Spin exchange effects on the physicochemical properties of tetraoxolene-bridged bimetallic complexes

The synthesis, physical, and spectroscopic properties of a series of metal complexes bridged by the redox-active chloranilate ligand are described. Compounds containing the (CAcat,cat)4- ligand, where (CAcat,cat)4- represents the fully reduced aromatic form of chloranilate, have been prepared by two different routes from H2CA and H4CA starting materials; the corresponding (CAsq,cat)3- analogue was obtained by one-electron oxidation with decamethylferrocenium tetrafluoroborate. Homo- and heterobimetallic complexes containing CrIII and GaIII with chloranilate have been prepared, yielding the following six complexes: [Ga2(tren)2(CAcat,cat)](BPh4)2 (1), [Ga2(tren)2(CAsq,cat)](BPh4)2(BF4) (2), [GaCr(tren)2(CAcat,cat)](BPh4)2 (3), [GaCr(tren)2(CAsq,cat)](BPh4)2(BF4) (4), [Cr2(tren)2(CAcat,cat)] (BPh4)2 (5), and [Cr2(tren)2(CAsq,cat)](BPh4)2(BF4) (6) (where tren is tris(2-aminoethyl)amine). Single-crystal X-ray structures have been obtained for complexes 1, 3, and 5; nearly identical C-C bond distances within the quinoidal ligand confirm the aromatic character of the bridge in each case. Complex 2 exhibits a temperature-independent magnetic moment of microeff = 1.64 +/- 0.04 microB in the solid state between 4 and 350 K, consistent with the CAsq,cat formulation of the ligand and an S = 1/2 ground state for complex 2. Complex 3 exhibits a value of microeff = 3.44 +/- 0.09 microB that is also temperature-independent, indicating an S = 3/2 ground state. Complexes 4-6 are all influenced by Heisenberg spin exchange. The temperature-independent behavior of complexes 4 and 6 indicate the presence of strong antiferromagnetic exchange between the CrIII and the (sq,cat) bridging radical yielding well-isolated ground states of S = 1 and 5/2 for 4 and 6, respectively. In contrast, complex 5 exhibits a weak intramolecular antiferromagnetic exchange interaction between the two CrIII centers (J = -2 cm-1 for H = -2J?1.?2) via superexchange through the diamagnetic CAcat,cat bridge. The absorption spectra of the CAsq,cat-containing complexes exhibit a number of sharp, relatively intense features in fluid solution. Group theoretical arguments coupled with a qualitative ligand-field analysis including the effects of Heisenberg spin exchange suggest that several of the observed transitions are a consequence of exchange interactions in both the ground- and excited-state manifolds of the compounds.     

Hadronic interactions and cosmic radiation at energies above 105 GeV

A selection of results in particle physics is given from experiments with the cosmic radiation from about 1960 to December 1974. Mostly, the results are from interactions of energy greater than 10⁵ GeV. The topics covered are: a) variation of charged particle multiplicity with energy, b) rise in the p-p cross section with energy, c) rise in the NN production cross section with energy, d) the mean p_T and its distribution at energies from 10⁵ to 10⁸ GeV, e) the mass spectrum of ‘fireballs” and the transverse momenta of decay products in the fireball CMS, f) other emulsion chamber results including the event ‘Centauro’, g) the search and evidence for very massive, comparatively stable particles, h) interactions between 10¹⁷ and 10²¹ eV.     

棉花植株及土壤中丙硫克百威及其代谢物残留气相色谱测定

研究超声波提取、毛细管气相色谱配氮磷检测器(NPD)检测棉花植株及土壤中丙硫克百威及其代谢物残留 ,各组分回收率83%～110 % ,线性范围0.1～100mg/L,最低检测质量分数0.01×10-6～0.02×10-6(w) ;该法具有简单、快速、节省试剂的特点     

Ultrafast dynamics of 2E state formation in Cr(acac)3

Femtosecond time-resolved absorption spectroscopy has been used to elucidate the excited-state dynamics associated with formation of the (2)E excited state in a Cr(III) transition metal complex. Cr(acac)(3) (where acac is the deprotonated monoanion of acetylacetone) exhibits monophasic decay kinetics with tau = 1.1 +/- 0.1 ps following excitation into the lowest-energy ligand-field absorption band; the time constant is found to be independent of both excitation and probe wavelength across the entire (4)A(2) --> (4)T(2) absorption envelope. The lack of a significant shift in the excited-state absorption spectrum combined with the observed spectral narrowing is consistent with an assignment of this process as vibrational cooling (k(vib)) in the (2)E state. The data on Cr(acac)(3) indicate that intersystem crossing associated with the (4)T(2) --> (2)E conversion occurs at a rate k(ISC) > 10(13) s(-)(1) and furthermore competes effectively with vibrational relaxation in the initially formed (4)T(2) state. Excitation into the higher energy (4)LMCT state (lambda(ex) = 336 nm) gives rise to biphasic kinetics with tau( 1) = 50 +/- 20 fs and tau( 2) = 1.2 +/- 0.2 ps. The slower component is again assigned to vibrational cooling in the (2)E state, whereas the subpicosecond process is attributed to conversion from the charge-transfer to the ligand-field manifold. In addition to detailing a process central to the photophysics of Cr(III), these results reinforce the notion that the conventional picture of excited-state dynamics in which k(vib) > k(IC) > k(ISC) does not generally apply when describing excited-state formation in transition metal complexes.     

W(CO)6-catalyzed oxidative carbonylation of primary amines to N,N'-disubstituted ureas in single or biphasic solvent systems. Optimization and functional group compatibility studies

Primary amines undergo carbonylation to N,N'-disubstituted ureas using W(CO)6 as the catalyst, I2 as the oxidant, and CO as the carbonyl source. Preparation of various N,N'-disubstituted ureas from aliphatic primary amines, RNH2 (R = n-Pr, n-Bu, i-Pr, sec-Bu, or t-Bu), was achieved in good to excellent yields. Studies of functional group compatibility using a series of substituted benzylamines demonstrated broad tolerance of functionality during the carbonylation reaction. Preparation of various N,N'-disubstituted ureas from substituted benzylamines, R-C6H4CH2NH2 (R = H, p-OCH3, p-CO2H, p-CO2Et, p-CH2OH, p-SCH3, p-vinyl, p-Cl, p-Br, m-I, p-NH2, p-NO2, or p-CN), was achieved in good yields. For many substituted benzylamines, yields of ureas were higher when a two-phase CH2Cl2/H2O solvent system was used.     

Femtosecond time-resolved optical and Raman spectroscopy of photoinduced spin crossover: temporal resolution of low-to-high spin optical switching

A combination of femtosecond electronic absorption and stimulated Raman spectroscopies has been employed to determine the kinetics associated with low-spin to high-spin conversion following charge-transfer excitation of a FeII spin-crossover system in solution. A time constant of tau = 190 +/- 50 fs for the formation of the 5T2 ligand-field state was assigned based on the establishment of two isosbestic points in the ultraviolet in conjunction with changes in ligand stretching frequencies and Raman scattering amplitudes; additional dynamics observed in both the electronic and vibrational spectra further indicate that vibrational relaxation in the high-spin state occurs with a time constant of ca. 10 ps. The results set an important precedent for extremely rapid, formally forbidden (DeltaS = 2) nonradiative relaxation as well as defining the time scale for intramolecular optical switching between two electronic states possessing vastly different spectroscopic, geometric, and magnetic properties.     

High‐Silica Zeolite SSZ‐61 with Dumbbell‐Shaped Extra‐Large‐Pore Channels

The synthesis of the high‐silica zeolite SSZ‐61 using a particularly bulky polycyclic structure‐directing agent and the subsequent elucidation of its unusual framework structure with extra‐large dumbbell‐shaped pore openings are described. By using information derived from a variety of X‐ray powder diffraction and electron microscopy techniques, the complex framework structure, with 20 Si atoms in the asymmetric unit, could be determined and the full structure refined. The Si atoms at the waist of the dumbbell are only three‐connected and are bonded to terminal O atoms pointing into the channel. Unlike the six previously reported extra‐large‐pore zeolites, SSZ‐61 contains no heteroatoms in the framework and can be calcined easily. This, coupled with the possibility of inserting a catalytically active center in the channel between the terminal O atoms in place of H⁺, afford SSZ‐61 intriguing potential for catalytic applications.     

Distinguishing between Dexter and rapid sequential electron transfer in covalently linked donor-acceptor assemblies

The syntheses, physical, and photophysical properties of a family of complexes having the general formula [M2(L)(mcb)(Ru(4,4'-(X)2-bpy)2)](PF6)3 (where M = Mn(II) or Zn(II), X = CH3 or CF3, mcb is 4'-methyl-4-carboxy-2,2'-bipyridine, and L is a Schiff base macrocycle derived from 2,6-diformyl-4-methylphenol and bis(2-aminoethyl)-N-methylamine) are described. The isostructural molecules all consist of dinuclear metal cores covalently linked to a Ru(II) polypyridyl complex. Photoexcitation of [Mn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (4) in deoxygenated CH2Cl2 solution results in emission characteristic of the 3MLCT excited state of the Ru(II) chromophore but with a lifetime (tau(obs) = 5.0 +/- 0.1 ns) and radiative quantum yield (Phi(r) approximately 7 x 10(-4)) that are significantly attenuated relative to the Zn(II) model complex [Zn2(L)(mcb)(Ru((CF3)2-bpy)2)](PF6)3 (6) (tau(obs) = 730 +/- 30 ns and Phi(r) = 0.024, respectively). Quenching of the 3MLCT excited state is even more extensive in the case of [Mn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (3), whose measured lifetime (tau(obs) = 45 +/- 5 ps) is >10(4) shorter than the corresponding model complex [Zn2(L)(mcb)(Ru((CH3)2-bpy)2)](PF6)3 (5) (tau(obs) = 1.31 +/- 0.05 micros). Time-resolved absorption measurements on both Mn-containing complexes at room-temperature revealed kinetics that were independent of probe wavelength; no spectroscopic signatures for electron-transfer photoproducts were observed. Time-resolved emission data for complex 4 acquired in CH2Cl2 solution over a range of 200-300 K could be fit to an expression of the form k(nr) = k0 + A x exp{-DeltaE/kB T} with k0 = 1.065 +/- 0.05 x 10(7) s(-1), A = 3.7 +/- 0.5 x 10(10) s(-1), and DeltaE = 1230 +/- 30 cm(-1). Assuming an electron-transfer mechanism, the variable-temperature data on complex 4 would require a reorganization energy of lambda approximately 0.4-0.5 eV which is too small to be associated with charge separation in this system. This result coupled with the lack of enhanced emission at temperatures below the glass-to-fluid transition of the solvent and the absence of visible absorption features associated with the Mn(II)2 core allows for a definitive assignment of Dexter transfer as the dominant excited-state reaction pathway. A similar conclusion was reached for complex 3 based in part on the smaller driving force for electron transfer (DeltaG0(ET) = -0.1 eV), the increase in probability of Dexter transfer due to the closer proximity of the donor excited state to the dimanganese acceptor, and a lack of emission from the compound upon formation of an optical glass at 80 K. Electronic coupling constants for Dexter transfer were determined to be approximately 10 cm(-1) and approximately 0.15 cm(-1) in complexes 3 and 4, respectively, indicating that the change in spatial localization of the excited state from the bridge (complex 3) to the periphery of the chromophore (complex 4) results in a decrease in electronic coupling to the dimanganese core of nearly 2 orders of magnitude. In addition to providing insight into the influence of donor/acceptor proximity on exchange energy transfer, this study underscores the utility of variable-temperature measurements in cases where Dexter and electron-transfer mechanisms can lead to indistinguishable spectroscopic observables.