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PubMed2004

Reaction products of W(CO)(6) with formamidines; electronic structure of a W(2)(mu-CO)(2) core with unsymmetric bridging carbonyls.

Reactions of W(CO)(6) with formamidines contrast with those of Mo(CO)(6) and Cr(CO)(6) in that the former do not yield quadruply bonded dimetal species. From the reaction of W(CO)(6) with HDAniF (HDAniF = N,N'-di-p-anisylformamidine), several new ditungsten carbonyl compounds (W(2)(mu-CO)(2)(mu-DAniF)(2)(eta(2)-DAniF)(2) (1), W(2)(mu-CO)(2)(mu-DAniF)(2)(eta(2)-DAniF)(eta(2)-CH(2)DAniF) (2), and W(2)(mu-CO)(mu-CNC(6)H(4)OCH(3))(mu-DAniF)(2)(eta(2)-DAniF)(2) (3)) have been isolated and fully characterized. In 2, CH(2)DAniF represents a DAniF ligand in which a methylene group has been added to one of the nitrogen atoms. This ligand binds to the tungsten atom using a nitrogen and a carbon atom. Compound 1 has a tungsten-tungsten bond distance of 2.476(1) A and a planar W(2)(mu-CO)(2) core structure which has C(2)(h)() symmetry with short and long W-C bond distances (1.99(1) and 2.28(1) A, respectively). DFT calculations on a model of 1 indicate that (a) the C(2)(h)() instead of D(2)(h)() symmetry of the ditungsten core may be attributed to W --> CO pi back-bonding interactions and (b) the bond between the tungsten atoms may be formulated as a double bond. The new tetragonal paddlewheel compound W(2)(DAniF)(4) (4) and the edge-sharing bioctahedron W(2)(mu-O)(mu-NC(6)H(3)Cl(2))(mu-D(Cl)PhF)(2)(eta(2)-D(Cl)PhF)(2) (5) (D(Cl)PhF = N,N'-di-(3,5-dichlorophenyl)formamidinate) have also been prepared.

Amidines↗
PubMed2006

Electronic structures and absorption spectra of linkage isomers of trithiocyanato (4,4',4' '-tricarboxy-2,2':6,2' '-terpyridine) ruthenium(II) complexes: a DFT study.

Black dye (BD), isomer 1 ([Ru(II)(H3-tctpy)(NCS)3](-1), where H3-tctpy = 4,4',4' '-tricarboxy-2,2':6,2' '-terpyridine) is known to be an excellent sensitizer for dye-sensitized solar cells and exhibits a very good near-IR photo response, compared to other ruthenium dyes. Because isothiocyanate is a linear ambidentate ligand, BD has three other linkage isomers, [Ru(H3-tctpy)(NCS)2(SCN)](-1), isomer 2 and 2', and [Ru(H3-tctpy))(SCN)3](-1), isomer 3. In this study, we have calculated the geometry of BD and its isomers by DFT. Further, we have analyzed the bonding in these isomers using NBO methods. TDDFT calculations combined with scalar relativistic zero-order regular approximations (SR-ZORA) have been carried out to simulate the absorption spectra. Calculations have been performed for the isomers both in vacuo and in solvent (ethanol). The inclusion of the solvent is found to be important to obtain spectra in good agreement with the experiment. The first absorption bands are dominated by the metal-to-ligand charge transfer (MLCT) and ligand-to-ligand charge transfer (LLCT).

Journal Article↗
PubMed2006

Experimental and theoretical investigations of IR spectra and electronic structures of the U(OH)2, UO2(OH), and UO2(OH)2 molecules.

Reactions of laser-ablated U atoms and H2O2 molecules produce UO2, H2UO2, and UO2(OH)2 as major products and U(OH)2 and HU(O)OH as minor products. Complementary information is obtained from similar reactions of U atoms with D2O2, with H2 + O2 mixtures, and with H2O in excess Ar. Through extensive relativistic density functional theory calculations, we have determined the geometry structures and ground states of these U species with a variety of oxidation states U(II), U(IV), U(V), and U(VI). The calculated vibrational frequencies, IR intensities, and isotopic frequency ratios are in good agreement with the experimental values, thus supporting assignments of the observed matrix IR spectra. We propose that the reactions proceed by forming an energized [U(OH)4] intermediate from reactions of the excited U atom with two H2O2 molecules. Because of the special stability of the U(VI) oxidation state, this intermediate decomposes to the UO2(OH)2 molecule, which reveals a distinctive difference between the chemistries of U and Th, where the major product in analogous Th reactions is the tetrahedral Th(OH)4 molecule owing to the stable Th(IV) oxidation state.

Journal Article↗
PubMed2006

Spectroscopic, magnetochemical, and crystallographic study of cesium iron phosphate hexahydrate: characterization of the electronic structure of the iron(II) hexa-aqua cation in a quasicubic environment.

Spectroscopic, magnetochemical, and crystallographic data are presented for CsFe(H2O)6PO4, a member of a little-known isomorphous series of salts that facilitates the study of hexa-aqua ions in a quasicubic environment. Above 120 K, the deviations from cubic symmetry are minimal, as shown by the first example of an iron(II) Mössbauer spectrum that exhibits no measurable quadrupole splitting. Two crystallographically distinct [Fe(OH2)6]2+ complexes are identified from inelastic neutron-scattering (INS) experiments conducted between 2 and 15 K. The data are modeled with the ligand-field Hamiltonian, H = lambdaLŝ + betaB(kL + 2ŝ) + Delta(tet){Lz2 - (1/3)L(L + 1)} + Delta(rhom){Lx2 - Ly2}, operating in the ground-term (5)T(2g) (Oh) basis. An excellent reproduction of INS, Mössbauer, HF-EPR, and magnetochemical data are obtained in the 2 and 15 K temperature regimes with the following parameters: lambda = -80 cm(-1); k = 0.8; site A Delta(tet) = 183 cm(-1), Delta(rhom)= 19 cm(-1); site B Delta(tet) = 181 cm(-1), Delta(rhom)= 12 cm(-1). The corresponding zero-field-splitting (ZFS) parameters of the conventional S = 2 spin Hamiltonian are as follows: site A D = 12.02 cm(-)(1), E = 2.123 cm(-1); site B D = 12.15 cm(-1), E = 1.37 cm(-1). A theoretical analysis of the variation of the energies of the low-lying states with respect to displacements along selected normal coordinates of the [Fe(OH2)6]2+, shows the zero-field splitting to be extremely sensitive to small structural perturbations of the complex. The expressions derived are discussed in the context of spin-Hamiltonian parameters reported for the [Fe(OH2)6]2+ cation in different crystalline environments.

Journal Article↗
PubMed1996

Electronic Structure of [Ta(5)(NH)(4)Cl(17)](6-): A Cluster with a Distorted Square Pyramidal Ta(5) Core.

We report approximate molecular orbital calculations on the [Ta(5)(NH)(4)Cl(17)](6-) cluster synthesized by Simon and Meyer. The cluster is based on a "flattened" square pyramid of tantalum atoms, basal bridging imides, and terminal chlorides. This cluster was of interest to us due to the unusual presence of imide ligands, the distorted nature of the metal core, and the possible resemblance to B(5)H(9). Our calculations indicate that metal-metal bonding is limited to Ta(apical)-Ta(basal) bonding, with no significant bonding between the basal metal atoms. The imide ligands, which bridge the base of the pyramid, were found to have a significant amount of capping character. The metal-metal bonding orbitals have some unusual features due to the pyramid's distortion. Additionally, the flattened nature of the pyramid leads to an interesting energy ordering of the metal-metal bonding orbitals.

Journal Article↗
PubMed1996

Study of the Electronic Structure of Ni(eta(5)-C(5)H(5))(NO) by Variable-Photon-Energy Photoelectron Spectroscopy and Density Functional Calculations.

Photoelectron spectra, with photon energies varying from 18 to 120 eV, have been measured for Ni(eta(5)-C(5)H(5))(NO). Relative partial photoelectron cross sections and branching ratios have been evaluated for the first three valence ionization bands. He I and He II photoelectron spectra have been remeasured for Ni(eta(5)-C(5)H(5))(NO) and Ni(eta(5)-C(5)H(4)CH(3))(NO). In the latter case, the fine structure on the first band differs from that in the previously published spectrum. Density functional calculations have been carried out to determine the ionization potentials of the lowest lying states of Ni(eta(5)-C(5)H(5))(NO) as well as the corresponding photoionization cross sections and the resulting branching ratios using the LCGTO-DF and LDKL-DF methods, respectively. Both experimental and theoretical investigations lead to an ion state ordering (2)E(1) < (2)E(2) approximately (2)A(1)< (2)E(1) and an assignment of (2)E(1) states to the first and third bands with the (2)A(1) and (2)E(2) states comprising the second band. This differs from the original assignment in the literature, where the (2)A(1) ionization was assigned to a high-energy shoulder on the first band. The separation of this shoulder from the main band maximum of 0.23 eV (1850 +/- 81 cm(-)(1)) suggests that it may be caused by excitation of the NO stretching vibration in the ion. The neutral molecule has a NO stretch of 1832 cm(-)(1); the calculated energies for the neutral molecule and the cation are 1845 and 1911 cm(-)(1), respectively. Agreement between calculated and experimental ionization energies and good matching of the theoretical and measured branching ratios support the new assignment of the photoelectron spectrum.

Journal Article↗
PubMed1996

Geometry and Electronic Structure of CuCl(6)(4-) Polyhedra Doped into (3-Chloroanilinium)(8)[CdCl(6)]Cl(4)-An EPR and Structural Investigation.

The EPR single-crystal and powder spectra of mixed crystals of (3-chloroanilinium)(8)(Cd(1-x)Cu(x)Cl(6))Cl(4) are measured as a function of temperature and x and analyzed with respect to the geometry and bonding properties of the CuCl(6) polyhedra. These undergo strong distortions due to vibronic Jahn-Teller coupling, with the resulting tetragonal elongation being superimposed by a considerable orthorhombic symmetry component induced by a host site strain acting as a compression along the crystallographic a axis. This strain becomes apparent in the cadmium compound (x = 0), whose crystal structure is also reported [a = 8.701(2) Å, b = 13.975(2) Å, c = 14.173(2) Å, alpha = 81.62(1) degrees, beta = 72.92(1) degrees, gamma = 77.57(1) degrees, triclinic P&onemacr;, Z = 1]. A calculation of the ground state potential surface and its vibronic structure nicely reproduces the g values, Cu-Cl spacings, and ligand field data. At high copper concentrations (including x = 1), the CuCl(6) polyhedra are coupled elastically, with the long axes of neighboring polyhedra having perpendicular orientations. The elastic correlation presumably is not of the long-range antiferrodistortive type, however. Above about 55 K, the angular Jahn-Teller distortion component becomes dynamically averaged within the time scale of the EPR experiment, leading to local tetragonally compressed CuCl(6) octahedra.

Journal Article↗
PubMed1999

Synthesis and Crystal and Electronic Structures of the Dinuclear Platinum Compounds [PEtPh(3)](2)[Pt(2)(&mgr;-PPh(2))(2)(C(6)F(5))(4)] and [Pt(2)(&mgr;-PPh(2))(2)(C(6)F(5))(4)]: A Computational Study by Density Functional Theory.

The electrolytic behavior of the dinuclear complexes [NBu(4)](2)[MM'(&mgr;-PPh(2))(2)(C(6)F(5))(4)] (M = M' = Pt (1), Pd (1a); M = Pt, M' = Pd (1b)) has been studied, showing electrochemically irreversible oxidation and related reduction processes. The chemical oxidation of the binuclear compound for M = M' = Pt, results in the formation of the binuclear Pt(III) compound [Pt(2)(&mgr;-PPh(2))(2)(C(6)F(5))(4)]. The crystal structure analysis of both complexes has been carried out, showing very similar structures with similar Pt-C and Pt-P distances and analogous skeletons. However the Pt-Pt distances are very different, 3.621(1) Å for the Pt(II) compound and 2.7245(7) Å for the Pt(III) derivative (as are the parameters geometrically related to this Pt-Pt distance), suggesting that, in the Pt(III) compound, there is a strong Pt-Pt bond. Results of DFT calculations on [Pt(2)(&mgr;-PH(2))(2)(C(6)F(5))(4)](n)()(-) (n = 2, 0) agree very well with the crystallographic data and indicate that, in the Pt(III) compound, there is approximately a single sigma bond between the metal atoms.

Journal Article↗
PubMed2000

Molecular and electronic structure of nitridocyanometalates of chromium(V) and manganese(V): a combined experimental and DFT study.

The six-coordinate complexes [M(N)(CN)5]3- (M = Cr, Mn) have been isolated as salts of robust rhodium amine complexes. [Rh(en)3][Mn(N)(CN)5].H2O (1) and [Rh(tn)3] [Cr(N)(CN)5].2H2O (2) have been characterized by single-crystal X-ray crystallography: 1 crystallizes in the hexagonal space group P6(3) with a = b = 15.810(2) A, c = 13.844(3) A, V = 2996.8(8) A3, and Z = 6; 2 crystallizes in the orthorhombic space group Pbcn with a = 9.723(1) A, b = 14.564(2) A, c = 31.498(4) A, V = 4460.3(8) A3, and Z = 8. In 1, all the anions are oriented with their Mn identical to N directions almost coparallel to the crystallographic 3-fold axis. Polarized single-crystal UV-vis spectroscopy of 1 confirms the validity of the Jørgensen-Ballhausen-Gray d-orbital splitting scheme with the lowest energy transition being dxy-->[dyz,dzx]. Single-crystal EPR spectroscopy of [Cr(N)(CN)5]3- diluted into 1 shows the hyperfine (53Cr) and super-hyperfine (14N) tensors to be quite anisotropic with different major axes. For the hyperfine interaction we observe A Parallel > A Perpendicular, whereas, for the super-hyperfine interaction to the terminal nitrido ligand, the reverse ordering is found: A Perpendicular > A Parallel. The complexes [M(N)(CN)5]3-, trans-[M(N)(CN)4(py)]2-, and [M(N)(CN)4]2- (M = Cr, Mn) were investigated by DFT methods. Good reproduction of the molecular structures, vibrational, and UV-vis spectra was obtained. However, pronounced differences between local density and gradient corrected functionals were observed in the description of the weak bonding to the ligands trans to the nitrido ligand. For the five-coordinate [M(N)(CN)4]2- complexes the LUMO is predicted to be a strongly admixed dz2(M)-pz(M) hybrid.

Journal Article↗
PubMed2001

A comparison of the influences of alkoxide and thiolate ligands on the electronic structure and reactivity of molybdenum(3+) and tungsten(3+) complexes. preparation and structures of M(2)(O(T)Bu)(2)(S(t)Bu)(4), [Mo(S(t)Bu)(3)(NO)](2), and W(S(t)Bu)(3)(NO)(py).

M(2)(O(t)Bu)(6) compounds (M = Mo, W) react in hydrocarbon solvents with an excess of (t)BuSH to give M(2)(O(t)Bu)(2)(S(t)Bu)(4), red, air- and temperature-sensitive compounds. (1)H NMR studies reveal the equilibrium M(2)(O(t)Bu)(6) + 4(t)BuSH <==> M(2)(O(t)Bu)(2)(S(t)Bu)(4) + 4(t)BuOH proceeds to the right slowly at 22 degrees C. The intermediates M(2)(O(t)Bu)(4)(S(t)Bu)(2), M(2)(O(t)Bu)(3)(S(t)Bu)(3), and M(2)(O(t)Bu)(5)(S(t)Bu) have been detected. The equilibrium constants show the M-O(t)Bu bonds to be enthalpically favored over the M-S(t)Bu bonds. In contrast to the M(2)(O(t)Bu)(6) compounds, M(2)(O(t)Bu)(2)(S(t)Bu)(4) compounds are inert with respect to the addition of CO, CO(2), ethyne, (t)BuC triple bond CH, MeC triple bond N, and PhC triple bond N. Addition of an excess of (t)BuSH to a hydrocarbon solution of W(2)(O(t)Bu)(6)(mu-CO) leads to the rapid expulsion of CO and subsequent formation of W(2)(O(t)Bu)(2)(S(t)Bu)(4). Addition of an excess of (t)BuSH to hydrocarbon solutions of [Mo(O(t)Bu)(3)(NO)](2) and W(O(t)Bu)(3)(NO)(py) gives the structurally related compounds [Mo(S(t)Bu)(3)(NO)](2) and W(S(t)Bu)(3)(NO)(py), with linear M-N-O moieties and five-coordinate metal atoms. The values of nu(NO) are higher in the related thiolate compounds than in their alkoxide counterparts. The bonding in the model compounds M(2)(EH)(6), M(2)(OH)(2)(EH)(4), (HE)(3)M triple bond CMe, and W(EH)(3)(NO)(NH(3)) and the fragments M(EH)(3), where M = Mo or W and E = O or S, has been examined by DFT B3LYP calculations employing various basis sets including polarization functions for O and S and two different core potentials, LANL2 and relativistic CEP. BLYP calculations were done with ZORA relativistic terms using ADF 2000. The calculations, irrespective of the method used, indicate that the M-O bonds are more ionic than the M-S bonds and that E ppi to M dpi bonding is more important for E = O. The latter raises the M-M pi orbital energies by ca. 1 eV for M(2)(OH)(6) relative to M(2)(SH)(6). For M(EH)(3) fragments, the metal d(xz)(),d(yz)() orbitals are destabilized by OH ppi bonding, and in W(EH)(3)(NO)(NH(3)) the O ppi to M dpi donation enhances W dpi to NO pi* back-bonding. Estimates of the bond strengths for the M triple bond M in M(2)(EH)(6) compounds and M triple bond C in (EH)(3)M triple bond CMe have been obtained. The stronger pi donation of the alkoxide ligands is proposed to enhance back-bonding to the pi* orbitals of alkynes and nitriles and facilitate their reductive cleavage, a reaction that is not observed for their thiolate counterpart.

Journal Article↗
PubMed2001

Solution 1H NMR of the molecular and electronic structure of the heme cavity and substrate binding pocket of high-spin ferric horseradish peroxidase: effect of His42Ala mutation.

Solution 1H NMR has been used to assign a major portion of the heme environment and the substrate-binding pocket of resting state horseradish peroxidase, HRP, despite the high-spin iron(III) paramagnetism, and a quantitative interpretive basis of the hyperfine shifts is established. The effective assignment protocol included 2D NMR over a wide range of temperatures to locate residues shifted by paramagnetism, relaxation analysis, and use of dipolar shifts predicted from the crystal structure by an axial paramagnetic susceptibility tensor normal to the heme. The most effective use of the dipolar shifts, however, is in the form of their temperature gradients, rather than by their direct estimation as the difference of observed and diamagnetic shifts. The extensive assignments allowed the quantitative determination of the axial magnetic anisotropy, Deltachi(ax) = -2.50 x 10(-8) m(3)/mol, oriented essentially normal to the heme. The value of Deltachi(ax) together with the confirmed T(-2) dependence allow an estimate of the zero-field splitting constant D = 15.3 cm(-1), which is consistent with pentacoordination of HRP. The solution structure was generally indistinguishable from that in the crystal (Gajhede, M.; Schuller, D. J.; Henriksen, A.; Smith, A. T.; Poulos, T. L. Nature Structural Biology 1997, 4, 1032-1038) except for Phe68 of the substrate-binding pocket, which was found turned into the pocket as found in the crystal only upon substrate binding (Henriksen, A.; Schuller, D. J.; Meno, K.; Welinder, K. G.; Smith, A. T.; Gajhede, M. Biochemistry 1998, 37, 8054-8060). The reorientation of several rings in the aromatic cluster adjacent to the proximal His170 is found to be slow on the NMR time scale, confirming a dense, closely packed, and dynamically stable proximal side up to 55 degrees C. Similar assignments on the H42A-HRP mutant reveal conserved orientations for the majority of residues, and only a very small decrease in Deltachi(ax) or D, which dictates that five-coordination is retained in the mutant. The two residues adjacent to residue 42, Ile53 and Leu138, reorient slightly in the mutant H42A protein. It is concluded that effective and very informative 1H NMR studies of the effect of either substrate binding or mutation can be carried out on resting state heme peroxidases.

Algorithms↗