An evaluation of least-squares fitting methods in XAFS spectroscopy: iron-based SBA-15 catalyst formulations.

A detailed comparison has been made of determinations by (57)Fe Mössbauer spectroscopy and four different XAFS spectroscopic methods of %Fe as hematite and ferrihydrite in 11 iron-based SBA-15 catalyst formulations. The four XAFS methods consisted of least-squares fitting of iron XANES, d(XANES)/dE, and EXAFS (k(3)chi and k(2)chi) spectra to the corresponding standard spectra of hematite and ferrihydrite. The comparison showed that, for this particular application, the EXAFS methods were superior to the XANES methods in reproducing the results of the benchmark Mössbauer method in large part because the EXAFS spectra of the two iron-oxide standards were much less correlated than the corresponding XANES spectra. Furthermore, the EXAFS and Mössbauer results could be made completely consistent by inclusion of a factor of 1.3+/-0.05 for the ratio of the Mössbauer recoilless fraction of hematite relative to that of ferrihydrite at room temperature (293K). This difference in recoilless fraction is attributed to the nanoparticle nature of the ferrihydrite compared to the bulk nature of the hematite. Also discussed are possible alternative non-least-squares XAFS methods for determining the iron speciation in this application as well as criteria for deciding whether or not least-squares XANES methods should be applied for the determination of element speciation in unknown materials.

Solid-state 13C NMR and quantum chemical investigation of metal diene complexes.

This paper presents novel measurements and calculations of the olefinic (13)C chemical shift tensor principal values in several metal diene complexes. The experimental values and the calculations show shifts as large as 70 ppm with respect to the values in the parent olefinic compounds. These shifts are highly anisotropic, with the largest ones observed in the less shielded principal components and the smallest ones in the most shielded principal components of the tensor. The orientations of the principal components of the tensors remain, within 10 degrees , at their directions in ethylene and other olefinic compounds. The calculations, performed using the GIAO method and the LanDZ pseudopotential basis set, show good agreement with the experiments, and were used to establish definite evidence for the existence of a Cl-bridge structure in the bicyclo[2.2.1]hepta-2,5-diene (BCHD)dichlororuthenium(II) polymer.

Enhancement in the reducibility of cobalt oxides on a mesoporous silica supported cobalt catalyst.

The silylation of SBA-15 enhances the reducibility of cobalt oxides on a SBA-15 supported cobalt catalyst, and consequently increases the catalytic activity for Fischer-Tropsch synthesis of hydrocarbons from syngas and selectivity for longer chain products.

Chemical reduction of 2,4,6-tricyano-1,3,5-triazine and 1,3,5-tricyanobenzene. Formation of novel 4,4',6,6'-Tetracyano-2,2'-bitriazine and its radical anion.

Chemical reduction of 2,4,6-tricyano-1,3,5-triazine, TCT, results in the formation of an unstable radical anion that undergoes immediate dimerization at a ring carbon to form [C(12)N(12)](2-), [TCT](2)(2-), characterized by a long 1.570 (4) A central C[bond]C. [TCT](2)(2-) can decompose into the radical anion of 4,4',6,6'-tetracyano-2,2'-bitriazine, [TCBT]*-, the one-electron reduced form of planar (D(2h)) TCBT, which is also structurally characterized as the [TMPD][TCBT] charge-transfer complex (TMPD = N,N,N',N'-tetramethyl-p-phenylenediamine) with a 1.492 (2) A central sp(2)[bond]sp(2) C[bond]C. Although crystals could not be obtained for the radical anion [TCBT]*-, the electrochemistry (E degrees = +0.03 V), EPR (g = 2.003, (2)A((14)N) = 3.347 G, and (4)A((14)N) = 0.765 G and a line width of 0.24 G), and theoretical calculations support the formation of [TCBT]*-. In addition, thermolysis of [TCT](2)(2-) yields [TCBT]*-. Chemical reduction of 2,4,6-tricyanobenzene, TCB, forms an unstable radical anion that immediately undergoes dimerization at a ring carbon to form [C(12)H(6)N(6)](2-), [TCB](2)(2-), which has a long 1.560 (5) A central C[bond]C. Reaction of TCT with tetrathiafulvalene (TTF) forms structurally characterized [TTF][TCT], and in the presence of water, TCT hydrolyzes to 2,4-dicyano-6-hydroxy-s-triazine, DCTOH. In contrast, the reaction of TCT with TMPD forms [TMPD][TCT], which in the presence of water forms structurally characterized [HTMPD](+)[DCTO](-).

Effect of Conformational Constraints on Gated Electron Transfer Kinetics. 2. Copper(II/I) Complexes with Phenyl-Substituted [14]aneS(4) Ligands in Acetonitrile(1).

Kinetic studies have been conducted in acetonitrile on the electron-transfer reactions of five copper(II/I) complexes involving ligands in which either a benzene or a cyclohexane ring, or both, have been substituted into the ligand backbone of the 14-membered tetrathiamacrocycle [14]aneS(4). The specific ligands utilized in this work include 2,3-benzo-1,4,8,11-tetrathiacyclotetradecane (bz-[14]aneS(4)), 2,3-trans-cyclohexano-1,4,8,11-tetrathiacyclotetradecane (trans-cyhx-[14]aneS(4)), 2,3-benzo-9,10-trans-cyclohexano-1,4,8,11-tetrathiacyclotetradecane (bz,trans-cyhx-[14]aneS(4)), 2,3-benzo-9,10-cis-cyclohexano-1,4,8,11-tetrathiacyclotetradecane (bz,cis-cyhx-[14]aneS(4)), and 2,3-cis-9,10-trans-dicyclohexano-1,4,8,11-tetrathiacyclotetradecane (cis, trans-dicyhx-[14]aneS(4)). Each Cu(II/I)L system has been reacted with three separate reducing agents and three separate oxidizing agents to examine the effect of driving force upon the kinetic parameters. The Marcus relationship has been applied to each cross-reaction rate constant to estimate the apparent self-exchange rate constant, k(11), for each Cu(II/I)L system. For all but one of the five systems, the k(11) values obtained from the three reduction reactions are in virtual agreement with the corresponding value obtained for the oxidation reaction with the smallest driving force. As the driving force for Cu(I)L oxidation increases, a smaller k(11) value is calculated for each system. This behavior is consistent with our previously proposed dual-pathway square scheme mechanism in which a significant conformational change occurs as a separate step preceding electron transfer in the case of Cu(I)L oxidation. Although direct observation of conformationally-gated electron transfer was not attained for any of the five systems included in the current work, limits for the rate constant for conformational change have been estimated from the conditions required to change the apparent pathway for the oxidation kinetics. These limits show that the Cu(I)L complex involving a single phenyl substituent (bz-[14]aneS(4)) exhibits a much slower conformational change than do any of the other systems included in this study. The implications of this observation are discussed.

Electron-Transfer Kinetics and Thermodynamic Characterization of Copper(II/I) Complexes with Acyclic Tetrathiaethers in Aqueous Solution.

The kinetics of a series of Cu(II/I)-acyclic tetrathiaether complexes reacting with several oxidizing and reducing reagents have been examined in aqueous solution at 25 degrees C. This investigation has included a re-examination of Cu(II/I)(Me(2)-2,3,2-S(4)) (Me(2)-2,3,2-S(4) = 2,5,9,12-tetrathiatridecane = L12a), containing the ethylene-trimethylene-ethylene bridging sequence, plus three newly synthesized ligands containing an alternate bridging sequence of trimethylene-ethylene-trimethylene: 2,6,9,13-tetrathiatetradecane (Me(2)-3,2,3-S(4) = L12b) and two cyclohexanediyl-substituted derivatives, viz., cis-1,2-bis[(3-methylthiopropyl)thio]cyclohexane (cis-cyhx-Me(2)-3,2,3-S(4) = L14) and trans-1,2-bis[(3-methylthiopropyl)thio]cyclohexane (trans-cyhx-Me(2)-3,2,3-S(4) = L15). The corresponding phenylene derivative, 1,2-bis[(3-(methylthio)propyl)thio]benzene (bz-Me(2)-3,2,3-S(4) = L13), was also synthesized but did not form a measurable copper complex. The conditional stability constants for Cu(II)L (K(Cu)()II(L)(')) and Cu(I)L (K(Cu)()I(L)(')) and the Cu(II/I)L formal redox potentials (E(f)) vs NHE at 25 degrees C (generally at &mgr; = 0.10 (NaClO(4))) are as follows: for L12b, 15 M(-)(1), 1.0 x 10(13) M(-)(1), 0.83 V; for L14, 2.8 x 10(2) M(-)(1), 0.9(5) x 10(13) M(-)(1), 0.75 V; for L15, 8.8 x 10(2) M(-)(1), 6.(3) x 10(13) M(-)(1), 0.77 V. Application of the Marcus relationship to the experimentally determined cross-reaction rate constants yielded self-exchange rate constants for all four Cu(II/I)L acyclic systems which were relatively constant for both oxidation and reduction under a wide range of conditions. This contrasts sharply with previous results obtained for corresponding macrocyclic ligand systems.