCl and (2-chloro-phenyl)-{5-ethyl-2-[(4-nitro-benzylidene)-amino]-thiophen-3-yl}-methanone (II): C20H15N2O3SCl.

Structures and energetics of (MgCO3)n clusters (n ≤ 16).

There is significant interest in the role of carbonate minerals for the storage of CO2 and the role of prenucleation clusters in their formation. Global minima for (MgCO3)n (n ≤ 16) structures were optimized using a tree growth-hybrid genetic algorithm in conjunction with MNDO/MNDO/d semiempirical molecular orbital calculations followed by density functional theory geometry optimizations with the B3LYP functional. The most stable isomers for (MgCO3)n (n < 5) are approximately 2-dimensional. Mg can be bonded to one or two O atoms of a CO3(2-), and the 1-O bonding scheme is more favored as the cluster becomes larger. The average C-Mg coordination number increases as the cluster size increases, and at n = 16, the average C-Mg coordination number was calculated to be 5.2. The normalized dissociation energy to form monomers increases as n increases. At n = 16, the normalized dissociation energy is calculated to be 116.2 kcal/mol, as compared to the bulk value of 153.9 kcal/mol. The adiabatic reaction energies for the recombination reactions of (MgO)n clusters and CO2 to form (MgCO3)n were calculated. The exothermicity of the normalized recombination energy ⟨RE⟩CO2 decreases as n increases and converged to the experimental bulk limit rapidly. The normalized recombination energy ⟨RE⟩CO2 was calculated to be -52.2 kcal/mol for the monomer and -30.7 kcal/mol for n = 16, as compared to the experimental value of -27.9 kcal/mol for the solid phase reaction. Infrared spectra for the lowest energy isomers were calculated, and absorption bands in the previous experimental infrared studies were assigned with our density functional theory predictions. The (13)C, (17)O, and (25) Mg NMR chemical shifts for the clusters were predicted. The results provide insights into the structural and energetic transitions from nanoclusters of (MgCO3)n to the bulk and the spectroscopic properties of clusters for their experimental identification.

Synthesis, Crystal and Molecular Structure Studies and DFT Calculations of Phenyl Quinoline-2-Carboxylate and 2-Methoxyphenyl Quinoline-2-Carboxylate; Two New Quinoline-2 Carboxylic Derivatives

The crystal and molecular structures of the title compounds, phenyl quinoline-2-carboxylate and 2-methoxyphenyl quinoline-2-carboxylate, two new derivatives of quinolone-2-carboxylic acid, are reported and confirmed by single crystal X-ray diffraction and spectroscopic data. Compound (I), C16H11NO2, crystallizes in the monoclinic space group P21/c, with 8 molecules in the unit cell. The unit cell parameters are a = 14.7910(3) Å; b = 5.76446(12) Å; c = 28.4012(6) Å; β = 99.043(2)°; V = 2391.45(9) Å3. Compound (II), C17H13NO5, crystallizes in the monoclinic space group P21/n with 4 molecules in the unit cell. The unit cell parameters are a = 9.6095(3) Å; b = 10.8040(3) Å; c = 13.2427(4) Å; β = 102.012(3)°; V = 1344.76(7) Å3. Density functional theory (DFT) geometry optimized molecular orbital calculations were performed and frontier molecular orbitals of each compound are displayed. Correlation between the calculated molecular orbital energies (eV) for the surfaces of the frontier molecular orbitals to the electronic excitation transitions from the absorption spectra of each compound has been proposed. Additionally, similar correlations observed among six closely related compounds examining small structural differences to their frontier molecular orbital surfaces and from their DFT molecular orbital energies, provide further support for the suggested assignments of the title compounds.

Erratum to "Geometries and Vertical Excitation Energies in Retinal Analogues Resolved at the CASPT2 Level of Theory: Critical Assessment of the Performance of CASSCF, CC2, and DFT Methods".

A systematic investigation of structural properties and vertical excitation energies of a series of structurally modified 11-cis-retinal chromophores in vacuo was performed by means of multiconfigurational second-order perturbation theory (CASPT2). CASPT2-based geometries agree reasonably well with Moller–Plesset second-order perturbation theory (MP2), local second-order approximate coupled cluster singles and doubles (LCC2), and density functional theory (DFT) geometries, while the complete active space self-consistent field (CASSCF) method exaggerates dramatically the bond length pattern in the polyene chain. The quality of the resulting vertical excitation energies obtained by employing CASSCF, second-order approximate coupled cluster singles and doubles (CC2), LCC2, and time-dependent density functional theory (TD-DFT) approaches is assessed with respect to the CASPT2 data. We show that the commonly used CASSCF/CASPT2 approach works reasonably well in the case of vertical excitation energies of planar...

Exploration of the transition temperatures and crystal structure of highly crystalline poly(1,3-cyclohexadiene): An experimental and computational investigation

Experimental determination of transition temperatures for highly crystalline polymers such as poly-1,3-cyclohexadiene (PCHD) can be difficult due to reduced solubility and thermalization processes which occur during data acquisition. In order to facilitate further understanding of these processes for PCHD, density functional theory (DFT) and molecular dynamics (MD) were used in conjunction with differential scanning calorimetry (DSC) and powder X-ray diffraction (XRD) to explore the oligomer microstructures, the crystal structure, and the temperature dependence of the specific volume (1/ρ). DFT geometry minimizations on isolated oligomers were used to identify the lowest energy confirmer; revealing that alternating R,R and S,S chiral bonds between monomer units afford the lowest energy structure. MD simulations of crystalline PCHD were constructed so as to replicate the experimental XRD pattern of crystalline PCHD, with the best fit producing a monoclinic crystal structure. The temperature dependence of the specific volume derived from MD simulations provided insight into the glass/vitrification (Tg) and melting (Tm) transition temperatures. Comparison of the simulation transition temperatures with differential scanning calorimetry data of PCHD polymerized with Ni(acac)2/MAO shows good agreement and solidifies the fidelity of the newly defined PCHD crystalline structure.

Spies Within Metal-Organic Frameworks: Investigating Metal Centers Using Solid-State NMR

Structural characterization of metal–organic frameworks (MOFs) is crucial, since an understanding of the relationship between the macroscopic properties of these industrially relevant materials and their molecular-level structures allows for the development of new applications and improvements in current performance. In many MOFs, the incorporated metal centers dictate the short- and long-range structure and porosity of the material. Here we demonstrate that solid-state NMR (SSNMR) spectroscopy targeting NMR-active metal centers at natural abundance, in concert with ab initio density functional theory (DFT) calculations and X-ray diffraction (XRD), is a powerful tool for elucidating the molecular-level structure of MOFs. 91Zr SSNMR experiments on MIL-140A are paired with DFT calculations and geometry optimizations in order to detect inaccuracies in the reported powder XRD crystal structure. 115In and 139La SSNMR experiments on sets of related MOFs at two different magnetic fields illustrate the sensitivit...

Molecular structure of W(PMe3)3H6 in the solid state and in solution

The molecular structure of W(PMe3)3H6 has been identified by low temperature (−123°C) X-ray diffraction studies as a classical hydride compound, with a tricapped trigonal prismatic geometry in which two of the PMe3 ligands adopt eclipsed positions on opposite triangular faces of the prism and one PMe3 ligand caps a rectangular face. The solid state structure is reproduced well by density functional theory geometry optimization calculations on the molecule in the gas phase. NMR spectroscopic studies provide evidence that W(PMe3)3H6 also exists as a classical hydride compound in solution. For example, the T1(min) value for the hydride ligands of W(PMe3)3H6 is 380ms at 500MHz and 310ms at 400MHz, values that are more in accord with a classical hydride formulation than a nonclassical one. Likewise, the W(PMe3)3H6−xDx isotopologues (obtained by isotope exchange with either deuterobenzene or deuterotoluene) exhibit upfield (+13ppb) deuterium secondary isotope effects on the hydride chemical shift, which are also consistent with a classical description.

Design of Ferroelectric Organic Molecular Crystals with Ultrahigh Polarization

Inspired by recent successful synthesis of room-temperature ferroelectric supramolecular charge-transfer complexes, i.e., tetrathiafulvalene (TTF)- and pyromellitic diimide (PMDI)-based crystals (Tayi et al. Nature 2012, 488, 485-489), three new ferroelectric two-component organic molecular crystals are designed based on the TTF and PMDI motifs and an extensive polymorph search. To achieve energetically favorable packing structures for the crystals, a newly developed computational approach that combines polymorph predictor with density functional theory (DFT) geometry optimization is employed. Tens of thousands of packing structures for the TTF- and PMDI-based crystals are first generated based on the limited number of asymmetric units in a unit cell as well as limited common symmetry groups for organocarbon crystals. Subsequent filtering of these packing structures by comparing with the reference structures yields dozens of promising crystal structures. Further DFT optimizations allow us to identify several highly stable packing structures that possess the space group of P2₁ as well as high to ultrahigh spontaneous polarizations (23-127 μC/cm(2)) along the crystallographic b axis. These values are either comparable to or much higher than the computed value (25 μC/cm(2)) or measured value (55 μC/cm(2)) for the state-of-the-art organic supramolecular systems. The high polarization arises from the ionic displacement. We further construct surface models to derive the electric-field-switched low-symmetry structures of new TTF- and PMDI-based crystals. By comparing the high-symmetry and low-symmetry crystal structures, we find that the ferroelectric polarization of the crystals is very sensitive to atomic positions, and a small molecular displacement may result in relatively high polarizations along the a and c axes, polarity reversal, and/or electronic contribution to polarization. If these newly designed TTF- and PMDI-based crystals with high polarizations are confirmed by experiments, the computer-aided ferroelectric material design on the basis of hydrogen-bonded charge-transfer complexes with flexible electron-donor and acceptor molecules would be proven valuable for expediting the search of room-temperature "displasive-type" ferroelectric organic crystals.

Geometries and Vertical Excitation Energies in Retinal Analogues Resolved at the CASPT2 Level of Theory: Critical Assessment of the Performance of CASSCF, CC2, and DFT Methods.

A systematic investigation of structural properties and vertical excitation energies of a series of structurally modified 11-cis-retinal chromophores in vacuo was performed by means of multiconfigurational second-order perturbation theory (CASPT2). CASPT2-based geometries agree reasonably well with Møller-Plesset second-order perturbation theory (MP2), local second-order approximate coupled cluster singles and doubles (LCC2), and density functional theory (DFT) geometries, while the complete active space self-consistent field (CASSCF) method exaggerates dramatically the bond length pattern in the polyene chain. The quality of the resulting vertical excitation energies obtained by employing CASSCF, second-order approximate coupled cluster singles and doubles (CC2), LCC2, and time-dependent density functional theory (TD-DFT) approaches is assessed with respect to the CASPT2 data. We show that the commonly used CASSCF/CASPT2 approach works reasonably well in the case of vertical excitation energies of planar structures, but lack of dynamic correlation leads to large errors in energetics for strongly strained structures. For example, the highly twisted conformers of 9,10-dimethyl and 9,10,13-trimethyl species are found as global minima at the CASSCF level, whereas they turn almost planar at the CASPT2, MP2, LCC2, and DFT levels of theory. The CC2 method has shown a remarkable performance, manifested by a maximum deviation of 0.05 eV from the reference CASPT2 results, whereas the local version of CC2 seems to fail to describe the charge-transfer character of the S0 → S1 transitions correctly. We believe that our CASPT2 benchmark set will provide a reference that can be utilized for validation and development studies on 11-cis-retinal protonated Schiff base chromophore analogues.

Improved ReaxFF Force Field Parameters for Au–S–C–H Systems

Evaluation and reparameterization of previously reported ReaxFF parameters (Järvi, T. T.; et al. J. Phys. Chem. A 2011, 115, 10315-10322) is carried out for Au-S-C-H systems. Changes in Au-S and Au-Au bond parameters and S-Au-S angle bending parameters yield improvements for bond bending potential energy surfaces. The new ReaxFF parameters lead to good agreement with density functional theory geometries of small clusters and gold-thiolate nanoparticles. The energies of Au38(SCH3)24 clusters are compared, and the new ReaxFF calculations are also in good agreement with PBE calculations for the isomer orderings. In addition, the relative energies of Au40(SCH3)24 nanoparticles and Au-thiolate SAMs are calculated using the updated parameters. These new ReaxFF parameters will enable the study of the geometries and reactivity of larger gold-thiolate nanoparticles.

Molecular modeling for Cu(II)-aminopolycarboxylate complexes: Structures, conformational energies, and ligand binding affinities

A ligand field molecular mechanics (LFMM) force field (FF) has been developed for d(9) copper(II) complexes of aminopolycarboxylate ligands. Training data were derived from density functional theory (DFT) geometry optimizations of 14 complexes comprising potentially hexadentate N2O4, tetrasubstituted ethylenediamine (ed), and propylenediamine cores with various combinations of acetate and propionate side arms. The FF was validated against 13 experimental structures from X-ray crystallography including hexadentate N2O4 donors where the nitrogens donors are forced to be cis and bis-tridentate ONO ligands which generate complexes with trans nitrogen donors. Stochastic conformational searches for [Cu{ed(acetate)n(propionate)(4-n)}](2-), n = 0-4, were carried out and the lowest conformers for each system reoptimized with DFT. In each case, both DFT and LFMM predict the same lowest-energy conformer and the structures and energies of the higher-energy conformers are also in satisfactory agreement. The relative interaction energies for n = 0, 2, and 4 computed by molecular mechanics correlate with the experimental log β binding affinities. Adding in the predicted log β values for n = 1 and 3 suggest for this set of complexes a monotonic decrease in log β as the number of propionate arms increases.

Computational studies of catalyst-free single walled carbon nanotube growth

Semiempirical tight binding (TB) and density functional theory (DFT) methods have been used to study the mechanism of single walled carbon nanotube (SWNT) growth. The results are compared with similar calculations on graphene. Both TB and DFT geometry optimized structures of relevance to SWNT growth show that the minimum energy growth mechanism is via the formation of hexagons at the SWNT end. This is similar to the result for graphene where growth occurs via the formation of hexagons at the edge of the graphene flake. However, due to the SWNT curvature, defects such as pentagons are more stable in SWNTs than in graphene. Monte Carlo simulations based on the TB energies show that SWNTs close under conditions that are proper for growth of large defect-free graphene flakes, and that a particle such as a Ni cluster is required to maintain an open SWNT end under these conditions. The calculations also show that the proper combination of growth parameters such as temperature and chemical potential are required to prevent detachment of the SWNTs from the Ni cluster or encapsulation of the cluster by the feedstock carbon atoms.

Structural, Spectroscopic, and Computational Characterization of the Azide Adduct of FeIII(2,6-diacetylpyridinebis(semioxamazide)), a Functional Analogue of Iron Superoxide Dismutase

We have prepared and thoroughly characterized, using X-ray crystallographic, spectroscopic, and computational methods, the diazide adduct of [Fe(III)(dapsox)(H2O)2](+) [dapsox = 2,6-diacetylpyridinebis(semioxamazide)], (1), a low-molecular weight, functional analogue of iron superoxide dismutase (FeSOD). The X-ray crystal structure of the dimeric form of 1, (Na[Fe(III)(dapsox)(N3)2]·DMF)2 (2) shows two axially coordinated, symmetry inequivalent azides with differing Fe-N3 bond lengths and Fe-N-N2 bond angles. This inequivalence of the azide ligands likely reflects the presence of an interdimer hydrogen bonding interaction between a dapsox NH group and the coordinated nitrogen of one of the two azide ligands. Resonance Raman (rR) data obtained for frozen aqueous solution and solid-state samples of 2 indicate that the azides remain inequivalent in solution, suggesting that one of the azide ligands of 1 engages in an intermolecular hydrogen bonding interaction with a water molecule. Density functional theory (DFT) and time-dependent DFT calculations have been used to study two different computational models of 1, one using coordinates taken from the X-ray crystal structure of 2, and the other generated via DFT geometry optimization. An evaluation of these models on the basis of electronic absorption, magnetic circular dichroism, and rR data indicates that the crystal structure based model yields a more accurate electronic structure description of 1, providing further support for the proposed intermolecular hydrogen bonding of 1 in the solid state and in solution. An analysis of the experimentally validated DFT results for this model reveals that the azides have both σ- and π-bonding interactions with the Fe(III) center and that more negative charge is located on the Fe-bound, rather than on the terminal, nitrogen atom of each azide. These observations are reminiscent of the results previously reported for the azide adduct of FeSOD and provide clues regarding the origin of the high catalytic activity of Fe-dapsox for superoxide disproportionation.