Enhanced translational diffusion of rubrene in sucrose benzoate

The translational diffusion of rubrene in the fragile molecular glass former, sucrose benzoate (SB) (fragility index m approximately 94), has been studied from T(g)+6 K to T(g)+71 K(T(g)=337 K) by using the technique of holographic fluorescence recovery after photobleaching. In the temperature range of the measurements, the translational relaxation functions were observed to decay exponentially, indicating that Fick's law of diffusion governs the translational motion of rubrene in sucrose benzoate. The value of the translational diffusion coefficient D(T) obtained from the 1e time of the translational relaxation function varied from 5.3 x 10(-15) cm2 s(-1) at 343 K to 5.0x10(-9) cm2 s(-1) at 408 K. The temperature dependence of D(T) for diffusion of rubrene in SB is compared with that of the viscosity and the dielectric relaxation time tau(D) of SB. The temperature dependence of D(T) is weaker than that of Teta for T<1.2T(g) but tracks the reciprocal of the dielectric relaxation time 1tau(D) for 1.05T(g)相似文献   

Mössbauer, electron paramagnetic resonance, and theoretical study of a high-spin, four-coordinate Fe(II) diketiminate complex

The iron(II) complex LFeCl 2Li(THF) 2 (L = beta-diketiminate), 1, has been studied with variable-temperature, variable-field Mossbauer spectroscopy and parallel mode electron paramagnetic resonance (EPR) spectroscopy in both solution and the solid state. In zero applied field the 4.2 K Mossbauer spectrum exhibits an isomer shift delta = 0.90 mm/s and quadrupole splitting Delta E Q = 2.4 mm/s, values that are typical for the high-spin ( S = 2) state anticipated for the iron in 1. Spectra recorded in applied magnetic fields yield an anisotropic magnetic hyperfine tensor with A x = +2.3 (+ 1.0) T, A y = A z = -21.5 T ( solution) and a nearly axial zero-field splitting of the spin quintet with D = D x approximately -14 cm (-1) and rhombicity E/ D approximately 0.1. The small, positive value for A x results from the presence of residual orbital angular momentum along x. The EPR analysis gives g x approximately 2.4 (and g y approximately g z approximately 2.0) and reveals a split " M S = +/- 2" ground doublet with a gap distributed around Delta = 0.42 cm (-1). The Mossbauer spectra of 1 show unusual features that arise from the presence of orientation-dependent relaxation and a distribution in the magnetic hyperfine field along x. The origin of the distribution has been analyzed using crystal field theory. The analysis indicates that the distribution in the magnetic hyperfine field originates from a narrow distribution, sigma phi approximately 0.5 degrees , in torsion angle phi between the FeN 2 and FeCl 2 planes, arising from minute inhomogeneities in the molecular environments.     

"Switching on" the properties of single-molecule magnetism in triangular manganese(III) complexes

The reaction between oxide-centered, triangular [MnIII3O(O2CR)6(py)3](ClO4) (R = Me (1), Et (2), Ph (3)) compounds and methyl 2-pyridyl ketone oxime (mpkoH) affords a new family of Mn/carboxylato/oximato complexes, [MnIII3O(O2CR)3(mpko)3](ClO4) [R = Me (4), Et (5), and Ph (6)]. As in 1-3, the cations of 4-6 contain an [MnIII3(mu3-O)]7+ triangular core, but with each Mn2 edge now bridged by an eta1:eta1:mu-RCO2- and an eta1:eta1:eta1:mu-mpko- group. The tridentate binding mode of the latter causes a buckling of the formerly planar [MnIII3(mu3-O)]7+ core, resulting in a relative twisting of the three MnIII octahedra and the central O2- ion now lying approximately 0.3 A above the Mn3 plane. This structural distortion leads to ferromagnetic exchange interactions within the molecule and a resulting S = 6 ground state. Fits of dc magnetization data for 4-6 collected in the 1.8-10.0 K and 10-70 kG ranges confirmed S = 6 ground states, and gave the following D and g values: -0.34 cm(-1) and 1.92 for 4, -0.34 cm(-1) and 1.93 for 5, and -0.35 cm(-1) and 1.99 for 6, where D is the axial zero-field splitting (anisotropy) parameter. Complexes 4-6 all exhibit frequency-dependent out-of-phase (chi" M) ac susceptibility signals suggesting them possibly to be single-molecule magnets (SMMs). Relaxation rate vs T data for complex 4 down to 1.8 K obtained from the chi" M vs T studies were supplemented with rate vs T data measured to 0.04 K via magnetization vs time decay studies, and these were used to construct Arrhenius plots from which was obtained the effective barrier to relaxation (Ueff) of 10.9 K. Magnetization vs dc field sweeps on single-crystals of 4.3CH2Cl2 displayed hysteresis loops exhibiting steps due to quantum tunneling of magnetization (QTM). The loops were essentially temperature-independent below approximately 0.3 K, indicating only ground-state QTM between the lowest-lying Ms = +/-6 levels. Complexes 4-6 are thus confirmed as the first triangular SMMs. High-frequency EPR spectra of single crystals of 4.3CH2Cl2 have provided precise spin Hamiltonian parameters, giving D = -0.3 cm(-1), B40 = -3 x 10(-5) cm(-1), and g = 2.00. The spectra also suggest a significant transverse anisotropy of E > or = 0.015 cm(-1). The combined work demonstrates the feasibility that structural distortions of a magnetic core imposed by peripheral ligands can "switch on" the properties of an SMM.     

Cluster oxalate complexes [M3(mu3-Q)(mu2-Q2)3(C2O4)3]2- and [Mo3(mu3-Q)(mu2-Q)3(C2O4)3(H2O)3]2- (M = Mo, W; Q = S, Se): mechanochemical synthesis and crystal structure

Mechanochemical reaction of cluster coordination polymers 1infinity[M3Q7Br4] (M = Mo, W; Q = S, Se) with solid K2C2O4 leads to cluster core excision with the formation of anionic complexes [M3Q7(C2O4)3]2-. Extraction of the reaction mixture with water followed by crystallization gives crystalline K2[M3Q7(C2O4)3].0.5KBr.nH2O (M = Mo, Q = S, n = 3 (1); M = Mo, Q = Se, n = 4 (2); M = W, Q = S, n = 5 (3)). Cs2[Mo3S7(C2O4)3].0.5CsCl.3.5H2O (4) and (Et4N)1.5H0.5K{[Mo3S7(C2O4)3]Br}.2H2O (5) were also prepared. Close Q...Br contacts result in the formation of ionic triples {[M3Q7(C2O4)3](2)Br}5- in 1-4 and the 1:1 adduct {[Mo3S7(C2O4)3]Br}3- in 5. Treatment of 1 or 2 with PPh(3) leads to chalcogen abstraction with the formation of [Mo3(mu3-Q)(mu2-Q)3(C2O4)3(H2O)3]2-, isolated as (Ph4P)2[Mo3(mu3-S)(mu2-S)3(C2O4)3(H2O)3].11H2O (6) and (Ph4P2[Mo3(mu3-Se)(mu2-Se)3(C2O4)3(H2O)3].8.5H2O.0.5C2H5OH (7). All compounds were characterized by X-ray structure analysis. IR, Raman, electronic, and 77Se NMR spectra are also reported. Thermal decomposition of 1-3 was studied by thermogravimetry.     

Synthesis and structural characterization of nonanuclear lanthanide complexes

A series of nonanuclear lanthanide oxo-hydroxo complexes of the general formula [Ln(9)(mu(4)-O)(2)(mu(3)-OH)(8)(mu-BA)(8)(BA)(8)](-)[HN(CH(2)CH(3))(3)](+).(CH(3)OH)(2)(CHCl(3)) (BA = benzoylacetone; Ln = Sm, 1; Eu, 2; Gd, 3; Dy, 4; Er, 5) were prepared by the reaction of hydrous lanthanide trichlorides with benzoylacetone in the presence of triethylamine in methanol and recrystallized from chloroform/methanol (1:10) at room temperature. These five compounds are isomorphous. Crystal data for 1: cubic, Pn3n; T = 180 K; a = 33.8652(4) A; V = 38838.4(8) A(3); Z = 6; D(calcd) = 1.125 g cm(-)(3); R1 = 3.37%. Crystal data for 2: cubic, Pn3n; T = 180 K; a = 33.8252(8) A; V = 38700.9(16) A(3); Z = 6; D(calcd) = 1.133 g cm(-)(3); R1 = 4.97%. Crystal data for 3: cubic, Pn3n; T = 180 K; a = 33.7061(6) A; V = 38293.5(12) A(3); Z = 6; D(calcd) = 1.157 g cm(-)(3); R1 = 5.13%. Crystal data for 4: cubic, Pn3n; T = 180 K; a = 33.5900(7) A; V = 37899.2(14) A(3); Z = 6; D(calcd) = 1.182 g cm(-)(3); R1 = 4.03%. Crystal data for 5: cubic, Pn3n; T = 180 K; a = 33.5054(8) A; V = 37613.6(16) A(3); Z = 6; D(calcd) = 1.202 g cm(-)(3); R1 = 4.86%. The core of the anionic cluster comprises two vertex-sharing square-pyramidal [Ln(5)(mu(4)-O)(mu(3)-OH)(4)](9+) units. The compounds were characterized by elemental analysis, IR, fast atom bombardment mass spectra, thermogravimetry, and differential scanning calorimetry. The thermal analysis indicated that the nonanuclear species were stable up to 150 degrees C. Luminescence spectra of 2 and magnetic properties of 1-5 were also studied.     

Mössbauer study of reduced rubredoxin as purified and in whole cells. Structural correlation analysis of spin Hamiltonian parameters

The [Fe(II)(Cys)(4)](2-) site of rubredoxin from Clostridium pasteurianum (Rd(red)) has been studied by M?ssbauer spectroscopy in both purified protein and whole cells of Escherichia coli overproducing it. Excellent fits were obtained to an S = 2 spin Hamiltonian for D = 5.7(3) cm(-1), E/D = 0.25(2), delta = 0.70(3) mm/s, DeltaE(Q) = -3.25(2) mm/s, eta = 0.75(5), A(x) = -20.1(7) MHz, A(y) = -11.3(2) MHz, and A(z) = -33.4(14) MHz. These parameters were analyzed with crystal-field theory for the (5)D manifold of iron(II), revealing a d(z(2)) orbital ground state that is admixed by approximately 0.21 d(x(2) - y(2)). The spin-Hamiltonian parameters are consistent within the (5)D theory, apart from the zero-field splitting parameter, D. This problem was solved by extending the crystal-field treatment with spin-orbit coupling to spin-triplet d-d excited states of the iron. Theoretical estimates are given for the spin-triplet (D(T)) and spin-quintet contributions (D(Q)) to D based on excitation energies derived from time-dependent density functional theory, TD-DFT. The computational results were interpreted in terms of crystal-field theory, yielding the Racah parameters B = 682 cm(-1) and C = 2583 cm(-1). The theoretical analysis gives the relative magnitudes D(Q):D(T):D(ss) = 51%: 42%:7% (D(ss) originates from spin-spin interaction). The DFT analysis corroborates the pivotal role of the torsion angles (omega(i)) of the C-S(i) bonds in shaping the electronic structure of the iron(II) site. Rd(red) in overexpressing whole cells accounts for 60% of the M?ssbauer absorption. The Rd(red) spectra from whole cells are virtually identical to those of the purified protein. By using the theoretical omega dependence of the spin Hamiltonian parameters, the torsions for Rd(red) in whole cells and purified protein samples are estimated to be the same within 2 degrees. These findings establish M?ssbauer spectroscopy as a structural tool for investigating iron sites in whole cells.     

The liquidlike ordering of lipid A-diphosphate colloidal crystals: the influence of Ca2+, Mg2+, Na+, and K+ on the ordering of colloidal suspensions of lipid A-diphosphate in aqueous solutions

A comprehensive study was performed on electrostatically stabilized aqueous dispersion of lipid A-diphosphate in the presence of bound Ca2+, Mg2+, K+, and Na+ ions at low ionic strength (0.10-10.0-mM NaCl, 25 degrees C) over a range of volume fraction of 1.0 x 10(-4)< or =phi< or =4.95 x 10(-4). These suspensions were characterized by light scattering (LS), quasielastic light scattering, small-angle x-ray scattering, transmission electron microscopy, scanning electron microscopy, conductivity measurements, and acid-base titrations. LS and electron microscopy yielded similar values for particle sizes, particle size distributions, and polydispersity. The measured static structure factor, S(Q), of lipid A-diphosphate was seen to be heavily dependent on the nature and concentration of the counterions, e.g., Ca2+ at 5.0 nM, Mg2+ at 15.0 microM, and K+ at 100.0 microM (25 degrees C). The magnitude and position of the S(Q) peaks depend not only on the divalent ion concentration (Ca2+ and Mg2+) but also on the order of addition of the counterions to the lipid A-diphosphate suspension in the presence of 0.1-microM NaCl. Significant changes in the rms radii of gyration (R2G) 1/2 of the lipid A-diphosphate particles were observed in the presence of Ca2+ (24.8+/-0.8 nm), Mg2+ (28.5+/-0.7 nm), and K+ (25.2+/-0.6 nm), whereas the Na+ salt (29.1+/-0.8 nm) has a value similar to the one found for the de-ionized lipid A-diphosphate suspensions (29.2+/-0.8 nm). Effective particle charges were determined by fits of the integral equation calculations of the polydisperse static structure factor, S(Q), to the light-scattering data and they were found to be in the range of Z*=700-750 for the lipid A-diphosphate salts under investigation. The light-scattering data indicated that only a small fraction of the ionizable surface sites (phosphate) of the lipid A-diphosphate was partly dissociated (approximately 30%). It was also discovered that a given amount of Ca2+ (1.0-5.0 nM) or K+ (100 microM) influenced the structure much more than Na+ (0.1-10.0-mM NaCl) or Mg2+ (50 microM). By comparing the heights and positions of the structure factor peaks S(Q) for lipid A-diphosphate-Na+ and lipid A-diphosphate-Ca2+, it was concluded that the structure factor does not depend simply on ionic strength but more importantly on the internal structural arrangements of the lipid A-diphosphate assembly in the presence of the bound cations. The liquidlike interactions revealed a considerable degree of ordering in solution accounting for the primary S(Q) peak and also the secondary minimum at large particle separation. The ordering of lipid A-diphosphate-Ca2+ colloidal crystals in suspension showed six to seven discrete diffraction peaks and revealed a face-centered-cubic (fcc) lattice type (a=56.3 nm) at a volume fraction of 3.2 x 10(-4)< or =phi< or =3.9 x 10(-4). The K+ salt also exhibited a fcc lattice (a=55.92 nm) at the same volume fractions, but reveals a different peak intensity distribution, as seen for the lipid A-diphosphate-Ca2+ salt. However, the Mg2+ and the Na+ salts of lipid A-diphosphate showed body-centered-cubic (bcc) lattices with a=45.50 nm and a=41.50 nm, respectively (3.2 x 10(-4)< or =phi< or =3.9 x 10(-4)), displaying the same intensity distribution with the exception of the (220) diffraction peaks, which differ in intensity for both salts of lipid A-diphosphate.     

New routes to polymetallic clusters: fluoride-based tri-, deca-, and hexaicosametallic MnIII clusters and their magnetic properties

The syntheses, structures and magnetic properties of three new MnIII clusters, [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1), [Mn(0O6(OH)2(bta)8(py)8F8] (2) and [NHEt3]2[Mn3O(bta)6F3] (3), are reported (bta=anion of benzotriazole), thereby demonstrating the utility of MnF3 as a new synthon in Mn cluster chemistry. The "melt" reaction (100 degrees C) between MnF(3) and benzotriazole (btaH, C6H5N3) under an inert atmosphere, followed by dissolution in MeOH produces the cluster [Mn26O17(OH)8(OMe)4F10(bta)22(MeOH)14(H2O)2] (1) after two weeks. Complex 1 crystallizes in the triclinic space group P1, and consists of a complicated array of metal tetrahedra linked by mu3-O2- ions, mu3- and mu2-OH- ions, mu2-MeO- ions and mu2-bta- ligands. The "simpler" reaction between MnF3 and btaH in boiling MeOH (50 degrees C) also produces complex 1. If this reaction is repeated in the presence of pyridine, the decametallic complex [Mn10O6(OH)2(bta)8(py)8F8] (2) is produced. Complex 2 crystallizes in the triclinic space group P1 and consists of a "supertetrahedral" [Mn(III)10] core bridged by six mu3-O2- ions, two mu3-OH- ions, four mu2-F- ions and eight mu2-bta- ions. The replacement of pyridine by triethylamine in the same reaction scheme produces the trimetallic species [NHEt3]2[Mn3O(bta)6F3] (3). Complex 3 crystallises in the monoclinic space group P2(1)/c and has a structure analogous to that of the basic metal carboxylates of general formula [M3O(RCO2)6L3]0/+, which consists of an oxo-centred metal triangle with mu2-bta- ligands bridging each edge of the triangle and the fluoride ions acting as the terminal ligands. DC magnetic susceptibility measurements in the 300-1.8 K and 0.1-7 T ranges were investigated for all three complexes. For each, the value of chi(M)T decreases with decreasing temperatures; this indicates the presence of dominant antiferromagnetic exchange interactions in 1-3. For complex 1, the low-temperature value of chi(M)T is 10 cm(3) K mol(-1) and fitting of the magnetisation data gives S=4, g=2.0 and D=-0.90 cm(-1). For complex 2, the value of chi(M)T falls to a value of approximately 5.0 cm(3) K mol(-1) at 1.8 K, which is consistent with a small spin ground state. For the triangular complex 3, the best fit to the experimental chi(M)T versus T data was obtained for the following parameters: Ja = -5.01 cm(-1), Jb = +9.16 cm(-1) and g=2.00, resulting in an S=2 spin ground state. DFT calculations on 3, however, suggest an S=1 or S=0 ground state with J(a)=-2.95 cm(-1) and J(b)=-2.12 cm(-1). AC susceptibility measurements performed on 1 in the 1.8-4.00 K range show the presence of out-of-phase AC susceptibility signals, but no peaks. Low-temperature single-crystal studies performed on 1 on an array of micro-SQUIDS show the time- and temperature-dependent hysteresis loops indicative of single-molecule magnetism behaviour.     

Systematic approach to the quantitative voltammetric analysis of the FeIII/FeII component of the [alpha2-Fe(OH2)P2W17O61]7-/8- reduction process in buffered and unbuffered aqueous media

The one-electron reduction of [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-) at a glassy carbon electrode was investigated using cyclic and rotating-disk-electrode voltammetry in buffered and unbuffered aqueous solutions over the pH range 3.45-7.50 with an ionic strength of approximately 0.6 M maintained. The behavior is well-described by a square-scheme mechanism P + e(-) <--> Q (E(1)(0/) = -0.275 V, k(1)(0/) = 0.008 cm s(-1), and alpha(1) = 1/2), PH(+) + e(-) <--> QH(+) (E(2)(0/) = -0.036 V, k(2)(0/) = 0.014 cm s(-1), and alpha(2) = 1/2), PH(+) <--> P + H(+) (K(P) = 3.02 x 10(-6) M), and QH(+) <--> Q + H(+) (K(Q) = 2.35 x 10(-10) M), where P, Q, PH(+), and QH(+) correspond to [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-), [alpha(2)-Fe(II)(OH)P(2)W(17)O(61)](9-), [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), and [alpha(2)-Fe(II)(OH(2))P(2)W(17)O(61)](8-), respectively; E(1)(0)' and E(2)(0)' are the formal potentials, k(1)(0)' and k(2)(0)' are the formal (standard) rate constants, and K(P) and K(Q) are the acid dissociation constants for the relevant reactions. The analysis for the buffered media is based on the approach of Laviron who demonstrated that a square scheme with fully reversible protonations, reversible or quasi reversible electron transfers with the assumption that alpha(1) = alpha(2), can be well-described by the behavior of a simple redox couple, ox + e(-) <--> red, whose formal potential, E(app)(0)', and standard rate constant, k(app)(0)', are straightforwardly derived functions of pH, as are the values of E(1)(0)', k(1)(0)', E(2)(0)', k(2)(0)', and K(P) (only three of the four thermodynamic parameters in a square scheme can be specified). It was assumed that alpha(app) = 1/2, and the simulation program DigiSim was used to determine the values of E(app)(0)' and k(app)(0)', which are required to describe the cyclic voltammograms obtained in buffered media in the pH range from 3.45 to 7.52 (buffer-related reactions which effect general acid-base catalysis are included in the simulations). DigiSim simulations of cyclic voltammograms obtained in unbuffered media yielded the values of E(1)(0)' and k(1)(0)'; K(Q) was then directly computed from thermodynamic constraints. These simulations included additional reactions between the redox species and H(2)O. The value of the diffusion coefficient of the [alpha(2)-Fe(III)(OH(2))P(2)W(17)O(61)](7-), 2.92 x 10(-6) cm(2) s(-1), was determined using DigiSim simulations of voltammograms at a rotating disk electrode in buffered and unbuffered media at pH 3.45. The diffusion coefficients of all redox species were assumed to be identical. When the pH is greater than 6, instability of P (i.e., [alpha(2)-Fe(III)(OH)P(2)W(17)O(61)](8-)) led to the loss of the reactant and precluded lengthy experimentation.     

Electron self-exchange in the solid-state: cocrystals of hydroquinone and bipyridyl triazole.

Solid-state voltammetry, spectroscopy, and microscopy studies have been used to probe the proton and electron conductivity within a self-assembled cocrystal, HQBpt. This crystallographically defined material contains 3,5-bis(pyridin-2-yl)-1,2,4-triazole, HBpt, dimers that are pi-stacked and hydrogen bonded to 1,4-hydroquinone, H(2)Q, in a herringbone arrangement. When deposited onto platinum microelectrodes, the cocrystal exhibits a well-defined voltammetric response corresponding to oxidation of H(2)Q to the quinone, Q, across a wide range of voltammetric time scales, electrolyte compositions, and pH values. Scanning electron microscopy reveals that redox cycling in aqueous perchlorate solutions in which the pH is systematically varied from 1 to 7 triggers electrocrystallization and the extensive formation of rodlike crystals. Fast scan rate voltammetry reveals that the homogeneous charge transport diffusion coefficient, D(app), is independent of the perchlorate concentration for 0.1 < [ClO(4)(-)] < 1.0 M (pH 6.6) at 3.14 +/- 0.11 x 10(-)(9) cm(2) s(-)(1). Moreover, D(app) is independent of the perchloric acid concentration for concentrations greater than approximately 2.0 M, maintaining a value of 4.81 +/- 0.07 x 10(-)(8) cm(2) s(-)(1). The observation that D(app) is independent of the supporting electrolyte suggests that the rate-determining step for homogeneous charge transport is not the availability of charge-compensating counterions or protons, but the dynamics of electron self-exchange between H(2)Q and Q. We have used the Dahms-Ruff formalism to determine electron self-exchange rate constants which are 2.84 +/- 0.22 x 10(9) and 9.69 +/- 0.73 x 10(10) M(-)(1) s(-)(1) for pH values greater than approximately 2.0 and less than -0.3, respectively. Significantly, these values are more than 2 orders of magnitude larger that those found for benzoquinone self-exchange reactions in aqueous solution. These results indicate that hydrogen bonds play an important role in supporting rapid electron transfer. The increase in D(app) between pH 1.0 and -0.3 is associated with protonation of the HBpt moieties, which triggers a reversible change in the material's structure.     

Photohydroxylation of 1,4-benzoquinone in aqueous solution revisited

In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH(2) which are formed with a quantum yield of Phi=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (lambda(max)=282 and approximately 410 nm) which is the precursor of these products increases nonlinearly (k=(2-->3.8) x 10(6) s(-1)) with increasing Q concentration ((0.2-->10) mM). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q(2)* (K=5500+/-1000 M(-1)). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)(3), Q(2)* decays by electron transfer and water addition yielding benzosemiquinone (.)QH and (.)OH adduct radicals (.)QOH. The latter enolizes to the 2-hydroxy-1,4-semiquinone radical (.)Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of (.)QH. On the post-millisecond time scale, that is, when (.)QH has decayed, Ph(OH)(3) is oxidized by Q yielding HOQ and QH(2) as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. This shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a non-radical one at low Q concentration. In agreement with this, the yield of Ph(OH)(3) is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H(2)PO(4) (-) giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that (.)OH is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the (.)OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q(2)* oxidizes DMPO (k approximately 1 x 10(8) M(-1) s(-1)) to its radical cation which subsequently reacts with water. Q*/Q(2)* react with alcohols by H abstraction (rates in units of M(-1) s(-1)): methanol (4.2 x 10(7)), ethanol (6.7 x 10(7)), 2-propanol (13 x 10(7)) and tertiary butyl alcohol ( approximately 0.2 x 10(7)). DMSO (2.7 x 10(9)) and O(2) ( approximately 2 x 10(9)) act as physical quenchers.     

Reflected shock tube studies of high-temperature rate constants for OH + C2H2 and OH + C2H4

The reflected shock tube technique with multi-pass absorption spectrometric detection of OH-radicals at 308 nm (corresponding to a total path length of approximately 4.9 m) has been used to study the reactions, OH + C(2)H(2)--> products (1) and OH + C(2)H(4)--> C(2)H(3) + H(2)O (2). The present optical configuration gives a S/N ratio of approximately 1 at approximately 0.5-1.0 x 10(12) radicals cm(-3). Hence, kinetics experiments could be performed at [OH](0) = approximately 4-20 ppm thereby minimizing secondary reactions. OH was produced rapidly from the dissociations of either CH(3)OH or NH(2)OH (hydroxylamine). A mechanism was then used to obtain profile fits that agreed with the experiment to within <+/-5%. The derived Arrhenius expressions, in units of cm(3) molecule(-1) s(-1) are: k(1) = (1.03 +/- 0.24) x 10(-10) exp(-7212 +/- 417 K/T) for 1509-2362 K and k(2) = (10.2 +/- 5.8) x 10(-10) exp(-7411 +/- 871 K/T) for 1463-1931 K. The present study is the first ever direct measurement for reaction (1) at temperatures >1275 K while the present results extend the temperature range for (2) by approximately 700 K. These values are compared with earlier determinations and with recent theoretical calculations. The calculations agree with the present data for both reactions to within +/-10% over the entire T-range.     

Slowdown of H/D exchange reaction rate and water dynamics in ionic liquids: deactivation of solitary water solvated by small anions in 1-butyl-3-methyl-imidazolium chloride

The H/D exchange reaction and the rotational dynamics of heavy water (D2O) are studied at 50 degrees C in the ionic liquid, 1-butyl-3-methylimidazolium chloride ([bmim][Cl]), in the [D2O] range of 3-55 M. The initial H/D exchange rates are observed as 1.0 x 10(-7), 4.5 x 10(-6), 1.0 x 10(-5), 4.1 x 10(-5), 1.1 x 10(-4), and 3.7 x 10(-4) s(-1), respectively, at [D2O] of 2.8, 7.1, 8.1, 11, 15, and 25 M. The rate is very slow and less than 10(-5) s(-1) at [D2O] below approximately 7 M. It steeply increases to the order of 10(-4)s(-1) for 7 M < [D2O] < 10 M, and linearly increases with [D2O] in the more water-rich region. The intercept of the linear region at [D2O] = approximately 9 M is interpreted by considering that each chloride anion deactivates 1.6 equiv water molecules due to the strong solvation. Correspondingly, the rotational correlation time of D2O at [D2O] < 7 M is 1 order of magnitude larger than that in water-rich conditions.     

Mechanisms of electron-ion recombination of N2H+/N2D+ and HCO+/DCO+ ions: temperature dependence and isotopic effect

The temperature dependencies of the rate coefficients, alpha(e), for electron-ion dissociative recombination (DR) of N2H+/N2D+ and HCO+/DCO+ ions with electrons have been measured over the range 100-500 K. Also, optical emissions have been detected at approximately 100 K from the N2(B3(pi)g) electronically excited products of N2H+/N2D+ recombination. The measurements were carried out using the classic FALP technique combined with an optical monochromator. For N2H+, there was no variation of alpha(e) with temperature above 200 K, with an average value of alpha(e)(N2H+) = 2.8 x 10(-7) cm3 s(-1). The temperature variation for T approximately 100-300 K observed for alpha(e)(HCO+) is similar to that of N2H+ ions for T approximately 300-500 K. The smaller rate coefficient measured for DCO+ and N2D+ ions shows the influence of an isotope effect. The substantial enhancement of the vibrational level, upsilon' = 6, from the N2B state for N2H+ recombination over N2D+ recombination is consistent with previous result at 300 K and implies the influence of a tunneling mechanism of DR.     

Pulsed Laval nozzle study of the kinetics of OH with unsaturated hydrocarbons at very low temperatures

The kinetics of reactions of the OH radical with ethene, ethyne (acetylene), propyne (methyl acetylene) and t-butyl-hydroperoxide were studied at temperatures of 69 and 86 K using laser flash-photolysis combined with laser-induced fluorescence spectroscopy. A new pulsed Laval nozzle apparatus is used to provide the low-temperature thermalised environment at a single density of approximately 4x10(16) molecule cm(-3) in N2. The density and temperature within the flow are determined using measurements of impact pressure and rotational populations from laser-induced fluorescence spectroscopy of NO and OH. For ethene, rate coefficients were determined to be k2=(3.22+/-0.46)x10(-11) and (2.12+/-0.12)x10(-11) cm3 molecule(-1) s(-1) at T=69 and 86 K, respectively, in good agreement with a master-equation calculation utilising an ab initio surface recently calculated for this reaction by Cleary et al. (P. A. Cleary, M. T. Baeza Romero, M. A. Blitz, D. E. Heard, M. J. Pilling, P. W. Seakins and L. Wang, Phys. Chem. Chem. Phys., 2006, 8, 5633-5642) For ethyne, no previous data exist below 210 K and a single measurement at 69 K was only able to provide an approximate upper limit for the rate coefficient of k3<1x10(-12) cm3 molecule(-1) s(-1), consistent with the presence of a small activation barrier of approximately 5 kJ mol(-1) between the reagents and the OH-C2H2 adduct. For propyne, there are no previous measurements below 253 K, and rate coefficients of k4=(5.08+/-0.65), (5.02+/-1.11) and (3.11+/-0.09)x10(-12) cm3 molecule(-1) s(-1) were obtained at T=69, 86 and 299 K, indicating a much weaker temperature dependence than for ethene. The rate coefficient k1=(7.8+/-2.5)x10(-11) cm3 molecule(-1) s(-1) was obtained for the reaction of OH with t-butyl-hydroperoxide at T=86 K. Studies of the reaction of OH with benzene and toluene yielded complex kinetic profiles of OH which did not allow the extraction of rate coefficients. Uncertainties are quoted at the 95% confidence limit and include systematic errors.     

Ordering of lipid A-monophosphate clusters in aqueous solutions

In this investigation, a study of the self-assembly of electrostatically stabilized aqueous dispersions of nanometric lipid A-monophosphate clusters from Escherichia coli was carried out in three different volume-fraction regimes. The experimental techniques used in the investigation were osmotic pressure, static and quasielastic light scattering, scanning electron microscopy and transmission electron microscopy, and small-angle x-ray scattering. Experiments were carried out at low ionic strength (I=0.1-5.0 mM NaCl) at 25 degrees C. At volume fractions between 1.5x10(-4)相似文献   

Mn(II) staircase structures stitched by water clusters to a 3D metal-organic open framework: X-ray structural and magnetic studies

4-Hydroxypyridine-2,6-dicarboxylic acid (chelidamic acid, cdaH2) reacts with Mn(OAc)2 x 4H2O to form a 1D staircase structure with dimeric Mn(II) units connected by water clusters to form a 3D framework, {[Mn2(cda)2 x 4H2O] x 4H2O}n, 1, in aqueous pyridine at room temperature. The compound crystallizes in the triclinic space group P1 with a = 9.495(3), b =10.733(5), c = 11.065(4) A, alpha = 87.42(5), beta = 74.14(5), gamma = 80.07(2) degrees, U = 1068.5(9) A3, Z = 2, rho(calcd) = 1.915 g cm(-3), T = 100 K, mu = 1.28 mm(-1), R1 = 0.0453 (I > 2sigma(I)), wR2 = 0.1046, GOOF = 1.282. Upon removal of the water molecules by heating, the 3D structure breaks down. Thermogravimetric analysis, infrared, X-ray powder diffraction studies, and X-ray crystallography were performed to characterize this compound. Since the coordination polymer has diaqua-bridged Mn(II) centers, it was subjected to variable-temperature magnetic studies.     

Characterization of SU-8 for electrokinetic microfluidic applications

The characterization of SU-8 microchannels for electrokinetic microfluidic applications is reported. The electroosmotic (EO) mobility in SU-8 microchannels was determined with respect to pH and ionic strength by the current monitoring method. Extensive electroosmotic flow (EOF), equal to that for glass microchannels, was observed at pH > or =4. The highest EO mobility was detected at pH > or =7 and was of the order of 5.8 x 10(-4) cm(2) V(-1) s(-1) in 10 mM phosphate buffer. At pH < or =3 the electroosmotic flow was shown to reverse towards the anode and to reach a magnitude of 1.8 x 10(-4) cm(2) V(-1) s(-1) in 10 mM phosphate buffer (pH 2). Also the zeta-potential on the SU-8 surface was determined, employing lithographically defined SU-8 microparticles for which a similar pH dependence was observed. SU-8 microchannels were shown to perform repeateably from day to day and no aging effects were observed in long-term use.     

A highly sensitive method for time-resolved detection of O(1D) applied to precise determination of absolute O(1D) reaction rate constants and O(3P) yields

We demonstrate detection, in the gas-phase, of O(1D2) at concentrations down to 10(7) cm(-3) and develop this new method for time-resolved kinetic studies allowing both the total removal rate of O(1D2), of up to 1.5 x 10(6) s(-1), and the fraction quenched to O(3P(J)) by species X, k(q)/k(X), to be determined precisely from a single time profile: at 295 K we find, k(O(1D2) + N2O) = (1.43 +/- 0.08) x 10(-10) cm3 s(-1) with k(q)/k(N2O) = 0.056 +/- 0.009; k(O(1D2) + C2H2) = (3.1 +/- 0.2) x 10(-10) cm3 s(-1) with k(q)/k(C2H2) = 0.020 +/- 0.010; k(q)/k(H2O) < 0.003 for O(1D2) + H2O.     

Heptanuclear and decanuclear manganese complexes with the anion of 2-hydroxymethylpyridine

The synthesis and magnetic properties are reported of two new clusters [Mn(10)O(4)(OH)(2)(O(2)CMe)(8)(hmp)(8)](ClO(4))(4) (1) and [Mn(7)(OH)(3)(hmp)(9)Cl(3)](Cl)(ClO(4)) (2). Complex 1 was prepared by treatment of [Mn(3)O(O(2)CMe)(6)(py)(3)](ClO(4)) with 2-(hydroxymethyl)pyridine (hmpH) in CH(2)Cl(2), whereas 2 was obtained from the reaction of MnCl(2).4H(2)O, hmpH, and NBu(n)(4)MnO(4) in MeCN followed by recrystallization in the presence of NBu(n)(4)ClO(4). Complex 1.2py.10CH(2)Cl(2).2H(2)O crystallizes in the triclinic space group P1. The cation consists of 10 Mn(III) ions, 8 mu(3)-O(2)(-) ions, 2 mu(3)-OH(-) ions, 8 bridging acetates, and 8 bridging and chelating hmp(-) ligands. The hmp(-) ligands bridge through their O atoms in two ways: two with mu(3)-O atoms and six with mu(2)-O atoms. Complex 2.3CH(2)Cl(2).H(2)O crystallizes in the triclinic space group P1. The cation consists of four Mn(II) and three Mn(III) ions, arranged as a Mn(6) hexagon of alternating Mn(II) and Mn(III) ions surrounding a central Mn(II) ion. The remaining ligation is by three mu(3)-OH(-) ions, three terminal chloride ions, and nine bridging and chelating hmp(-) ligands. Six hmp(-) ligands contain mu(2)-O atoms and three contain mu(3)-O atoms. The Cl(-) anion is hydrogen-bonded to the three mu(3)-OH(-) ions. Variable-temperature direct current (dc) magnetic susceptibility data were collected for complex 1 in the 5.00-300 K range in a 5 kG applied field. The chi(M)T value gradually decreases from 17.87 cm(3) mol(-1) K at 300 K to 1.14 cm(3) mol(-1) K at 5.00 K, indicating an S = 0 ground state. The ground-state spin of complex 2 was established by magnetization measurements in the 0.5-3.0 T and 1.80-4.00 K ranges. Fitting of the data by matrix diagonalization, incorporating only axial anisotropy (DS(z)(2)), gave equally good fits with S = 10, g = 2.13, D = -0.14 cm(-1) and S = 11, g = 1.94, D = -0.11 cm(-1). Magnetization versus dc field scans down to 0.04 K reveal no hysteresis attributable to single-molecule magnetism behavior, only weak intermolecular interactions.