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251.
Measurements of the third-order nonlinear optical responses of solutions of the metal-metal multiply bonded complexes Mo(2)(OPr(i))(6), W(2)(OBu(t))(6), M(2)(NMe(2))(6), M(2)(O(2)CBu(t))(4), and M(2)Cl(4)(PMe(3))(4) (M = Mo, W), using picosecond degenerate four-wave mixing at 1064 nm, are reported. These complexes display only very small instantaneous electronic polarizations when excited with cross-polarized beams. When the excitation beams are similarly polarized, a significant third-order optical response is detected, which is attributable to the formation of bulk thermal excitation gratings. Time-dependent measurements support this view. 相似文献
252.
The covariant derivative of a single massive fermion field on a Riemannian manifold is defined. The standard method of defining free bosonic Lagrangians from the fermion covariant derivative does not give the usual Lagrangian density for the free gravitational field. We express the fermion Lagrangian mass term as a frame field term added to the covariant derivative; this extended covariant derivative defines a gravitational Lagrangian density proportional to the usual scalar curvatureR, plus a term quadratic in the curvature components. The quadratic term is expected to be negligible at distances much greater than the fermion Compton wavelength, and is of a general form widely studied in recent years. The frame field term used to derive this gravitational Lagrangian is essentially the same as that used previously to derive the electroweak interaction boson mass matrix without using the Higgs-Kibble mechanism. 相似文献
253.
[reaction: see text] An efficient macrolactonization protocol devoid of any base was developed derived from the use of vinyl esters in transesterification. Subjecting a hydroxy acid and ethoxyacetylene to 2 mol % [RuCl(2)(p-cymene)](2) in toluene followed by addition of camphorsulfonic acid or inverse addition provided macrolactones in good yields. 相似文献
254.
Chisholm MH D'Acchioli JS Pate BD Patmore NJ Dalal NS Zipse DJ 《Inorganic chemistry》2005,44(4):1061-1067
With the aid of density function theory, the molecular and electronic structures of the molecules Mo2(O2CMe)4, MoW(O2CMe)4, and W2(O2CMe)4 and their single-electron oxidized radical cations have been determined; this includes calculated observables such as v(MM) and the delta --> delta* electronic transition energies. The calculated properties are compared with those for the corresponding pivalates, M2(O2CtBu)4 (M = Mo or W) and MoW(O2CtBu)4 and their radical cations prepared in situ by oxidation with Cp2FePF6. The EPR spectra of the radical cations are also reported. The EPR spectrum of the MoW(O2CtBu)4+ cation reveals that the unpaired electron is in a polarized MM delta orbital having 70% Mo and 30% W character. The MM stretching frequencies show good correlation with the MM bond lengths obtained from single-crystal X-ray diffraction studies of MoW(O2CtBu)4, W2(O2CtBu)4, and W2(O2CtBu)4+PF6- compounds, along with previously reported structures. These data provide benchmark parameters for valence trapped dicarboxylate bridged radical cations of the type [(tBuCO2)3M2]2(micro-O2C-X-CO2)+ (X = conjugated spacer). 相似文献
255.
Chisholm MH Pate BD Wilson PJ Zaleski JM 《Chemical communications (Cambridge, England)》2002,(10):1084-1085
Electron paramagnetic resonance, electronic absorption, and resonance Raman spectroscopy reveal that in the oxalate-bridged compounds, [[(tBuCO2)3M2]2(mu-O2CCO2)]+[PF6]-, the unpaired electron is delocalized over four metal centers (M = Mo or W) as a result of M2 delta to bridge pi conjugation, but in the related cationic perfluoroterephthalate-bridged species, the tungsten complex is delocalized and the molybdenum analogue valence trapped. 相似文献
256.
Chisholm MH Feil F Hadad CM Patmore NJ 《Journal of the American Chemical Society》2005,127(51):18150-18158
Toluene solutions of M2(O2C(t)Bu)4 (M = Mo, W; 2 equiv) react with a range of functionalized terephthalic acids, HO2CArCO2H (Ar = C6H4, C6F4, C6Cl4, C6H2-2,5-Cl2, C6H2-2,5-(OH)2, C6H3-2-F), to give [(tBuCO2)3M2]2[mu-O2CArCO2]. These compounds show intense ML(bridge)CT absorptions in the visible region of the electronic spectrum, and the terephthalate bridge serves to electronically couple the two M2 units via interactions between the M2 delta and bridge pi orbitals. Electronic structure calculations reveal how the degree of electronic coupling is controlled by the dihedral angles between the terephthalate C6 ring and the two CO2 units and the degree of interaction between the M4 delta MOs and the LUMO of the bridge. Both of these factors are controlled by the aryl substituents, and collectively these determine the thermochromism displayed by these complexes in solution together with the physical properties of the oxidized radical cations as determined by electrochemical studies (CV, DPV), UV-vis-NIR and EPR spectroscopic methods. 相似文献
257.
258.
Bursten BE Chisholm MH Hadad CM Li J Wilson PJ 《Chemical communications (Cambridge, England)》2001,(22):2382-2383
The electronic structures of oxalate-bridged, quadruply-bonded dimolybdenum and ditungsten compounds have been investigated by a variety of computational methods employing density function theory (gradient corrected and time-dependent) which reveal the consequences of strong mixing of M2 delta and oxalate pi orbitals within extended chains and cyclic structures. 相似文献
259.
Greig Chisholm Alan R. Kennedy Laura Beaton Eve Brook 《Acta Crystallographica. Section C, Structural Chemistry》2002,58(11):o645-o648
The structures of three fluoro‐substituted acetoacetanilides, namely 2′‐, 3′‐ and 4′‐fluoroacetoacetanilide, all C10H10FNO2, are presented and discussed. We observe a planar structure with intramolecular hydrogen bonding when the F atom is in the ortho position of the aromatic ring, and a twisted structure with intermolecular hydrogen bonding when the F atom is in the meta or para positions. It can be predicted which of these two structural motifs will be adopted by considering the position of any aromatic substituents. In this regard, fluorine appears to mimic the steric effect of a larger substituent, which we attribute to its high electronegativity. 相似文献
260.
Malcolm H. Chisholm David L. Clark Mark J. Hampden-Smith David H. Hoffman 《Angewandte Chemie (International ed. in English)》1989,28(4):432-444
Alkoxide and carbonyl ligands complement each other because they both behave as “π buffers” to transition metals. Alkoxides, which are π donors, stabilize early transition metals in high oxidation states by donating electrons into vacant dπ orbitals, whereas carbonyls, which are π acceptors, stabilize later transition elements in their lower oxidation states by accepting electrons from filled dπ orbitals. Both ligands readily form bridges that span M? M bonds. In solution fluxional processes that involve bridge–terminal ligand exchange are common to both alkoxide and carbonyl ligands. The fragments [W(OR)3], [CpW(CO)2], [Co(CO)3], and CH are related by the isolobal analogy. Thus the compounds [(RO)3W ? W(OR)3], [Cp(CO)2W?W(CO)2Cp], hypothetical [(CO)3Co?Co(CO)3], and HC?CH are isolobal. Alkoxide and carbonyl cluster compounds often exhibit striking similarities with respect to substrate binding—e.g., [W3(μ3-CR)(OR′)9] versus [Co3(μ3-CR)(CO)9] and [W4(C)(NMe)(OiPr)12] versus [Fe4(C)(CO)13]—but differ with respect to M? M bonding. The carbonyl clusters use eg-type orbitals for M? M bonding whereas the alkoxide clusters employ t2g-type orbitals. Another point of difference involves electronic saturation. In general, each metal atom in a metal carbonyl cluster has an 18-electron count; thus, activation of the cluster often requires thermal or photochemical CO expulsion or M? M bond homolysis. Alkoxide clusters, on the other hand, behave as electronically unsaturated species because the π electrons are ligand-centered and the LUMO metal-centered. Also, access to the metal centers may be sterically controlled in metal alkoxide clusters by choice of alkoxide groups whereas ancillary ligands such as tertiary phosphanes or cyclopentadienes must be introduced if steric factors are to be modified in carbonyl clusters. A comparison of the reactivity of alkynes and ethylene with dinuclear alkoxide and carbonyl compounds is presented. For the carbonyl compounds CO ligand loss is a prerequisite for substrate uptake and subsequent activation. For [M2(OR)6] compounds (M = Mo and W) the nature of substrate uptake and activation is dependent upon the choice of M and R, leading to a more diverse chemistry. 相似文献