The homolytic bond dissociation energies of a series of five- and six-coordinate mono- and dihydride complexes of the type HM(diphosphine)2 and [H2M(diphosphine)2]+ (where M = Co, Rh, and Ir) are calculated and compared with experimental values. This work probes the relationship between the homolytic bond dissociation energies (HMBDEs) of these complexes in these two different coordination environments and formal oxidation states. The results of these calculations and previous experimental observations suggest that for M = Rh the HMBDE of the five-coordinate HM(diphosphine)2 species are 0-2 kcal/mol larger than the HMBDE of the corresponding six-coordinate [H2M(diphosphine)2]+ species. For M = Ir the bond energies of the five- and six-coordinate complexes are nearly the same and for M = Co the six-coordinate species are 1-5 kcal/mol less than the corresponding five-coordinate species. Simplified models of large and complicated ligands seem to capture the essential trends and give very good estimates of these thermodynamic properties compared with experimentally available data that are difficult to obtain.

The kinetics of the hydrogenation of 2 CpCr(CO)3•/[CpCr(CO)3]2 to CpCr(CO)3H has been investigated. The reaction is second-order in Cr and first-order in H2, with a rate constant (if the rate law is written with [CpCr(CO)3•]2) of 12(2) M–2 s–1 at 25 °C in benzene. DFT calculations rule out a side-on H2 complex as an intermediate and suggest either (1) homolytic cleavage via a collinear Cr–H–H–Cr transition state or (2) end-on approach of H2 to one Cr as charge is transferred to the other, followed by heterolytic cleavage of the coordinated H2 between the first Cr and the O of a carbonyl ligand on the second Cr, and eventual isomerization of the resulting O-protonated intermediate to CpCr(CO)3H.

The [Ni((P2NR)-N-R'2)2](2+) complexes (where PR2NR'2 is 1,5-R'-3,7-R-1,5-diaza-3,7-diphosphacyclooctane) are fast electrocatalysts for H2 production and oxidation. Binding of a fifth ligand (CH3CN or BF4) or chair/boat isomerization has the potential to slow catalysis by blocking the addition of H-2 or by incorrectly positioning the pendant amines. We report the structural dynamics of a series of nickel complexes characterized by NMR spectroscopy and theoretical modeling to examine the effects of the fifth ligand for the Ni(II) complexes, including CH3CN, BF4, Cl, and H, as well as the differences in dynamics between the Ni(II) and Ni(0) oxidation states. A fast exchange process was observed for the [Ni(CH3CN)((P2NR)-N-R'2)2](2+) complexes, with rates ranging from 104 to 107 s1 depending on the phosphorus and nitrogen substituents on the PR2NR'2 ligand. This exchange process was identified to occur through a multistep mechanism, which consists of dissociation of the acetonitrile, boat/chair isomerization of each of the four rings (including nitrogen inversion), and reassociation of an acetonitrile on the opposite side of the complex. The rate of the chair/boat inversion was found to be influenced by varying the substituent on the nitrogen atom, but the rate of the overall exchange process is at least an order of magnitude faster than the catalytic rate in acetonitrile, demonstrating that the structural dynamics of the [Ni(CH3CN)((P2NR)-N-R'2)2](2+) complexes do not hinder catalysis. Possible catalytic implications of the coordination of a fifth ligand to the Ni(II) complex are discussed.

In this study, a novel approach was developed to formulate surrogate fuels having characteristics that are representative of diesel fuels produced from real-world refinery streams. Because diesel fuels typically consist of hundreds of compounds, it is difficult to conclusively determine the effects of fuel composition on combustion properties. Surrogate fuels, being simpler representations of these practical fuels, are of interest because they can provide a better understanding of fundamental fuel-composition and property effects on combustion and emissions-formation processes in internal-combustion engines. In addition, the application of surrogate fuels in numerical simulations with accurate vaporization, mixing, and combustion models could revolutionize future engine designs by enabling computational optimization for evolving real fuels. Dependable computational design would not only improve engine function, it would do so at significant cost savings relative to current optimization strategies that rely on physical testing of hardware prototypes. The approach in this study utilized the state-of-the-art techniques of C-13 and H-1 nuclear magnetic resonance spectroscopy and the advanced distillation curve to characterize fuel composition and volatility, respectively. The ignition quality was quantified by the derived cetane number. Two well-characterized, ultra-low-sulfur #2 diesel reference fuels produced from refinery streams were used as target fuels: a 2007 emissions certification fuel and a Coordinating Research Council (CRC) Fuels for Advanced Combustion Engines (FACE) diesel fuel. A surrogate was created for each target fuel by blending eight pure compounds. The known carbon bond types within the pure compounds, as well as models for the ignition qualities and volatilities of their mixtures, were used in a multiproperty regression algorithm to determine optimal surrogate formulations. The predicted and measured surrogate-fuel properties were quantitatively compared to the measured target-fuel properties, and good agreement was found.

To understand the electrical properties and behavior of amorphous silicon, it is imperative to understand the behavior of hydrogen in the amorphous silicon lattice. Although considerable effort has been extended to study hydrogen trapping in a-Si:H, the energetics and populations of the traps remain unresolved. We have developed a reaction diffusion model which includes multiple bulk trapping mechanisms, bulk transport, and realistic surface processes. The model results are compared with existing hydrogen temperature programmed evolution data, isothermal hydrogen evolution experiments, and thermal quenching experiments. We find consistently good fits for all three types of experiments with a shallow trap depth of 1.5 eV and deep trap depth of 1.8–1.9 eV below the mobility level, with 20–30% of hydrogen present in the deep trap. The difference of 0.3–0.4 eV between the two trap depths is similar to the defect formation activation energy and suggests a defect formation mechanism which involves movement of hydrogen from a deep to a shallow trap.

Three hydrotreated bio-oils with different oxygen contents (8.2, 4.9, and 0.4 w/w) were distilled to produce light, naphtha, jet, diesel, and gas oil boiling range fractions that were characterized for oxygen-containing species by a variety of analytical methods. The bio-oils were originally generated from lignocellulosic biomass in an entrained-flow fast pyrolysis reactor. Analyses included elemental composition, carbon type distribution by C-13 nuclear magnetic resonance, acid number, gas chromatography/mass spectroscopy, volatile organic acids by liquid chromatography, and carbonyl compounds by 2,4-dinitrophenylhydrazine derivatization and liquid chromatography. Acid number titrations employed an improved titrant electrode combination with faster response that allowed the detection of multiple end points in many samples and allowed for acid values attributable to carboxylic acids and to phenols to be distinguished. The results of these analyses showed that the highest oxygen content bio-oil fractions contained oxygen as carboxylic acids, carbonyls, aryl ethers, phenols, and alcohols. Carboxylic acids and carbonyl compounds detected in this sample were concentrated in the light, naphtha, and jet fractions (<260 degrees C boiling point). The carboxylic acid content of all of the high oxygen content fractions was likely too high for these materials to be considered as fuel blendstocks, although the potential for blending with crude oil or refinery intermediate streams may exist for the diesel and gas oil fractions. The 4.9% oxygen sample contained, almost exclusively, phenolic compounds found to be present throughout the boiling range fractions, which imparted measurable acidity primarily in the light, naphtha, and jet fractions. Additional study is required to understand what levels of the weakly acidic phenols could be tolerated in a refinery feedstock. The diesel and gas oil fractions from this upgraded oil had low acidity but still contained 3-4 wt % oxygen present as phenols that could not be specifically identified. These materials appear to have excellent potential as refinery feedstocks and some potential for blending into finished fuels. Fractions from the lowest oxygen-content oil exhibited some phenolic acidity but generally contained very low levels of oxygen functional groups. These materials would likely be suitable as refinery feedstocks and potentially as fuel blend components. Paraffins, isoparaffins, olefins, naphthenes, and aromatics (PIONA) analysis of the light and naphtha fractions showed benzene contents of 0.5 and 0.4 vol % and predicted (research octane number (RON) + motor octane number (MON))/2 of 63 and 70, respectively.

The importance of 17-electron metal radicals is now wellrecognized. Highly reactive Cp(CO)3Mo • and Cp(CO)3W • are produced by photochemical homolysis of the M M bonded dimers [Cp(CO)3M]2. 3 Tyler and co-workers reported extensive studies of the reactions of Cp(CO)3Mo • with phosphines, resulting in disproportionation proceeding through 19-electron intermediates. Cp(CO)3W • and related radicals can also abstract halogen atoms from halogenated hydrocarbons, and rate constants for many such reactions have been reported. In the absence of reactive substrates, these metal-centered radicals dimerize at (or close to) diffusion-controlled rates, with dimerization rate constants typically around 3 10 M 1 s . Reactions of the radicals Cp(CO)3Mo • 8 and Cp(CO)3W • 9 have been observed by time-resolved IR spectroscopy, and recent studies have used ultrafast spectroscopy and two-dimensional IR to study their reactivity. The M M bonded dimers [Cp(CO)3M]2 (M = Mo, W) do not thermally homolyze to Cp(CO)3M • to an appreciable extent;ΔG 0 = 22( 3 kcal mol 1 for homolysis of the Mo Mo bond in [Cp(CO)3Mo]2. In contrast, Tyler and co-workers found that [(C5Ph5)Mo(CO)3]2, having a bulky pentaphenyl-substituted Cp ring, undergoes Mo Mo bond dissociation to a greater extent. The Cr Cr bond in [Cp(CO)3Cr]2 is weaker than the analogous Mo Mo or W Wbonds, and the 17-electron radical Cp(CO)3Cr • is in equilibrium with the dimer. The chromium radical Cp(CO)3Cr • has been found to dimerize at somewhat less than diffusion-controlled rate constants (k = 3 10 M 1 s ), making it more persistent than its Mo

A series consisting of a tungsten anion, radical, and cation, supported by the N-heterocyclic carbene 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes) and spanning formal oxidation states W(0), W(I), and W(II), has been synthesized, isolated, and characterized. Reaction of the hydride CpW(CO)(2)(IMes)H with KH and 18-crown-6 gives the tungsten anion [CpW(CO)(2)(IMes)](-)[K(18-crown-6)](+). Electrochemical oxidation of [CpW(CO)(2)(IMes)](-) in MeCN (0.2 M (n)Bu(4)N(+)PF(6)(-)) is fully reversible (E(1/2) = -1.65 V vs Cp(2)Fe(+•/0)) at all scan rates, indicating that CpW(CO)(2)(IMes)(•) is a persistent radical. Hydride transfer from CpW(CO)(2)(IMes)H to Ph(3)C(+)PF(6)(-) in MeCN affords [cis-CpW(CO)(2)(IMes)(MeCN)](+)PF(6)(-). Comproportionation of [CpW(CO)(2)(IMes)](-) with [CpW(CO)(2)(IMes)(MeCN)](+) gives the 17-electron tungsten radical CpW(CO)(2)(IMes)(•). This complex shows paramagnetically shifted resonances in the (1)H NMR spectrum and has been characterized by IR spectroscopy, low-temperature EPR spectroscopy, and X-ray diffraction. CpW(CO)(2)(IMes)(•) is stable with respect to disproportionation and dimerization. NMR studies of degenerate electron transfer between CpW(CO)(2)(IMes)(•) and [CpW(CO)(2)(IMes)](-) are reported. DFT calculations were carried out on CpW(CO)(2)(IMes)H, as well as on related complexes bearing NHC ligands with N,N' substituents Me (CpW(CO)(2)(IMe)H) or H (CpW(CO)(2)(IH)H) to compare to the experimentally studied IMes complexes with mesityl substituents. These calculations reveal that W-H homolytic bond dissociation energies (BDEs) decrease with increasing steric bulk of the NHC ligand, from 67 to 64 to 63 kcal mol(-1) for CpW(CO)(2)(IH)H, CpW(CO)(2)(IMe)H, and CpW(CO)(2)(IMes)H, respectively. The calculated spin density at W for CpW(CO)(2)(IMes)(•) is 0.63. The W radicals CpW(CO)(2)(IMe)(•) and CpW(CO)(2)(IH)(•) are calculated to form weak W-W bonds. The weakly bonded complexes [CpW(CO)(2)(IMe)](2) and [CpW(CO)(2)(IH)](2) are predicted to have W-W BDEs of 6 and 18 kcal mol(-1), respectively, and to dissociate readily to the W-centered radicals CpW(CO)(2)(IMe)(•) and CpW(CO)(2)(IH)(•).

Hydride transfer from Cp(CO)(2)(PCy3)MoH to Ph3C+BF4- gives Cp(CO)(2)(PCy3)MoFBF3, and the crystal structure of this complex was determined. In the weakly bound FBF3 ligand, the B-F(bridging) bond length is 1.475(8) angstrom, which is 0.15 angstrom longer than the average length of the three B-F(terminal) bonds. The PCy3 and FBF3 ligands are cis to each other in the four-legged piano stool structure. Electronic structure (DFT) calculations predict the trans isomer of Cp(CO)(2)(PCy3)MoFBF3 to be 9.5 kcal/mol (in Delta G(g)degrees(,298)) less stable than the cis isomer that was crystallographically characterized. Hydride transfer from Cp(CO)(2)(PCy3)MoH to Ph3C+BAr4'-[Ar'-3,5 - bis(trifluoromethyl)phenyl] in CH2Cl2 solvent produces Cp(CO)(2()PCy3)Mo(ClCH2Cl)](+)[BAr4'](-), in which CH2Cl2 is coordinated to the metal. (C) 2009 Elsevier B.V. All rights reserved.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

Reported herein are thermochemical studies of hydrogen atom transfer (HAT) reactions involving transition metal H-atom donors M(II)LH and oxyl radicals. [Fe(II)(H(2)bip)(3)](2+), [Fe(II)(H(2)bim)(3)](2+), [Co(II)(H(2)bim)(3)](2+), and Ru(II)(acac)(2)(py-imH) [H(2)bip = 2,2'-bi-1,4,5,6-tetrahydropyrimidine, H(2)bim = 2,2'-bi-imidazoline, acac = 2,4-pentandionato, py-imH = 2-(2'-pyridyl)imidazole)] each react with TEMPO (2,2,6,6-tetramethyl-1-piperidinoxyl) or (t)Bu(3)PhO(*) (2,4,6-tri-tert-butylphenoxyl) to give the deprotonated, oxidized metal complex M(III)L and TEMPOH or (t)Bu(3)PhOH. Solution equilibrium measurements for the reaction of [Co(II)(H(2)bim)(3)](2+) with TEMPO show a large, negative ground-state entropy for hydrogen atom transfer, -41 +/- 2 cal mol(-1) K(-1). This is even more negative than the DeltaS(o)(HAT) = -30 +/- 2 cal mol(-1) K(-1) for the two iron complexes and the DeltaS(o)(HAT) for Ru(II)(acac)(2)(py-imH) + TEMPO, 4.9 +/- 1.1 cal mol(-1) K(-1), as reported earlier. Calorimetric measurements quantitatively confirm the enthalpy of reaction for [Fe(II)(H(2)bip)(3)](2+) + TEMPO, thus also confirming DeltaS(o)(HAT). Calorimetry on TEMPOH + (t)Bu(3)PhO(*) gives DeltaH(o)(HAT) = -11.2 +/- 0.5 kcal mol(-1) which matches the enthalpy predicted from the difference in literature solution BDEs. A brief evaluation of the literature thermochemistry of TEMPOH and (t)Bu(3)PhOH supports the common assumption that DeltaS(o)(HAT) approximately 0 for HAT reactions of organic and small gas-phase molecules. However, this assumption does not hold for transition metal based HAT reactions. The trend in magnitude of |DeltaS(o)(HAT)| for reactions with TEMPO, Ru(II)(acac)(2)(py-imH) << [Fe(II)(H(2)bip)(3)](2+) = [Fe(II)(H(2)bim)(3)](2+) < [Co(II)(H(2)bim)(3)](2+), is surprisingly well predicted by the trends for electron transfer half-reaction entropies, DeltaS(o)(ET), in aprotic solvents. This is because both DeltaS(o)(ET) and DeltaS(o)(HAT) have substantial contributions from vibrational entropy, which varies significantly with the metal center involved. The close connection between DeltaS(o)(HAT) and DeltaS(o)(ET) provides an important link between these two fields and provides a starting point from which to predict which HAT systems will have important ground-state entropy effects.

An extensive family of thermochemical data is presented for a series of complexes derived from Cp*Mo(mu-S)(2)(mu-SMe)(mu-SH)MoCp* and Cp*Mo(mu-S)(2)(mu-SH)(2)MoCp*. These data include electrochemical potentials, pK(a) values, homolytic solution bond dissociation free energies (SBDFEs), and hydride donor abilities in acetonitrile. Thermochemical data ranged from +0.6 to -2.0 V vs FeCp(2)(+/o) for electrochemical potentials, 5 to 31 for pK(a) values, 43 to 68 kcal/mol for homolytic SBDFEs, and 44 to 84 kcal/mol for hydride donor abilities. The observed values for these thermodynamic parameters are comparable to those of many transition metal hydrides, which is consistent with the many parallels in the chemistry of these two classes of compounds. The extensive set of thermochemical data is presented in free energy landscapes as a useful approach to visualizing and understanding the relative stabilities of all of the species under varying conditions of pH and H(2) overpressure. In addition to the previously studied homogeneous reactivity and catalysis, Mo(2)S(4) complexes are also models for heterogeneous molybdenum sulfide catalysts, and therefore, the present results demonstrate the dramatic range of S-H bond strengths available in both homogeneous and heterogeneous reaction pathways.

Homolytic solution bond dissociation free energies (SBDFE) for S-H bonds have been determined for soluble dimolybdenum tetrasulfide complexes through thermochemical cycles using electrochemical potentials and pK(a) values in acetonitrile. In spite of the importance and extensive use of metal sulfide catalysts, these S-H bond strengths are among the first experimentally determined values for metal sulfide systems. For [CP*MO(mu-S)(mu-SMe)(2)(mu-SH)MoCp*](+) (S4Me2H+), [Cp*Mo(mu-S)(mu-SMe)(mu-SH)(2)MoCP*](+) (S4MeH2+), and [Cp*Mo(mu-S)(mu-SH)(3)MoCP*](+) (S4H3+), the pK(a) values were determined to be 5.6 +/- 0.4, 5.3 +/- 0.3, and 4.9 +/- 0.3, respectively. The E-1/2 values for S4Me2 center dot+/o, S4MeH center dot+/o, and S4H2 center dot+/o were measured to be -0.02 +/- 0.02, +0.04 +/- 0.05, and +0.07 +/- 0.07 V vs FeCp2+/o, respectively. Using these experimental values, the homolytic S-H SBDFE for S4Me2H+ to S4Me2 center dot+, S4MeH2+ to S4MeH center dot+, and S4H3+ to S4H2 center dot+ were determined to be 60.8 +/- 1.0, 61.8 +/- 1.6, and 61.9 +/- 2.0 kcal/mol, respectively. These SBDFE values can be used to estimate gas phase bond dissociation enthalpies of 65.6, 66.6, and 66.7 kca/mol, respectively. Solid state structures are presented for S4MeH and S4H2.

The CRC Fuels for Advanced Combustion Engines working group has worked to identify a matrix of research diesel fuels for use in advanced combustion research applications. Nine fuels were specified and formulated to investigate the effects of cetane number aromatic content and 90% distillation fraction. Standard ASTM analyses were performed on the fuels as well as GC/MS and /u1H//u1/u3C NMR analyses and thermodynamic characterizations. Details of the actual results of the fuel formulations compared with the design values are presented, as well as results from standard analyses, such as heating value, viscosity and density. Cetane number characterizations were accomplished by using both the engine method and the Ignition Quality Tester (IQT/sT) apparatus.