Experimentally calibrated computational chemistry of tryptophan hydroxylase: Trans influence, hydrogen-bonding, and 18-electron rule govern O2-activation

Insight into the nature of oxygen activation in tryptophan hydroxylase has been obtained from density functional computations. Conformations of O₂-bound intermediates have been studied with oxygen trans to glutamate and histidine, respectively. An O₂-adduct with O₂trans to histidine (O_his) and a peroxo intermediate with peroxide trans to glutamate (P_glu) were found to be consistent (0.57-0.59 mm/s) with experimental Mössbauer isomer shifts (0.55 mm/s) and had low computed free energies. The weaker trans influence of histidine is shown to give rise to a bent O₂ coordination mode with O₂ pointing towards the cofactor and a more activated O-O bond (1.33 Å) than in O_glu (1.30 Å). It is shown that the cofactor can hydrogen bond to O₂ and activate the O-O bond further (from 1.33 to 1.38 Å). The O_his intermediate leads to a ferryl intermediate (F_his) with an isomer shift of 0.34 mm/s, also consistent with the experimental value (0.25 mm/s) which we propose as the structure of the hydroxylating intermediate, with the tryptophan substrate well located for further reaction 3.5 Å from the ferryl group. Based on the optimized transition states, the activation barriers for the two paths (glu and his) are similar, so a two-state scenario involving O_his and P_glu is possible. A structure of the activated deoxy state which is high-spin implies that the valence electron count has been lowered from 18 to 16 (glutamate becomes bidentate), giving a “green light” that invites O₂-binding. Our mechanism of oxygen activation in tryptophan hydroxylase does not require inversion of spin, which may be an important observation.     

A ferromagnetically coupled tetranuclear pseudo-cubane copper(II) cluster: magnetic and ESR studies

A tetranuclear copper(II) complex [Cu₄(NSI)₄] · 2C₂H₅OH · 2H₂O (NSI=hydroxethylsalicydeneimine) has been synthesized and characterized by X-ray diffraction analysis. The compound crystallizes in the monoclinic system, space group P2(1), a=9.494(3) Å, b=18.687(5) Å, c=13.149(4) Å, β=110.162(5)°, Z=2, R₁=0.0482 and wR₂=0.0978. The crystal structure contains a tetranuclear pseudo-cubane core based on an approximately cubane array of alternating copper and oxygen atoms. Each copper atom resides in a distorted square planar coordination environment with one nitrogen and three oxygen atoms from two NSI ligands. The tetranuclear units are linked in the crystal by O-H?O hydrogen bonds and weak Cu?O co-ordination bonds into one-dimensional structure. Variable temperature (5-300 K) magnetic measurements indicate the existence of ferromagnetic interactions among copper atoms. The IR and ESR spectra have also been investigated.     

Mono- and dinuclear Fe(III) complexes with the N2O2 donor 5-chlorosalicylideneimine ligands; synthesis, X-ray structural characterization and magnetic properties

Two novel monomeric [C₁₈H₁₇Cl₃N₂O₂Fe] (1) and dimeric [C₃₈H₃₆N₄O₄Cl₆Fe₂] (2) Fe(III) tetradentate Schiff base complexes have been synthesized and their crystal structures have been determined by single crystal X-ray diffraction analysis. In complex (1) the Schiff base ligand coordinates toward one iron atom in a tetradentate mode and each iron atom is five coordinated with the coordination geometry around iron atom which can be described as a distorted square pyramid. The presence of a short (2.89 Å) non-bonding interatomic Fe···O distances between adjacent monomeric Fe(III) complexes results in the formation of a dimer. Structural analysis of compound (2) shows that the structure is a centrosymmetric dimer in which the six coordinated Fe(III) atoms are linked by μ-phenoxo bridges from one of the phenolic oxygen atoms of each Schiff base ligand to the opposite metal center. The variable-temperature (2-300 K) magnetic susceptibility (χ) data of these two compounds have been investigated. The results show that for both complexes Fe(III) centers are in the high spin configuration (S = 5/2) and indicate antiferromagnetic spin-exchange interaction between Fe(III) ions. The obtained results are briefly discussed using magnetostructural correlations developed for other class of iron(III) complexes.     

Ferromagnetic trinuclear carbonato-bridged and tetranuclear hydroxo-bridged Cu(II) compounds with 4,4′-dimethyl-2,2′-bipyridine as ligand. X-ray structure, spectroscopy and magnetism

A controlled synthesis, characterisation and single-crystal X-ray analysis of two novel copper(II) compounds with the ligand 4,4′-dimethyl-2,2′-bipyridine (abbreviated dmbipy) is described. In a CO₂ atmosphere, with sodium hydroxide added, the carbonato-bridged triangular trinuclear compound [Cu₃(dmbipy)₆(μ₃-CO₃)](BF₄)₄(C₂H₅OH)(H₂O) (1) is obtained. Compound 1 crystallises in the monoclinic space group P2₁/n with a=16.169(6), b=23.351(11), c=21.312(7) Å, β=91.26(3), Z=4. The three copper ions are connected via the oxygen atoms from the symmetrically bridging carbonato group, resulting in a triangular array of copper atoms. Each copper has a distorted square-pyramidal environment with a basal plane formed by three nitrogen atoms of the two chelating bipyridine groups and the oxygen atom of the bridging carbonato group (Cu-N/O distances about 2.0 Å). The apical position at each copper is occupied by the fourth nitrogen atom of the bipyridines with distances varying from 2.100(11) to 2.146(11) Å. In all other experimental conditions the tetranuclear hydroxo-bridged compound [Cu₄(dmbipy)₄(μ₃-OH)₂(μ-OH)₂(H₂O)₂](BF₄)₄(H₂O)₄ (2) is obtained. Compound 2 crystallises in the monoclinic space group P2₁/c with a=13.274(8), b=21.685(7), c=11.266(7) Å, β=107.71(4), Z=2. The structure consists of two bis(hydroxo)-bridged dinuclear planar units which are connected with long Cu-O bonds to form a tetranuclear unit. Each Cu ion has a similar square-pyramidal coordination geometry: the equatorial plane of each Cu ion consists of two nitrogen atoms of the dmbipy ligand (Cu-N distances 1.945-2.003 Å), and two bridging hydroxo oxygen atoms (Cu-O distances 1.945-1.973 Å). The apical position of Cu1 is occupied by an oxygen atom of a water molecule with a distance of 2.262 Å. The second copper atom, Cu2, has the apical position occupied by an oxygen atom of a bridging hydroxo group at a distance of 2.349 Å; this bond is responsible for the formation of the tetranuclear unity. Compound 1 exhibits a ferromagnetic interaction between the copper ions with a J=29.3 cm⁻¹ and a very weak ferromagnetic intercluster interaction with zj′=2.4 cm⁻¹. Compound 2 also exhibits a ferromagnetic interaction between the copper ions with a J=31.1 cm⁻¹ and an overall magnetic interaction between the two dimeric units J′=8.76 cm⁻¹     

The dynamic motion of a M (M = Ca, Yb) atom inside the C74 (D 3h) cage: a relativistic DFT study

The interaction between M (M?=?Ca, Yb) atom and C₇₄ (D _3h) has been investigated by all electron relativistic density function theory. With the aid of the representative patch of C₇₄ (D _3h), we studied the interaction between C₇₄ (D _3h) and M (M?=?Ca, Yb) atom and obtained the interaction potential. Optimized structures show that there are three equivalent stable isomers and there is one transition state between every two stable isomers. According to the minimum energy pathway, the possible movement trajectory of M (M?=?Ca, Yb) atom in the C₇₄ (D _3h) cage is explored. The calculated energy barrier for Yb atoms moving from the stable isomer to the transition state is 10.4 kcal mol^?1 and the energy barrier for Ca atoms is 6.1 kcal mol^?1. The calculated NMR spectra of M@C₇₄ (M?=?Ca, Yb) are in good agreement with the experimental data. There are nine lines in the spectra: one 1/6 intensity signal, four half intensity signals and four full intensity signals.     

Dynamic motion of La atom inside the C74 (D 3h) cage: a relativistic DFT study

The interaction between lanthanum atom (La) and C₇₄ (D _3h) was investigated by all-electron relativistic density function theory (DFT). With the aid of the representative patch of C₇₄ (D _3h), we studied the interaction between C₇₄ (D _3h) and La and obtained the interaction potential. Optimized structures show that there are three equivalent stable isomers, with La located about 1.7 Å off center. There is one transition state between every two stable isomers. According to the minimum energy pathway, the possible movement trajectory of La atoms in the C₇₄ (D _3h) cage was explored. The calculated energy barrier for La atoms moving from the stable isomer to the transition state is 18.4 kcal mol^?1. In addition, the dynamic NMR spectra of La@C₇₄ according to the trajectory was calculated.

Iron(III), chromium(III) and cobalt(II) complexes with squarate: Synthesis, crystal structure and magnetic properties

The preparation and variable temperature-magnetic investigation of three squarate-containing complexes of formula [Fe₂(OH)₂(C₄O₄)₂(H₂O)₄]·2H₂O (1) [Cr₂(OH)₂(C₄O₄)₂(H₂O)₄]·2H₂O (2) and [Co(C₄O₄)(H₂O)₄]_n (3) [H₂C₄O₄ = 3.4-dihydroxycyclobutene-1,2-dione (squaric acid)] together with the crystal structures of 1 and 3 are reported. Complex 1 contains discrete centrosymmetric [Fe₂(OH)₂(C₄O₄)₂(H₂O)₄] diiron(II) units where the iron pairs are joined by a di-μ-hydroxo bridge and two squarate ligands acting as bridging groups through adjacent oxygen atoms. Two coordinated water molecules in cis position complete the octahedral environment at each iron atom in 1. The iron-iron distance with the dinuclear unit is 3.0722(6) Å and the angle at the hydroxo bridge is 99.99(7)°, values which compare well with the corresponding ones in the isostructural compound 2 (2.998 Å and 99.47°) whose structure was reported previously. The crystal structure of 3 contains neutral chains of squarato-O¹,O³-bridged cobalt(II) ions where four coordinated water molecules complete the six-coordination at each cobalt atom. The cobalt-cobalt separation across the squarate bridge is 8.0595(4) Å. A relatively important intramolecular antiferromagnetic coupling occurs in 1 whereas it is very weak in 2, the exchange pathway being the same [J = −14.4 (1) and −0.07 cm⁻¹ (2), the spin Hamiltonian being defined as ]. A weak intrachain antiferromagnetic interaction between the high-spin cobalt(II) ions occurs in 3 (J = −0.30 cm⁻¹). The magnitude and nature of these magnetic interactions are discussed in the light of their respective structures and they are compared with those reported for related systems.     

Enzymatic formation of (20R, 22R)-20,22-dihydroxycholesterol from cholesterol and a mixture of 16O2 and 18O2: random incorporation of oxygen atoms

[4-¹⁴C]Cholesterol was incubated with an adrenocortical preparation in the presence of ¹⁶O₂ and ¹⁸O₂ devoid of significant ¹⁶O¹⁸O. Isolated (20R,22R)-20,22-dihydroxycholesterol was converted to a trimethylsilyl derivative and analyzed by gas chromatography - mass spectrometry to determine the isotope distribution of the oxygen atoms at C-20 and C-22. The ions of $\text{m}\text{e}$ 289, 291, and 293 (comprising the C₈ C-20 to C-27 side-chain and containing, respectively, ¹⁶O₂, ¹⁶O¹⁸O, and ¹⁸O₂) exhibited a binomial distribution indicating that the oxygen atoms of the vicinal glycol were drawn at random from the atomic pool of the oxygen molecules. If both side-chain hydroxyl groups had originated from the atoms of the same oxygen molecule, the ion of $\text{m}\text{e}$ 291 would have been absent.     

Dihydroxyacetone phosphate, DHAP, in the crystalline state: monomeric and dimeric forms

It was shown that dihydroxyacetone phosphate may exist in both monomeric DHAP (C₃H₇O₆P) and dimeric DHAP-dimer (C₆H₁₄O₁₂P₂) form. Monomeric DHAP was obtained in the form of four crystalline salts: CaCl(DHAP)·2.9H₂O (7a), Ca₂Cl₃(DHAP)·5H₂O (7b), CaCl(DHAP)·2H₂O (7c), and CaBr(DHAP)·5H₂O (7d) by crystallization from aqueous solutions containing DHAP acid and CaCl₂ or CaBr₂, or by direct crystallization from a solution containing DHAP precursor and CaCl₂. At least one of the salts is stable and may be stored in the crystalline state at room temperature for several months. The dimeric form was obtained by slow saturation of free DHAP syrup with ammonia at −18 °C and isolated in the form of its hydrated diammonium salt (NH₄)₂(DHAP-dimer)·4H₂O (8). The synthesis of the compounds, their crystallization, and crystal structures determined by X-ray crystallography are described. In all 7a-d monomeric DHAP exists in the monoanionic form in an extended (in-plane) cisoid conformation, with both hydroxyl and ester oxygen atoms being synperiplanar to the carbonyl O atom. The crucial structural feature is the coordination manner, in which the terminal phosphate oxygen atoms act as chelating as well as bridging atoms for the calcium cations. Additionally, the DHAP monoanions chelate another Ca²⁺ by the α-hydroxycarbonyl moiety, in a manner observed previously in dihydroxyacetone (DHA) calcium chloride complexes. In dimeric 8 the anion is a trans isomer with the dioxane ring in a chair conformation with the hydroxyl groups in axial positions and the phosphomethyl group in an equatorial position.     

Structures and aromaticity of X2Y 2 − (X = C, Si, Ge and Y = N, P, As) anions

The equilibrium geometries, total energies, and vibrational frequencies of anions X₂Y₂ ⁻ (X = C, Si, Ge and Y = N, P, As) are theoretically investigated with density functional theory (DFT) method. Our calculation shows that for C₂N₂ ⁻ species, the D _2h isomer is the most stable four-membered structure, and for other species the C _2v isomer in which two X atoms are contrapuntal is the most stable structure at the B3LYP/6-311 +G_* level. Wiberg bond index (WBI) and negative nucleus-independent chemical shift (NICS) value indicate the existence of delocalization in stable X₂Y₂ ⁻ structures. A detailed molecular orbital (MO) analysis further reveals that stable isomers of these species have strongly aromatic character, which strengthens the structural stability and makes them closely connected with the concept of aromaticity.     

Hydrothermal synthesis and structural investigation of silver magnesium complex of benzenehexacarboxylic acid (mellitic acid), Ag2Mg2[C6(COO)6] · 8H2O with two-dimensional layered structure

Crystal and molecular structure of silver magnesium mellitate, Ag₂Mg₂[C₆(COO)₆] · 8H₂O, was synthesized hydrothermally and characterized by X-ray structure analysis. The complex crystallizes in the monoclinic system, space group P2/n, with unit cell dimensions of a=7.4347(2), b=9.9858(2), c=14.4248(3) Å, β=99.2429(5)°, V=1055.01(4) ų, and Z=2. The structure was solved and refined to R=0.036 (R_w=0.045) for 1707 independent reflections [I_o>2σ(I_o)]. The Ag cations are coordinated by six carboxylic oxygen atoms of mellitate anions with composition of [C₆(COO)₆]⁶⁻ on the (1 0 1) plane; each mellitate anion linking three neighboring Ag distorted trigonal prisms produces a two-dimensional layered structure parallel to (1 0 1). The Mg cations, which are coordinated by four water molecules and two carboxylic oxygen atoms, are intercalated between the two-dimensional layer stacks. The carboxylate group coordinated to Mg and Ag cations serve as a tridentate ligand in that structure. The number of water molecules incorporated into the mellitate compound is controlled mainly by ionic radii of metal cation in the structure. Furthermore, the ionic radii of metal cations in the mellitate compound play an essential role in arrangement of mellitate anions in the structure, whether as a one-dimensional infinite chain, a two-dimensional layered structure, or a three-dimensional framework structure.     

Structures, spectroscopic and thermodynamic properties of U2On (n = 0 ∼ 2, 4) molecules: a density functional theory study

The equilibrium structures, spectroscopic and thermodynamic parameters [entropy (S), internal energy (E), heat capacity (C _p)] of U₂, U₂O, U₂O₂ and U₂O₄ uranium oxide molecules were investigated systematically using density functional theory (DFT). Our computations indicated that the ground electronic state of U₂ is the septet state and the equilibrium bond length is 2.194 Å; the ground electronic state of U₂O and U₂O₂ were found to be $ {\tilde{X}}^3\varPhi $ and $ {\tilde{X}}^3{\sum}_{\mathrm{g}} $ with stable C _∞v and D _∞h linear structures, respectively. The bridge-bonded structure with D _2h symmetry and $ {\tilde{X}}^3{\mathrm{B}}_{1\mathrm{g}} $ state is the most stable configuration for the U₂O₄ molecule. Mulliken population analyses show that U atoms always lose electrons to become the donor and O atoms always obtain electrons as the acceptor. Molecular orbital analyses demonstrated that the frontier orbitals of the title molecules were contributed mostly by 5f atomic orbitals of U atoms. Vibrational frequencies analyses indicate that the maximum absorption peaks stem from the stretching mode of U–O bonds in U₂O, U₂O₂ and U₂O₄. In addition, thermodynamic data of U₂O_n (n?=?0?～?4) molecules at elevated temperatures of 293.0 K to 393.0 K was predicted.     

The molecular structure of epoxide hydrolase B from Mycobacterium tuberculosis and its complex with a urea-based inhibitor

Mycobacterium tuberculosis (Mtb), the intracellular pathogen that infects macrophages primarily, is the causative agent of the infectious disease tuberculosis in humans. The Mtb genome encodes at least six epoxide hydrolases (EHs A to F). EHs convert epoxides to trans-dihydrodiols and have roles in drug metabolism as well as in the processing of signaling molecules. Herein, we report the crystal structures of unbound Mtb EHB and Mtb EHB bound to a potent, low-nanomolar (IC₅₀ ≈ 19 nM) urea-based inhibitor at 2.1 and 2.4 Å resolution, respectively. The enzyme is a homodimer; each monomer adopts the classical α/β hydrolase fold that composes the catalytic domain; there is a cap domain that regulates access to the active site. The catalytic triad, comprising Asp104, His333 and Asp302, protrudes from the catalytic domain into the substrate binding cavity between the two domains. The urea portion of the inhibitor is bound in the catalytic cavity, mimicking, in part, the substrate binding; the two urea nitrogen atoms donate hydrogen bonds to the nucleophilic carboxylate of Asp104, and the carbonyl oxygen of the urea moiety receives hydrogen bonds from the phenolic oxygen atoms of Tyr164 and Tyr272. The phenolic oxygen groups of these two residues provide electrophilic assistance during the epoxide hydrolytic cleavage. Upon inhibitor binding, the binding-site residues undergo subtle structural rearrangement. In particular, the side chain of Ile137 exhibits a rotation of around 120° about its C^α-C^β bond in order to accommodate the inhibitor. These findings have not only shed light on the enzyme mechanism but also have opened a path for the development of potent inhibitors with good pharmacokinetic profiles against all Mtb EHs of the α/β type.     

Synthesis, isolation and spectroscopic characterization of Dawson polyoxotungstate-supported, organometallic complex, [{(C6H6)Ru}P2W15V3O62]: The two positional isomers

The Dawson polyoxotungstate (POM)-based, organometallic ruthenium(II) complex, [{(C₆H₆)Ru}P₂W₁₅V₃O₆₂]⁷⁻, was synthesized as two materials, i.e. 1 · 2Bu₄NCl and 1 · 1Bu₄NCl (1 = (Bu₄N)₇[{(C₆H₆)Ru}P₂W₁₅V₃O₆₂]), which contained two positional isomers a and b as major or minor species. In isomer a with the overall C_s symmetry, the (C₆H₆)Ru²⁺ group was supported on one vanadium(V) octahedral site (two V-O-V bridging oxygens and one OV terminal oxygen) of the three edge-shared vanadium(V) octahedra (V₃ site, B-site) in the Dawson POM-support [1,2,3-P₂W₁₅V₃O₆₂]⁹⁻, whereas in the other isomer b with the overall C_3v symmetry, the (C₆H₆)Ru²⁺ group was supported on the center of the V₃ site in the Dawson POM-support. Material 1 · 2Bu₄NCl was prepared by a stoichiometric reaction in CH₂Cl₂ at ambient temperature of the Dawson POM-support (Bu₄N)₉[1,2,3-P₂W₁₅V₃O₆₂] with the precursor [(C₆H₆)RuCl₂]₂, whereas material 1 · 1Bu₄NCl was prepared by a stoichiometric reaction in CH₃CN under refluxing conditions. The temperature-varied ³¹P NMR spectra revealed that b was thermodynamically more stable thana.     

Computational synthesis of C<Subscript>60</Subscript> cyano- and azopolyderivatives

The cyanation of C₆₀ to C₆₀(CN)₁₈ and the aziridination of C₆₀ to C₆₀(NH)₉ were studied by an unrestricted broken spin symmetry Hartree–Fock approach implemented in semiempirical codes based on the AM1 technique. The calculations focused on the successive addition of CN and NH moieties to the fullerene cage following the identification of the target cage atoms as those with the highest atomic chemical susceptibilities calculated at each step. The results obtained were analyzed from the viewpoint of the parallelism between these derivatives as well as C₆₀ fluorides and hydrides. The difference between the first-stage C₆₀ chlorination and other sterically free processes is discussed.     

Supramolecular assemblies of [Mo3Se4Clx(H2O)9−x] with cucurbituril; complementarity control through the variation of x

From solutions of seleno bridged triangular cluster Mo₃Se₄(aq)⁴⁺ in HCl, crystalline adducts with cucurbituril (Cuc, C₃₆H₃₆N₂₄O₁₂) of different composition, depending on HCl concentration, were isolated. From 2 M HCl, a monosubstituted cationic cluster crystallizes as {[Mo₃Se₄Cl(H₂O)₈]₂(C₃₆H₃₆N₂₄O₁₂)}Cl₆·16H₂O (1). Increase in HCl concentration to 6 M gives a pentasubstituted anionic species, (H₃O)₂[Mo₃Se₄Cl₅(H₂O)₄]₂(C₃₆H₃₆N₂₄O₁₂)·15H₂O (2). The crystal structures of 1 and 2 were determined by X-ray crystallography. Each portal of Cuc in 1 is covered with cluster cations [Mo₃Se₄Cl(H₂O)₈]³⁺ like a ‘lid’ on a ‘barrel’. Six water molecules in the trans position to the core μ₂-Se form complementary hydrogen bonds with oxygen atoms of Cuc (O?O, 2.713-3.067 Å). In 2 the complementarity is lost and the main structure building factor is short Se?Se interactions (Se?Se, 2.96-3.43 Å) between two adjacent anionic clusters. Stereochemistry of halide substitution in the triangular clusters M₃Q₄ is analyzed.     

Gas phase assembly and X-ray crystal structure of copper(I) 3,5-bis(trifluoromethyl)benzoate network with corannulene

Gas-phase co-deposition of [Cu(O₂C(3,5-CF₃)₂C₆H₃)] (1) with C₂₀H₁₀ at 170 °C affords crystals of the first copper(I)-corannulene adduct [Cu₆(O₂C(3,5-CF₃)₂C₆H₃)₆](C₂₀H₁₀)₂ (2). The X-ray crystallographic characterization of 2 reveals its main structural building blocks: a planar hexanuclear metal core supported by bridging 3,5-bis(trifluoromethyl)benzoate groups and two corannulene molecules. The Cu?Cu distances within the core of 2.6826(8)-2.7607(8) Å fall within the range of cuprophilic interactions. Several intermolecular Cu?C contacts between the cyclic Cu₆-unit and corannulene ranging from 2.799(5) to 3.266(5) Å can be identified. The shortest ones lying within the sum of the van der Waals radii for Cu and C (Σr_vdW (Cu, C) = 3.10 Å) are to the rim sites of corannulene. A noticeable flattening of the C₂₀H₁₀-bowl in 2 is also observed.     

Hydrothermal syntheses, structural characterizations and magnetic properties of cobalt(II) and manganese(II) coordination polymeric complexes containing pyrazinecarboxylate ligand

Two novel coordination polymeric complexes [Co(pzca)₂(H₂O)]_n (1) and [Mn(pzca)₂]_n (2) (pzca=2-pyrazinecarboxylate) have been synthesized by hydrothermal reaction of M(CH₃COO)₂·4H₂O (M=Co, Mn) and 2-pyrazinecarboxylic acid. The complex 1 displays an infinite zigzag chain structure in which each cobalt(II) center was coordinated by three nitrogen and three oxygen atoms to generate a CoN₃O₃ octahedral geometry. The existence of hydrogen bond leads to the formation of the interpenetrating stacking structure. Complex 2 indicates a two-dimensional layer structure through the linkage of bridging oxygen atom of pzca ligand. Each Mn(II) center exhibits a distorted octahedral coordination environment with four oxygen atoms and two nitrogen atoms. The distances of adjacent Mn(II) atoms are 3.503 and 5.654 Å, respectively. The magnetic property analyses reveal that both complexes show weak antiferromagnetic exchange interactions between the metal centers.     

95Mo NMR studies of complexes containing the Mo2O52+ core and crystal structure of Mo2O5[SC6H4NHCH2C5H4N]2(C3H7NO)3

The crystal structure of Mo₂O₅[SC₆H₄NHCH₂C₅H₄N]₂(C₃N₇NO)₃ is reported and seen to consist of a single oxo-bridged species with each Mo atom bonded to cis dioxo groups and the nitrogen atoms and thiolate group of the tridentate ligand. ⁹⁵Mo NMR spectra of this and three related complexes are presented and attempts made to interpret them in terms of their crystal structures.     

Structure, stability and spectroscopic properties of isomers of C48B6N6 heterofullerene with isolated and sequential BN substitutional patterns

Doping of fullerenes has received considerable attention, both experimentally and theoretically, as a tool to fine tuning their physical and chemical properties. In this contribution, we report the results of quantum-chemical calculations on several isomers of the boron and nitrogen doped fullerene derivative C₄₈B₆N₆. Optimized structures, relative stability and spectroscopic properties were computed at the B3LYP/6-31G^∗ level of theory. The more stable structures were characterized by computing vibrational frequencies along with Raman and IR intensities and by modeling their absorption spectra with semiempirical and TD-DFT calculations of excitation energies and intensities. Owing to the symmetry lowering with respect to C₆₀, the first allowed transitions of the more stable C₄₈B₆N₆ cages appear at lower energies. Despite this, the HOMO-LUMO energy gap, a measure of the semiconducting property, is only slightly reduced, compared to C₆₀. The calculated atomic charge distributions indicate considerable localization of charge on the heteroatoms. As a result, these isomers are expected to have more interesting condensed phase properties than C₆₀ owing to their enhanced intermolecular interactions. Among the isomers considered, the reduced structural deformation and favorable electrostatic interactions lead to a preference for the S₆ structure in which two B₃N₃ aggregates are located on opposite hexagons on the cage.