SEARCH · Search PubMed
Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.
Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.
This paper deals with the electrochemical production of hydrogen by depolarizing the oxygen evolution reaction using human feces and urine, which contains 30-40% bacteria and yeast. The electroactivity of graphite, tungsten carbide, perovskite and RuO2-coated Ebonex (Ti4O7) as anode materials are compared. The scale-up of the process in a laboratory-scale three-dimensional packed bed cell is discussed.
This study evaluated the results of polishing composite resin by using the following finishing and polishing techniques: Arkansas stone burs; eight-blade tungsten-carbide burs; diamond burs; aluminum-oxide disks; no polishing; and 12- and 30-blade tungsten-carbide burs. The results were compared to ascertain which technique delivers the smoothest surface. The study was done with 120 class V restorations on extracted human teeth distributed in two groups (I and II) of 60 restorations each. Group I teeth samples were filled with microfilled composite resin and Group II with a hybrid composite resin. Readings were made with a profilometer to evaluate the final polishing. The conclusions from the results of the study were that microfilled composite resins provided a better finish when treated with aluminum-oxide disks. These results were statistically significant (p < 0.001). When hybrid composites were used the control group recorded the lowest values.
This study evaluated the effects of four polishing methods on the occlusal surfaces of direct Class II composite restorations under clinical conditions. Forty premolars and 40 molars were treated with direct Class II restorations using the hybrid composite Herculite XRV (Kerr). After placement of the restorations, all of which were on occlusal surfaces, they were finished with a sequence of 30 microm diamonds and tungsten carbide instruments. Twenty restorations each, consisting of 10 premolars and 10 molars, were polished with one of the four following methods: (1) Diafix-oral (Mueller-Dental), (2) MPS gel (Premier), (3) P 403-W (Dentsply) and (4) Occlubrush (KerrHawe). Selection of the polishing methods followed a randomized protocol. Replicas of the restored teeth were fabricated and the occlusal surfaces were evaluated quantitatively for roughness with the help of profilometry. Qualitative assessment of the surfaces by SEM was done with respect to roundness of contours and surface roughness. The results were analyzed statistically by two-way ANOVA, chi-squared test for crosstables and Kruskal-Wallis test. Analysis of the quantitative data showed that there was no significant effect of the polishing methods on occlusal surface roughness (p>0.05). Localization of the restoration in premolars or molars had no effect on surface roughness (p>0.05). With respect to occlusal relief, SEM examination revealed that the use of the Occlubrush resulted in significantly more edged contours compared to the other polishing methods (p=0.008). Qualitative roughness evaluation showed that there were no significant differences among the four polishing methods (p>0.05).
Recent protein crystallographic results on tungsten enzymes and primary sequence relationships between certain molybdenum and tungsten enzymes provoke interest in the generalized bis(dithiolene) complexes [WIV(QR)(S2C2R'2)2]1- and [WVIO(QR)(S2C2R'2)2]1- (Q = O, S, Se) as minimal representations of enzyme sites. The existence and stability of W(IV) complexes have been explored by synthesis. Reaction of [W(CO)2(S2C2Me2)2] (1) with PhO- results in complete CO substitution to give [W(OPh)(S2C2Me2)2]1- (2). Reaction of 1 with PhQ- affords the monocarbonyls [W(CO)(QPh)(S2C2Me2)2]1- (Q = S (3), Se (5)). The use of sterically demanding 2,4,6-Pri3C6H2Q- also yields monocarbonyls, [W(CO)(QC6H2-2,4,6-Pri3)(S2C2Me2)2]1- (Q = S (4), Se (6)). The X-ray structures of square pyramidal 2 and trigonal prismatic 3-6 (with unidentate ligands cis) are described. The tendency to substitute one or both carbonyl ligands in 1 in the formation of [MIV(QAr)(S2C2Me2)2]1- and [MIV(CO)(QAr)(SeC2Me2)2]1- with M = Mo and W is related to the M-Q bond length and ligand steric demands. The results demonstrate a stronger binding of CO by W(IV) than Mo(IV), a behavior previously demonstrated by thermodynamic and kinetic features of zerovalent carbonyl complexes. Complexes 3-6 can be reversibly reduced to W(III) at approximately -1.5 V versus SCE. On the basis of the potential for 2(-2.07 V), monocarbonyl ligation stabilizes W(III) by approximately 500 mV. This work is part of a parallel investigation of the chemistry of bis(dithiolene)-molybdenum (Lim, B. S.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 2000, 39, 263) and -tungsten complexes related to enzyme active sites.
The cellular localization of the polyoxometalates, K12H2[P2W12O48].24H20 (JM 1591), K10[P2W18-Zn4(H2O)2O68].20H2O (JM 1596), and [Me3NH]8[Si2W18Nb6O77] (JM 2820) were examined in cultured J774 cells by inhibition of cellular uptake of acetylated low-density lipoprotein (LDL) and by electron microscopy. All three polyoxometalates inhibited the cellular uptake of acetylated LDL, suggesting that the polyoxometalates block the association of acetylated LDL with cellular scavenger receptors. Fluorescence microscopy showed increased numbers of vacuoles in the presence of polyoxometalates, suggesting their uptake by cells. Using scanning electron microscopy (SEM), no significant cell surface morphological differences were observed between treated and non-treated J774 cells, suggesting that the compounds are not toxic to J774 cells up to a concentration of 200 micrograms/ml. Transmission electron microscopy (TEM) revealed large amounts of high electron dense granules were observed in the ramifying system of tubular cavities and vacuoles. TEM-energy dispersive spectroscopy (EDS) X-ray microanalysis was unable to differentiate the dense particles, most likely because the amount of tungsten in the cells was below the limit of detection. X-ray microanalysis conducted using the SEM-wavelength dispersive spectroscopy (WDS) detected tungsten, averaging 0.45 +/- 0.16% (mean +/- S.D.), in the J774 cells treated with JM 2820, suggesting that this polyoxometalate was taken up by the macrophages or was bound to their surface. Polyoxometalates interact at the cell surface and appear to be taken up by J774 macrophages. The cellular localization of polyoxometalates may be associated with anti-HIV activity.
Reactions of bis(thio)-W(VI) complexes, LWS2X (L = hydrotris (3,5-dimethylpyrazol-1-yl)borate, X = monoanion), with alkynes produce dithiolene complexes, LWX(dithiolene). The synthesis and characterization of orange LW(OPh){S2C2(CO2Me)2} (1) and burgundyred LW(SePh) {S2C2(Ph)(2-quinoxalinyl)} (2) and the X-ray crystal structure of 1.0.5CH2Cl2, are described in detail. Crystals of 1.0.5CH2Cl2 are orthorhombic, space group Pbcn, with a = 29.826(6), b = 13.291(4), c = 16.078(4) A, V = 6373(5) A3, and Z = 8. The six-coordinate, distorted-octahedral complex features a tridentate L ligand, a monodentate phenoxide ligand, and a bidentate dithiolene ligand. The short W-S bonds (2.267(4) and 2.279(4) A) and the parameters associated with the phenoxide ligand (W-O = 1.850(8) A, W-O-C = 146(1) degree), point to a considerable degree of W-ligand multiple bonding in the [W(OPh)(dithiolene)]+ unit. For 2, W-Se and average W-S distances of 2.49(2) A and 2.30(2) A, respectively, have been obtained from EXAFS studies. The formation of yellow 3,3'-dithiobis[2-(phenyl)thieno[2,3-b]quinoxaline] (3) upon oxidation of 2 supports the likely generation of urothione upon oxidative degradation of molybdopterin-containing tungsten enzymes from hyperthermophilic organisms.
1. In vivo 59Fe absorption from intrinsically labelled Fe-containing fractions of liver and blood were measured in rats by intragastric dosing. All rats were fed on a low-Fe diet for 3 d before dosing in order to standardize the Fe status of the intestinal mucosal cells. 2. An increase in digestion time from 2 to 12 h increased 59Fe absorption (P less than 0.01) from all fractions except ferritin. 3. Fe-deficient rats when compared with essentially Fe-replete rats showed decreased gastric retention for all fractions, but increased 59Fe absorption over 2 h only from ferritin. Ferritin showed several unusual absorption characteristics. 4. Dietary tungsten supplementation of Fe-deficient rats reduced the ferroxidase activity of intestinal mucosal xanthine oxidase. In addition, gastric retention and 59Fe absorption (P less than 0.05) from all fractions were increased.
Two complexes that contain the racemic or enantiomerically pure (S) form of the 3,3'-di-tert-butyl-5,5',6,6'-tetramethyl-1,1'-biphenyl-2,2'-diolate (Biphen(2-)) ligand, W(NAr)(CHCMe(2)Ph)(Biphen) (2a) and W(NAr')(CHCMe(2)Ph)(Biphen) (2b) (Ar = 2,6-i-Pr(2)C(6)H(3); Ar' = 2,6-Me(2)C(6)H(3)), were prepared and shown to be viable catalysts for several representative ring-closing reactions to give products in good yields in most cases and high % ee in asymmetric reactions. Exploration of the reaction between 2a and a stoichiometric amount of one desymmetrization substrate allowed two intermediate tungstacyclobutane complexes to be observed, in addition to the final and quite stable tungstacyclobutane complex formed in a reaction between the ring-closed product and a tungsten methylene complex. Reactions involving (13)C labeled ethylene allowed for the observation of an unsubstituted tungstacyclobutane complex, an ethylene complex, an unsubstituted tungstacyclopentane complex, and a heterochiral dimeric form of a methylene complex. The tungstacyclopentane complex was found to catalyze the dimerization of ethylene to 1-butene slowly.
The amperometric detection of thiols following their separation by reversed-phase chromatography and reaction into a post-column mercury carrier stream is shown to be a sensitive method when using a tungsten wire sensor as the working electrode in a three-electrode flow cell. Five organic thiols (cysteine, homocysteine, reduced glutathione, D,L-penicillamine and 2-mercaptopropionic acid) and thiourea could be separated within approx. 18 min. The analytical performance is comparable, and stability superior, to chemically modified electrodes previously reported.
The multicomponent heteropoly compounds [Me4N]4 + x[SiVxMoyW12 - x - yO40].n H2O with Keggin structure have been synthesized and characterized by FTIR. The vibrational frequencies of containing-oxygen bonds (Si-Oa,M=Od, M-Ob-M and M-Oc-M) in these heteropoly compounds have been studied. The vibrational frequencies nu as(M=Od) and nu as(M-Ob-M) are demonstrated to explain why the acidity of the heteropoly compounds becomes weaker and the oxidative ability becomes stronger when W atoms are substituted by V atoms. The rule of the change is similar with [PVxMoyW12 - x - yO40](3 + x)-. In addition, the acidity of P series is stronger than the acidity of the corresponding Si series.
Endo- and exo-cycloisomerizations of 4-pentyn-1-ol have been studied computationally with density functional theory, in conjunction with double-zeta and triple-zeta basis sets, both in the absence and in the presence of tungsten carbonyl catalyst. In the absence of the catalyst, both endo- and exo-cycloisomerizations have been calculated to have a very high activation barrier of approximately 50-55 kcal/mol and cannot take place. With tungsten pentacarbonyl catalyst, endo-cycloisomerization becomes a complex multiple-step reaction and proceeds with a rate-determining barrier of 26 kcal/mol at the C(alpha) --> C(beta) hydride migration step to form a vinylidene intermediate. The primary role of the tungsten catalyst is to stabilize the vinylidene intermediate, thus lowering the rate-determining barrier. The second important role of the tungsten catalyst in endo-cycloisomerization is to assist the OH hydride migration to C(alpha) by making it a multistep process with small activation barriers. The exo-cycloisomerization with the catalyst still has a high rate-determining barrier of 47 kcal/mol. These findings clearly explain the experimentally observed endo-selectivity in the cycloisomerization of 4-pentyn-1-ol derivatives and support the experimentally proposed mechanism.
The reaction between trisdiolatotungsten(VI) complex [W(eg)(3)] (1) (eg = 1,2-ethanediolato dianion) and phenolic ligand precursor methylamino-N,N-bis(2-methylene-4,6-dimethylphenol) (H(2)L(Me)) or methylamino-N,N-bis(2-methylene-4-methyl-6-tert-butylphenol) (H(2)L(tBu)) affords monomeric oxotungsten complex [WO(eg)(L(Me))] (2) or [WO(eg)(L(tBu))] (3), respectively. These complexes react further with chlorinating reagents, which leads to the displacement of ethanediolato ligands from the complex units and formation of cis and trans isomers of the corresponding dichloro complexes [WOCl(2)(L(Me))] (4) and [WOCl(2)(L(tBu))] (5), respectively. Identical dichloro complexes were also prepared by the reaction between the above-mentioned phenolic ligand precursors and [WOCl(4)]. Molecular structures of 3, cis-4, trans-4, and cis-5 were verified by X-ray crystallography. Complexes 2-5 can be activated by Et(2)AlCl to catalyze ring-opening metathesis polymerization of norbornene.
Explore the source record for details and available documents.
Explore the source record for details and available documents.
A series of compounds of the form [M(2)L(4)] and [[((t)()BuCO(2))(3)M(2)](2)(mu-L')] have been made where M = Mo or W, L = a thienyl, bithienyl, or terthienyl carboxylate, and L' = a corresponding thienyl dicarboxylate. The electronic absorption spectra are reported and the electronic structures discussed. Intense metal-to-ligand charge transfer bands traverse the visible and near-IR regions of the electronic absorption spectrum. The compounds show reversible metal-based oxidations and quasireversible ligand-based reductions. The molecular structure of Mo(2)(O(2)C-2-Th)(4).2THF is reported, on the basis of a single crystal X-ray diffraction study. These compounds provide insight into the expected properties of related dimetalated polythiophenes incorporating MM quadruple bonds.
A far superior synthesis is reported for W(2)(hpp)(4)Cl(2), a key intermediate in the synthesis of the most easily ionized closed-shell molecule W(2)(hpp)(4) (hpp = the anion of the bicyclic guanidine compound 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine). At 200 degrees C, the one-pot reaction of the air-stable and commercially available compounds W(CO)(6) and Hhpp in o-dichlorobenzene produces W(2)(hpp)(4)Cl(2) in multigram quantities with isolated yields of over 90%. At lower temperatures, the reaction can lead to other compounds such as W(Hhpp)(2)(CO)(4) or W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2), which are isolable in good purity depending upon the specific conditions employed. These compounds provide insight into the reaction pathway to W(2)(hpp)(4)Cl(2) and W(2)(hpp)(4). Two additional derivatives, W(2)(hpp)(4)X(2) where X is PF(6)(-) or the anion tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (TFPB), have also been synthesized and structurally characterized. A comparison of the electrode potentials of W(2)(mu-CO)(2)(mu-hpp)(2)(eta(2)-hpp)(2) and the di-p-anisylformamidinate analogue shows that oxidation of the hpp compound is significantly displaced (1.12 V) and shows that the bicyclic guanidinate ligand is considerably better than the formamidinate anion at stabilizing high oxidation states. A differential pulse voltammogram of W(2)(hpp)(4)(TFPB)(2) in THF shows two reduction processes with an E(1/2) of -0.97 V for the first and -1.81 V (vs Ag/AgCl) for the second. DFT calculations on the W(2)(hpp)(4)(2+) units in W(2)(hpp)(4)X(2) compounds show that the metal-metal bonding orbitals are destabilized by the axial ligands, which accounts for significant variations in the W-W distances. The low-energy gas-phase ionizations of W(2)(hpp)(4) are also reported and discussed.
The thermodynamic hydride donor abilities of [HW(CO)(5)](-) (40 kcal/mol), [HW(CO)(4)P(OMe(3))](-) (37 kcal/mol), and [HW(CO)(4)(PPh(3))](-) (36 kcal/mol) have been measured in acetonitrile by either equilibrium or calorimetric methods. The hydride donor abilities of these complexes are compared with other complexes for which similar thermodynamic measurements have been made. [HW(CO)(5)](-), [HW(CO)(4)P(OMe(3))](-), and [HW(CO)(4)(PPh(3))](-) all react rapidly with [CpRe(PMe(3))(NO)(CO)](+) to form dinuclear intermediates with bridging formyl ligands. These intermediates slowly form [CpRe(PMe(3))(NO)(CHO)] and [W(CO)(4)(L)(CH(3)CN)]. The structure of cis-[HW(CO)(4)(PPh(3))](-) has been determined and has the expected octahedral structure. The hydride ligand bends away from the CO ligand trans to PPh(3) and toward PPh(3).
Explore the source record for details and available documents.