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Thermochemical Investigations of Nearly Ideal Binary Solvents. 3. Solubility in Systems of Nonspecific Interactions
This article discusses thermochemical investigations of nearly ideal binary solvents. Abstract: The simple model which has previously led to successful predictive equations for the partial molar excess enthalpy of a solute in nearly ideal binary solvents has been slightly modified for application to the partial molar excess Gibbs free energy (excess chemical potential) of the solute in these systems. Three predictive equations are derived and tested for their ability to predict solubility in mixed solvents from measurements in the pure solvents. The most successful equation involves volumetrically weighted interaction parameters for the excess Gibbs free energy relative to the Flory-Huggins entropy of mixing, and predicts solubility in 22 systems containing naphthalene, iodine, and stannic iodide as solutes with an average deviation of 1.5% and a maximum deviation of 4%, using no adjustable parameters. digital.library.unt.edu/ark:/67531/metadc152443/
Comment on the Prediction of Gas Chromatographic Retention Behavior with Mixed Liquid Phases
This article comments on the prediction of gas chromatographic retention behavior with mixed liquid phases. digital.library.unt.edu/ark:/67531/metadc146582/
Thermochemical Investigations of Gas-Liquid Chromatography. Partition Coefficients of Inert Solutes on Self-Associating Binary Solvent Mixtures
This article discusses partition coefficients of inert solutes on self-associating binary solvent mixtures. Abstract: A conventional nonelectrolyte solution model which has led to successful predictive equations for the partial molar excess properties of a solute in simple binary solvent systems is extended to include self-associating solvent components. An expression is developed and tested for its ability to describe gas-liquid partition coefficients in mixed solvents from measurements in the pure solvents. For n-hexane, n-heptane, and cyclohexane on blended mixtures of n-hexadecane and n-octadecane with N,N-dibutyl-2-ethylhexylamide, the newly derived expression is found to describe the chromatographic data to within 2%. digital.library.unt.edu/ark:/67531/metadc152444/
Solubility of Phenylacetic Acid in Binary Solvent Mixtures
This article discusses the solubility of phenylacetic acid in binary solvent mixtures. Abstract: Solubilities are reported for phenylacetic acid at 25.0 °C in binary mixtures of carbon tetrachloride with cyclohexane, n-heptane, n-octane, or isooctane and mixtures of cyclohexane with n-heptane or isooctane. The results are compared to the predictions of equations developed previously for solubility in systems of purely nonspecific interactions, with phenylacetic acid considered as either monomeric or dimeric molecules in solution. The dimer model provided the more accurate predictions and described the 15-fold range of solubilities in the carbon tetrachloride + isooctane system to within a maximum deviation of 15%. digital.library.unt.edu/ark:/67531/metadc152445/
Excess Molar Volumes of Binary Mixtures of Cyclohexane and y-Butyrolactone with Aromatic Hydrocarbons
This article discusses excess molar volumes of binary mixtures of cyclohexane and y-butyrolactone with aromatic hydrocarbons. Abstract: Excess molar volumes of cyclohexane + benzene, cyclohexane + toluene, cyclohexane + p-xylene, y-butyrolactone + benzene, y-butyrolactone + toluene, and y-butyrolactone + m-xylene have been measured at 298.15 K. For the first three systems, Vᴱ is positive throughout the entire concentration range. Vᴱ is negative for binary mixtures containing y-butyrolactone, which indicates significant interaction between y-butyrolactone and the aromatic hydrocarbon. digital.library.unt.edu/ark:/67531/metadc152457/
Experimental Artifacts and Determination of Accurate Py Values
This article discusses experimental artifacts and determination of accurate Py values. Abstract: Recently the Py solvent scale has been introduced and a large body of data has been generated using fluorescence measurements. Numerous problems exist in the correct determination of these values, including instrumental and chemical artifacts. Among the instrumental problems associated with correct Py assignments are slit width effects, inner filtering and efficiencies associated with double-pass vs. single-pass cell compartment designs. These instrumental problems, together with chemical artifacts related to adequate blank correction and temperature control, were investigated in the work reported in this paper. digital.library.unt.edu/ark:/67531/metadc152446/
Solubility of Pyrene in Binary Solvent Mixtures Containing Cyclohexane
This article discusses solubility of pyrene in binary solvent mixtures containing cyclohexane. Abstract: Solubilities are reported for pyrene at 26.0 °C in binary mixtures of cyclohexane with n-hexane, n-heptane, n-octane, isooctane, and cyclooctane. The results of these measurements are compared to the predictions of equations developed previously for solubility in systems of nonspecific interactions. The nearly ideal binary solvent (NIBS) model predicts these solubilities with a maximum deviation of 4.8%, using as input data the solubility of pyrene in each pure solvent. The NIBS model correctly predicts a small maxima for the mole fraction solubility of pyrene in cyclohexane + n-heptane mixtures. digital.library.unt.edu/ark:/67531/metadc152448/
Solubility of Anthracene in Binary Solvent Mixtures Containing Dibutyl Ether
This article discusses the solubility of anthracene in binary solvent mixtures containing dibutyl ether. Abstract: Experimental solubilities are reported for anthracene in binary solvent mixtures containing dibutyl ether with n-hexane, cyclohexane, n-heptane, methylcyclohexane, n-octane, isooctane, and cyclooctane at 25 °C. Results of these measurements, combined with estimates for the excess Gibbs free energies of the binary solvents, are used to test predictive expressions derived from the nearly ideal binary solvent (NIBS) model. Expressions based on a volume fraction average of solute properties in the two pure solvents predict anthracene solubilities to within a maximum deviation of 5.1% and an overall average deviation of 2.1%. digital.library.unt.edu/ark:/67531/metadc152449/
Solubility of Pyrene in Binary Solvent Mixtures Containing Dibutyl Ether
This article discusses the solubility of pyrene in binary solvent mixtures containing dibutyl ether. Experimental solubilities are reported for pyrene in binary solvent mixtures containing dibutyl ether with n-hexane, cyclohexane, n-heptane, methylcyclohexane, n-octane, isooctane, and tert-butylcyclohexane at 26 °C. Results of these measurements, combined with estimates for the excess Gibbs free energies of the binary solvents, are used to test predictive expressions derived from the nearly ideal binary solvent (NIBS) model. Expressions based on a volume fraction average of solute properties in the two pure solvents predict pyrene solubilities to within a maximum deviation of 10% and an overall average deviation of 3.2%. digital.library.unt.edu/ark:/67531/metadc152453/
Polycyclic Aromatic Hydrocarbon Solute Probes Part II. Effect of Solvent Polarity on the Fluorescence Emission Fine Structures of Coronene Derivatives
This article discusses the effect of solvent polarity on the fluorescence emission fine structures of coronene derivatives. Abstract: The fluorescence properties of coronene (Co), benzo[a]coronene (BCo), naphtho[2,3-a]coronene (NCo), dibenzo[a,j]coronene (DCo), naphtho[1,2,3,4-ghi]perylene, benzo[pqr]naphtho[8,1,2-bcd]perylene and dibenzo[cd,lm]perylene dissolved in solvents of varying polarity are reported. Measurements indicated that the emission intensities of the four coronene derivatives depended on solvent polarity. The Co, BCo and NCo scales have been defined as the ratio of the fluorescence intensities of bands I and III of the vibronic spectra. Band III of dibenzo[a,j]coronene was not clearly identifiable in all the solvents studied, and the DCo scale was therefore defined as the intensity ratio of band I and IV. Emission intensity ratios of the three perylene derivatives remained nearly constant, irrespective of solvent polarity. digital.library.unt.edu/ark:/67531/metadc157299/
Comments on the Competitive Preferential Solvation Theory
This article offers comments on the competitive preferential solvation theory. Abstract: Simple additive relationships for the physico-chemical properties of a solute dissolved in binary solvent mixtures are developed from the competitive preferential solvation model. Additive expressions for solute mole fraction solubility and logarithm of solute solubility are mathematically identical to equations derived previously from the microscopic partition and basic nearly ideal binary solvent models. Calculated values based on the various additive relationships are compared to carbazole solubilities in ten binary solvent mixtures containing dibutyl ether with n-hexane, n-heptane, n-octane, cyclohexane, cyclo-octane, methylcyclohexane, iso-octane, n-hexadecane, squalane and t-butylcyclohexane. digital.library.unt.edu/ark:/67531/metadc157300/
Principal Resonance Contributors to High-Valent, Transition-Metal Alkylidene Complexes
This article discusses principal resonance contributors to high-valent, transition-metal alkylidene complexes. The results of ab initio calculations are reported for prototypical high-valent, alkylidene complexes. Stationary points on each potential energy surface are characterized and compared to experimental information where available; as long as a suitably flexible valence basis set is used, good agreement between theoretically calculated and experimentally determined geometries is obtained. The complexes of interest include group IVB (Ti, Zr and Hf) and group VB (Nb and Ta) alkylidenes with hydride ligands as well as models for the four-coordinate, olefin metathesis catalysts (Mo-, W-, and Re-alkylidenes) which have been recently synthesized and characterized. In light of the fact that much of the discussion concerning the reactivity of transition-metal carbene complexes has been presented in terms of the resonance contributors derived from rearranging the electrons in the M-C σ and π orbitals, the minima obtained from the portion of the study are then subjected to a further procedure to calculate these contributions. Resonance structures in which the carbon is the negative end of the M-C bond (i.e., nucleophilic resonance structures) contribute 50% to the ground-state wave function of these complexes. Those in which the carbon is formally neutral account for much of the remainder (45%). Only 5% is comprised of electrophilic resonance structures, i.e., those in which the carbon is the positive end of the M-C bond. Furthermore, the metal-carbon double bond is predominantly comprised of five resonance structures. Four of these resonance structures correspond to models of carbene bonding which have been discussed previously in the literature. The other resonance structure, which contributes roughly 33% to the ground-state wave function, has hitherto not been considered when examining the chemical reactivity of carbenes. This large resonance contributor can be described as arising from a dative carbon-to-metal σ bond plus a covalent M-C π bond. digital.library.unt.edu/ark:/67531/metadc107773/
High-Valent Transition-Metal Alkylidene Complexes: Effect of Ligand and Substituent Modification
This article discusses high-valent transition-metal alkylidene complexes. Abstract: An ab initio investigation into the effects of ligand and substituent modification on the metal-carbon double bond is reported. Prototypical group IVB (Ti, Zr, Hf) and Group VB (Nb, Ta) alkylidenes are chosen for this study. The MC/LMO/CI (multiconfiguration/localized molecular orbital/configuration interaction) procedure is used to examine the electronic structures of these complexes in terms of the prime resonance contributors to the ground-state wave function. The main conclusion drawn from this work is that the intrinsic nature of the metal-carbon double bond can typically be changed only within certain limits by modification of the electronegativity of the ligands (L) and substituents (Z). In other words, the Ta=C bond in H₃TaCCl₂ and Cl₃TaCH₂ and presumably in experimentally characterized analogues with larger ligands and substituents, e.g., Cp and neopentyl. Significant changes in the electronic structure are effected in three ways: The first way is through the introduction of a highly electropositive substituent, e.g., Li. This makes the metal-carbon bond closer to a triple bond for the Ta-alkylidenes. The second way to change the electronic structure of the alkylidenes significantly is to change the central metal atom. The heaviest members of groups IVB (Hf) and VB (Ta) are the most nucleophilic at the α-carbon. The third way in which the metal-carbon bond could be significantly altered is through the use of π-donor substituents. The introduction of π-donor substituents on Cα increases the electrophilicity of the α-carbon. digital.library.unt.edu/ark:/67531/metadc107774/
Methane Activation by Group IVB Imido Complexes
This article discusses methane activation by group IVB imido complexes. An ab initio study of methane activation by group IVB imido complexes, when coupled with available experimental data, reveals an interesting picture of this important reaction. Initial interaction of methane and (H)₂M=NH leads to the formation of alkane complexes bound by ≈9 kcal mol⁻¹. Experiment indicates that the polarity of the metal-ligand bond upon which the C-H is activated plays an important role in facilitating subsequent scission. Calculations support this hypothesis and suggest that formation of the alkane complex acts to increase Cδ-Hδ polarization, setting the stage for C-H cleavage. Calculated methane elimination barriers for (H)₂M(CH₃)(NH₂) (M=Ti, Zr, Hf) are in good agreement with experimental models in terms of absolute numbers and trends as a function of metal. Calculated methane activation barriers follow the order Ti > Zr > Hf, in line with calculated exothermicities. Calculated geometries indicate a late transition state for methane elimination, in agreement with experimentally determined activation parameters. The TSs have a kite-shaped geometry with an obtuse angle about the H of the C-H bond being activated (Ht) and a short MHt distance, 1-2% greater than normal. The short MHt distance suggests a stabilizing interaction, supported by a positive bond overlap population. Calculation of the intrinsic reaction coordinate demonstrates the importance of agostic interactions between N-H and M along the reaction coordinate. digital.library.unt.edu/ark:/67531/metadc107807/
Transition Metal Imido Complexes
This articles discusses transition metal imido complexes. A wide range of transition metal imido (TMI) complexes is studied using ab initio molecular orbital (MO) calculations. The main computational point of interest is the further testing of effective core potentials (ECPs) and valence basis sets to allow for the accurate calculation of properties for reasonably sized transition metal complexes. On the chemical side, several results from the study are to be noted. The agreement between geometries for calculated models and their experimental counterparts ranges from very good to excellent, as found in previous studies of multiply bonded transition metal-group IVA complexes. Taken together, these data suggest that the accuracy one has come to expect for the prediction of structural properties for main-group compounds may yet become a reality for transition metal complexes. The correct prediction of molecular structure also leads one to infer that the bonding in these complexes is also accurately described. The MC/LMO/CI (multiconfigurational/localized MO/configuration interaction) technique shows that eight resonance structures are most significant in the description of the metal-imido linkage. Three of these account for roughly two-thirds of the total contributions; two resonance structures, both possessing a dative σ bond, correspond to novel bonding descriptions for TMI complexes. The dative character of the σ bond has been largely unappreciated. The MC/LMO/CI results also indicate that the metal-imido bond is intermediate between a double and a triple bond. Nucleophilic resonance structures decrease as one goes to the right in the transition series while electrophilic ones show the reverse behavior, in agreement with observed reactivity. digital.library.unt.edu/ark:/67531/metadc107775/
Small Molecule Elimination from Group IVB (Ti, Zr, Hf) Amido Complexes
This article discusses small molecule elimination from group IVB (Ti, Zr, Hf) amido complexes. An ab initio quantum chemical analysis of HX (X = H, CH₃, Cl, NH₂, SiH₃) elimination by group IVB (Ti, Zr, Hf) amidos (H₂(X)M - NH₂ → H₂M = NH + HX), of interest in the context of CVD precursor design, is reported. Several deductions may be drawn from the calculations. First, in the transition state (TS) for HX elimination, electropositive and electroneutral X give rise to metal-transannular hydrogen (Ht) distances only slightly longer than normal metal-terminal hydride bonds lengths, while electronegative X groups yield substantially longer MHt distances. Second, the HX elimination barrier (∆Hǂelim) is lower when HX is polarized Hδ- • Hδ+ (X = SiH₃) or nonpolar (X = H). Third, a plot of calculated ∆Hǂelim versus MHt distances in the TS. Fourth, analysis of the electronic structure along the intrinsic reaction coordinate (IRC) supports the importance of N-H•••M agostic interactions preceding N-H scission. Fifth, the IRC shows the MHt distance decreasing as Ht is transferred from N to X, reaching a minimum when the transfer is roughly half complete, and then increasing once more is HX is eliminated. These results point to the leaving group (X) playing a crucial role in tuning the bonding the energetics of the TS, and thus the rate of HX elimination. The results lead to the conclusion that materials precursors designed to enhance MHt interaction, through the intermediacy of X, in the TS and along the reaction coordinate will lead to lower activation barriers to XH elimination. digital.library.unt.edu/ark:/67531/metadc107776/
Calculation of a Methane C-H Oxidative Addition Trajectory: Comparison to Experiment and Methane Activation by High-Valent Complexes
This article discusses the calculation of a methane C-H oxidative addition trajectory. Abstract: An effective core potential (ECP), parallel supercomputing study of methane activation by 14-electron, Ir(PH₃)₂(X) complexes (X = H, Cl) is presented. Considerable weakening of the coordinated methane C-H bond occurs upon formation of an ɳ²-CH coordinated (X)(PH₃)₂Ir•••HCH₃ adduct. A more strongly bound adduct (with greater weakening of the coordinated C-H bond) occurs when X = Cl versus X = H. The calculated Ir(PH₃)₂(H) + CH₄ → Ir(PH₃)₂(H)₂(Me) reaction enthalpy is -12.8 kcal mol⁻¹, and -41.6 kcal mol⁻¹ for the chloro analogue. The intrinsic reaction coordinate is calculated and compared to an experimental trajectory. Analysis of the wave function along the intrinsic reaction coordinate (IRC) suggests that although donation of electron density from methane to metal is essential for adduct formation, it is not until backdonation to σ* сʜ increases that the C-H bond is activated and cleaved. The electronic and molecular structure of the reacting system along the IRC suggest a two-stage mechanism: substrate to complex donation is important in the early part of the reaction (electrophilic stage) while complex to substrate backdonation is necessary later on (nucleophilic stage) for C-H scission. Finally, comparison of IRCs for low- and high-valent methane-activating complexes shows similar topology in the early portion of the activation event; differentiation between oxidative addition and σ-bond metathesis occurs at the point at which there is a shift from the electrophilic to nucleophilic stage of the reaction. digital.library.unt.edu/ark:/67531/metadc107777/
An Effective Core Potential Study of Transition-Metal Chalcogenides. 1. Molecular Structure
This article discusses an effective core potential study of transition-metal chalcogenides. Abstract: A structural analysis is reported of roughly 150 transition-metal (TM)-chalcogenido complexes in a variety of chemical environments. With few exceptions, agreement between calculated and experimental geometries is excellent. The research provides convincing evidence that computational methods employed are adequately describing the bonding in these diverse TM complexes. Interesting trends in relative TMCh (Rмсh-Rмсh) bond lengths are found. Experimental and computational data show that other than the zirconocene-and halfnocene-oxos there is similar behavior in relative bond lengths for widely varying TM-chalcogenido complexes. Relative bond lengths versus oxo (S-O, Se-O, and Te-O) in group IVB metallocenes tend to be larger than for other families of complexes and show less variation among the heavier chalcogens (Se-S, Te-S, and Te-Se). Analysis of localized wave functions for Cp₂ZrCh point to a greater contribution from a singly-bonded Zr-Ch structure (relative to Zr=Ch) when Ch is O compared to heavier chalcogens. Taken together, the data suggest that there is a fundamental difference in the Zr-oxo (and Hf-oxo) bond in relation to heavier chalcogens, consistent with recent experimental data. In previous studies of multiply bonded TM complexes the authors have focused on the ability of ECPs to make computations feasible for complexes incorporating even the heaviest transition metals. The present work also evaluates ECP methods for heavier main group (MG) elements. The chalcogens (CH) O, S, Se, and Te are included in this study. digital.library.unt.edu/ark:/67531/metadc107778/
Theoretical Estimation of Vibrational Frequencies Involving Transition Metal Compounds
This article discusses the theoretical estimation of vibrational frequencies involving transition metal compounds. The reliability of effective core potentials (ECPs) for estimating vibrational frequencies of transition metal (TM) complexes is assessed in relation to all-electron methods for main group compounds. Complexes with a multiple bond between a transition metal and chalcogen (O,S, or Se), chalcogenides, are investigated using the Stevens ECP/valence basis set scheme. Statistical treatment of the data indicates that ECPs, in addition to reliably modeling electronic structure, can be successful in estimating vibrational frequencies for TM complexes. As expected, theoretical prediction of vibrational data is not as accurate as the prediction of metric data for chalcogenides. However, agreement with experiment is still very good at the Hartree-Fock level of theory and is in even better accord upon the use of simple correlations to model well-known computational deficiencies (e.g., the neglect of anharmonic effects). Analysis of the data show interesting differences in predictive ability for first row transition metals versus second- and third-row analogues and oxo complexes versus their congeners with heavier chalcogens. digital.library.unt.edu/ark:/67531/metadc107803/
[Review] Deciphering the Chemical Code: Bonding Across the Periodic Table
This book review discusses 'Deciphering the Chemical Code: Bonding Across the Periodic Table' by Nicolaos D. Epiotis from the University of Washington. The reviewer describes the work as a new theoretical framework for describing chemical bonding and gives specific information on what's covered in the book, the themes, and ideal audiences. digital.library.unt.edu/ark:/67531/metadc107796/
Molecular Modeling of Vanadium-Oxo Complexes. A Comparison of Quantum and Classical Methods
This article discusses molecular modeling of vanadium-oxo complexes. A force field for vanadium-oxos was developed and tested with a variety of complexes with coordination numbers of 5 or 6 and formal oxidations states of +4 or +5 on the metal. Similarly, a semiempirical quantum mechanical method for transition metals was extended to vanadium. In this research soft and hard ligands were studied, as were ligands coordinated through single, multiple, and dative bonds. Despite the diversity of vanadium coordination chemistry, generally good modeling is achieved in a fraction of the time with less computational resources using molecular mechanics and semiempirical quantum mechanics. The L₄V⁴⁺O and L₅V⁵⁺O groups were emphasized given their prevalence and importance. In general, the predictive ability was superior for the former structural motif. The combination of molecular mechanics and semiempirical quantum calculations provide an effective and efficient tool for analysis of the steric and electronic energy differences between isomers. digital.library.unt.edu/ark:/67531/metadc107804/
Modeling Nonlinear Optical Properties of Transition Metal Complexes. Basis Set, Effective Core Potential, and Geometry Effects
This article discusses modeling nonlinear optical properties of transition metal complexes. Nonlinear optical (NLO) properties of transition metal complexes are studied using quantum chemical calculations. By comparison with all electron calculations, effective core potentials have been shown to be competent for the calculation of NLO properties as long as the valence basis sets are comparable. While overall the basis set effects are important for calculation of NLO properties, they are found to be less important for the central transition metal than for the surrounding ligands. Augmenting the basis set of main group elements with diffuse, s, p, and d functions in a proper way could provide the best compromise between speed and accuracy of the computation. Interesting trends are found in the calculation of NLO properties of [MO₄]q⁻. Both polarizability (α) and second hyperpolarizability (y) decrease toward the right across the transition series. The second series [MO₄]q⁻ have the largest α among the three metalates in a triad. For group IVB and VB complexes with larger charges (-4 and -3, respectively), the second series [MO₄]q⁻ have the largest y, while for groups VIB, VIIB, and VIII, with less anionic metalates (-2, -1, and 0, respectively), the third series metalates have the largest y. The relative difference in both α and y values among the three series in the same group is much smaller than between different groups. Overall, variations in the calculated values of NLO properties correlate with M-O bond lengths and hence the size of the metalate ion. digital.library.unt.edu/ark:/67531/metadc107805/
Structural Dichotomy in Six-Coordinate d⁰ Complexes: Trigonal Prismatic (ᵗBu₃SiC≡C)₆Ta⁻ and Octahedral (ᵗBu₃SiC≡C)₆M²⁻ (M = Zr, Hf)
This article discusses structural dichotomy in six-coordinate d⁰ complexes. Utilization of the bulky acetylide, ᵗBu₃SiC≡C⁻, enabled the synthesis of several early metal polyacetylides. Addition of NaC≡CH to ᵗBu₃SiBr in dimethyl sulfoxide afforded ᵗBu₃SiC≡CH, which was deprotonated to yield ᵗBu₃SiC≡CLi. Treatment of ZrCl₄, HfCl₄, adn TaCl₅ with varying amounts of ᵗBu₃SiC≡CLi gave {(THF)₂Li(ᵗBu₃SiC≡C)₂}Zr(C≡CSiᵗBu₃)₃(THF) (1; THF = tetrahydrofuran), {Et₂O)Li(ᵗBu₃SiC≡C)₂}Hf(C≡CSiᵗBu₃)₃(OEt₂) (2), {Li(ᵗBu₃SiC≡C)₃}₂M(M = Zr, 6; Hf, 7), and {Li(ᵗBu₃SiC≡C)₃}Ta(C≡CSiᵗBu₃)₃ (3). Metathesis of 3 with KOTf generated KTa(C≡CSiᵗBu₃)₆ (4) and cation sequestration of 4 with crypt 2.2.2 provided [K(crypt 2.2.2)][Ta(C≡CSiᵗBu₃)₆](5). Single-crystal X-ray structural studies determined the structures (core symmetry) of 1 (Oh), 2, (Oh), 3 (D₃), 5 (D₃), 6 (Oh), and 7 (Oh). The D₃h to D₃ twist in 3 and 5 has a steric origin, and the counterion position appears inconsequential. Origins of the structural preferences illustrated by the dichotomous twisted trigonal prismatic and octahedral cores of the d⁰ hexaacetylides 5 and 6 were probed through density functional (ADF) and effective core potential (GAMESS) calculations. The structural difference results from a lessening electronic preference for the trigonal prism-primarily a greater HOMO/LUMO gap-upon moving from Ta to Zr, minor steric factors, and increased interligand repulsions in the dianion (VSEPR). digital.library.unt.edu/ark:/67531/metadc107797/
[Review] Chemistry of Advanced Materials: An Overview
This book review discusses 'Chemistry of Advanced Materials: An Overview', edited by Leonard V. Interrante from Renssalaer Polytechnic Institute, and Mark J. Hampden-Smith from the University of New Mexico. digital.library.unt.edu/ark:/67531/metadc107798/
Estimating Solid-Liquid Phase Change Enthalpies and Entropies
This article discusses estimating solid-liquid phase change enthalpies and entropies. Abstract: A group additivity method based on molecular structure is described that can be used to estimate solid-liquid total phase change entropy (∆₀ᵀ(fus)S(tpce)) and enthalpy (∆₀ᵀ(fus)H(tpce)) of organic molecules. The estimation of these phase changes is described and numerous examples are provided to guide the user in evaluating these properties for a broad range of organic structures. A total of 1858 compounds were used in deriving the group values and these values are tested on a database of 260 additional compounds. The absolute average and relative errors between experimental and calculated values for these 1858 compounds are 9.9 J-mol⁻¹·K⁻¹ and 3.52 kJ·mol⁻¹, and 0.154 and 0.17 for ∆₀ᵀ(fus)S(tpce) and ∆₀ᵀ(fus)H(tpce), respectively. For the 260 test compounds, standard deviations of ± 13.0 J·mol⁻¹·K⁻¹ (∆₀ᵀ(fus)S(tpce)) and ±4.88 kJ mol⁻¹ (∆₀ᵀ(fus)H(tpce)) between experimental and calculated values were obtained. Estimations are provided for both databases. Fusion enthalpies for some additional compounds not included in the statistics are also included in the tabulation. digital.library.unt.edu/ark:/67531/metadc152463/
Computational Study of Polarizabilities and Second Hyperpolarizabilities of Inorganic Transition Metal Thiometalates and Metalates in Solution
This article discusses a computational study of polarizabilities and second hyperpolarizabilities of inorganic transition metal thiometalates and metalates in solution. Abstract: A systematic study of nonlinear optical (NLO) properties of inorganic transition metal (TM) thiometalates and metalates is reported. Polarizabilities (α) and second hyperpolarizabilities (y) are calculated in solution within the polarizable continuum model. It is found that NLO properties of anionic inorganic complexes can be successfully modeled in solution, when this cannot be done so in the gas phase. Solvent effects are found to significantly increase α and y. The effects are stronger on y (up to 80%) than on α (up to 40%) and stronger on TM thiometalates than on metalates. For α, solvent effects are found to be more important than electron correlation effects. For y, the two effects are similarly important. Solvent effects on α and y caused by subordinate factors other than the dominant electrostatic solute-solvent interactions were studied and assessed to be negligible. Upon solvation, large TM and ligand modification effects on α and y are found. One oxo-to-sulfido substitution results in an increase in α by 38 au and y by 10 000 au. digital.library.unt.edu/ark:/67531/metadc107806/
Inter- and Intramolecular Experimental and Calculated Equilibrium Isotope Effects for (silox)₂(ᵗBu₃SiND)TiR + RH (silox = ᵗBu₃SiO): Inferred Kinetic Isotope Effects for RH/D Addition to Transient (silox)₂Ti=NSiᵗBu₃
This article discusses inter- and intramolecular experimental and calculated equilibrium isotope effects. Abstract: Intermolecular equilibrium isotope effects (EIEs) were measured (26.5 ˚C) and calculated for (silox)₂₋ (ᵗBu₃SiND)TiRᴅ (1-ND-Rᴅ) + RʜH ⇆ (silox)₂(ᵗBu₃SiNH)TiRʜH/RᴅD = CH₄/CD₄, 2.00(6), calcd 1.88; C₂H₆/C₂D₆, 2.22(8), 1.93; ᶜC₃H₆/C₃D₆, 1.71(4), 1.48; C₂H₄/C₂D₄, 1.41(11), 1.34; C₆H₆/C₆D₆, 1.22(7), 1.26; C₇H₈/C₇D₈ (50.0 ˚C), 1.59(6), 1.55. Related intramolecular EIEs for (silox)₂(ᵗBu₃SiND)- TiR' (1-ND-R') ⇆ (silox)₂(ᵗBu₃SiNH)TiR (1-R) are provided: RH = R'D = CH₃D, 3.16(25), calcd 2.60; CH₂D₂, 1.13(8), 0.911; CHD₃, 0.389(20), 0.303; CH₃CD₃, 1.53(3), 1.44; 1, 1-ᶜC₃H₄D₂, 2.58(6), 2.53; trans-HDC=CHD, 1.00(2), 1.00; 1,3,5-C₆H₃D₃, 1.273(4), 1.25; PhCH₂D (50.0 ˚C), 2.06(2), 1.98. Calculations of pertinent model complexes (e.g., (HO)₂(H₂N)TiR (1'R)) generated the vibrational frequencies necessary to interpret the EIEs in terms of a statistical mechanics description utilizing gas-phase partition functions; EIE = SYM x MMI x EXC x EXP[-(∆∆ZPE/kʙT)]. The large MMI term in the intermolecular cases-a consequence of using perprotio vs perdeuterio small molecule substrates-is attenuated by EXC and EXP[-(∆∆ZPE/kʙT)] contributions derived from low-energy core vibrations. The EIEs are differentiated on the basis of the EXP [-(∆∆ZPE/kʙT)] term, with CH-based bending vibrations playing the major role. Substrate bending vibrations that are absent in intramolecular cases are primarily responsible for the greater intermolecular values. Using measured (or calculated) EIEs and kinetic isotope effects for 1,2-RH-elimination (KIEelim) from 1-R, KIEaddn values for 1,2-RH-addition to putative intermediate (silox)₂Ti=NSiᵗBu₃ (2) were inferred via EIE = KIEaddn/KIEelim. Extraordinary intermolecular KIEaddn values ranging from ~ 30(CH₄/CD₄, C₂H₆/C₂D₆) to ~ 9 (C₆H₆/C₆D₆) are consistent with previous mechanistic accounts and may be conventionally rationalized. digital.library.unt.edu/ark:/67531/metadc107772/
Deoxygenations of (silox)₃WNO and R₃PO by (silox)₃M (M = V, Ta) and (silox)₃NbL (silox = ᵗBu₃SiO): Consequences of Electronic Effects
This article discusses the consequences of electronic effects. Oxygen atom transfers involving metal-oxo functionalities are central to many biological transformations , prominent in applications to organic synthesis, and of increasing importance in inorganic systems as synthetic tools, objectives in biomimicry, and targets of fundamental studies. As a synthetic route to (silox)₃WN (4, silox = ᵗBu₃SiO), the deoxygenation of (silox)₃WNO (2) by (silox)₃Ta (1-Ta) was attempted without success, despite ample precedent in cleavages of epoxides, N₂O, NO, CO₂, and CO. A comparison study involving sources of M(silox)₃ (1-M; M = V, Nb, Ta) revealed that features of deoxygenations of 2 and R₃PO) (R = Me, Ph, ᵗBu) are the consequences of electronic effects enforced by a limiting steric environment. digital.library.unt.edu/ark:/67531/metadc107779/
Stepwise Reduction of Dinitrogen Bond Order by a Low-Coordinate Iron Complex
This article discusses stepwise reduction of dinitrogen bond order by a low-coordinate iron complex. Conversion of atmospheric N₂ into NH₃ is one of the most important chemical processes, because ammonia is the industrial and biological precursor to many nitrogen-containing compounds. Large-scale transformation of N₂ and H₂ into ammonia is performed in industry by the Haber-Bosch process, using "potassium-promoted" porous iron. A view of the N₂-reducing active site of iron-molybdenum nitrogenase, which contains unusual iron atoms with only three sulfur donors, is shown in Chart 1. The presence of iron in the active sites of this and other nitrogenases suggests that iron is again important for activating dinitrogen. Thus iron plays a major role in both natural and industrial N₂ reduction catalysis. digital.library.unt.edu/ark:/67531/metadc107780/
Discovering "Rediscovery"
Article containing an interview of James Marshall by Brian Coppola regarding Marshall's "Rediscovery of the Elements" articles. Marshall discusses his motivations for traveling to European locations where elements were discovered, his plans for the future, and other projects. digital.library.unt.edu/ark:/67531/metadc111173/
Manaccan Church, Lizard Peninsula, Cornwall, England.
Cover of The Hexagon bearing the image of the Church of Manacca in Cornwall, England. The church is a stone building with arched windows visible on two levels. A crenelated tower extends up from the nearest portion of the church. Tombstones are visible near the bottom of the image, with a fronded plant to the left. The picture has the logo of The Hexagon across its top, a photo of James and Virginia Marshall on the right, and a photo from an article about pharmaceutical jobs near the center. digital.library.unt.edu/ark:/67531/metadc111172/
Rediscovery of the Elements: Tellurium and Fata Baii (Fascebanya), Romania.
Article describing the discovery of the element tellurium. The authors travel through Romania to visit the Transylvanian mine where tellurium was discovered. They also visit the Brukenthal Museum of Sibiu and describe the museum's mineral collection. digital.library.unt.edu/ark:/67531/metadc111174/
Carbon-Hydrogen Bond Activation by Titanium Imido Complexes. Computational Evidence for the Role of Alkane Adducts in Selective C-H Activation
This article discusses carbon-hydrogen bond activation by Titanium imido complexes. Abstract: This paper reports calculations that probe the role of R (hydrocarbon) and R' (ligand substituent) effects on the reaction coordinate for C-H activation: Ti(OR')₂(=NR') + RH → adduct → transition state → (OR')₂Ti(N(H)R')(R). Compounds with R = H, Me, Et, Vy, cPr, Ph, Cy, Bz, and cubyl are studied using quantum (R' = H, SiH₃, SiMe₃) and classical (R' = SiᵗBu₃) techniques. Calculated geometries are in excellent agreement with data for experimental models. There is little variability in the calculated molecular structure of the reactants, products, and most interestingly, transition states as R and R' are changed. Structural flexibility is greatest in the adducts Ti(OR')₂(=NR')•••HR. Despite the small structural changes observed for Ti(OR')₂(=NR') with different R', significant changes are manifested in calculated electronic properties (the Mulliken charge on Ti becomes more positive and the Ti=N bond order decreases with larger R'), changes that should facilitate C-H activation. Substantial steric modification of the alkane complex is expected from R-R' interactions, given the magnitude of ∆Gadd and the conformational flexibility of the adduct. Molecular mechanics simulations of Ti(OSiᵗBu₃)₂(=NSiᵗBu₃)•••isopentane adducts yield an energy ordering as a function of the rank of the C-H bond coordinated to Ti that is consistent with experimental selectivity patterns. Calculated elimination barriers compare very favorably with experiment; larger SiH₃ and TMS ligand substituents generally yield better agreement with experiment, evidence that the modeling of the major contributions to the elimination barrier (N-H and C-H bond making) is ostensibly correct. Calculations indicate that weakening the C-H bond of the hydrocarbon yields a more strongly bound adduct. Combining the different conclusions, the present computational research points to the adduct, specifically the structure and energetics of the substrate/Ti-imido interaction, as the main factor in determining the selectivity of hydrocarbon (R) C-H activation. digital.library.unt.edu/ark:/67531/metadc107781/
Rediscovery of the Elements: Germanium
Article recounting the history of the element Germanium, including background material on mining in Germany and the isolation of Germanium by Clemens A. Winkler. Tourist information is provided regarding Winkler's laboratory and mines open to the public as museums. digital.library.unt.edu/ark:/67531/metadc111177/
Rediscovery of the Elements: Montpellier and Bromine
Article recounting the history of the element Bromine, including its discovery in the salt lagoons of the Montpellier region of France by Antoine-Jérome Balard. Maps of the region and of Balard's birthplace are presented. digital.library.unt.edu/ark:/67531/metadc111178/
Rediscovery of the Elements: Titanium, Manaccan, Cornwall, England
Article wherein James and Virginia Marshall visit Manaccan, England, to explore the site where titanium was first discovered. The area is described, as well as the man who discovered titanium, Reverend William Gregor. digital.library.unt.edu/ark:/67531/metadc111176/
Electronically Unsaturated Three-Coordinate Chloride and Methyl Complexes of Iron, Cobalt, and Nickel
This article discusses electronically unsaturated three-coordinate chloride and methyl complexes of iron, cobalt, and nickel. Abstract: Three-coordinate organometallic complexes are rare, especially with the prototypical methyl ligand. Using a hindered, rigid bidentate ligand (L), it is possible to create 12-electron methyliron (II) and 13-electron methylcobalt (II) complexes. These complexes are thermally stable, and ¹H NMR spectra suggest that the low coordination number is maintained in solution. Attempts to create the 14-electron LNiCH₃ led instead to the three-coordinate nickel(I) complex LNi(THF). Single crystals of LMCH₃ are isomorphous with the new three-coordinate chloride complexes LNiCl and LCoCl. Along with the recently reported LFeCl (Smith, J.M.; Lachicotte, R.J.; Holland, P.L. Chem Commun. 2001, 1542), these are the only examples of three-coordinate iron(II), cobalt(II), and nickel(II) complexes with terminal chloride ligands, enabling the systematic evaluation of the effect of coordination number and metal identity on M-Cl bond lenghts. Electronic structure calculations predict the ground states of the trigonal complexes. digital.library.unt.edu/ark:/67531/metadc107783/
Rediscovery of the Elements: Thorium
Article recounting the discovery of the element Thorium in Norway by Hans Morten Thrane Esmark. Maps and tourist information regarding the area are provided. digital.library.unt.edu/ark:/67531/metadc111179/
A Priori Assessment of the Stereoelectronic Profile of Phosphines and Phosphites
This article discusses research that has demonstrated the utility of a rigorously calibrated, molecular mechanics/semiempirical quantum mechanical protocol for developing stereoelectronic (Tolman) maps for phosphine ligands. A computational analysis of alkyl and aryl phosphines in common usage suggests that these ligands are quite similar stereoelectronically. A noticeable gap int he Tolman map for common phosphines is observed for large, electron-poor phosphines. Several candidates meeting these criteria were identified, the most promising of which is P(t-C₄F₉)₃. Phosphines in which the phosphorus participates in a ring, which comprise a very small subset of reported phosphines, have very interesting stereoelectronic properties, particularly those in which the ligating phosphorus is part of a three-membered ring. In terms of steric properties, the symmetric deformation coordinate proposed by Orpen and co-workers on the basis of crystallographic studies is calculated with sufficient accuracy using PM3(tm) to allow good confidence in predictions of novel phosphines. For quantification of the electronic properties of phosphines, the authors analyzed changes in the CO stretching frequency upon changing the ancillary phosphine ligands. digital.library.unt.edu/ark:/67531/metadc107782/
Rediscovery of the Elements: Althofen, Austria and Auer von Welsbach
Article recounting a visit to sites related to Baron Carl Auer von Welsbach, discoverer of lutetium and ytterbium and inventor of the incandescent "Welsbach mantle." Maps and tourist information are provided regarding the Welsbach Museum in Treibach, Austria. digital.library.unt.edu/ark:/67531/metadc111184/
Rediscovery of the Elements: Phosphoro di Bologna
Article recounting a visit to sites related to the discovery of phosphorescent stone (barium sulfide) in Bologna, Italy. Maps with driving directions are included. digital.library.unt.edu/ark:/67531/metadc111185/
Rediscovery of the Elements: Strontium
Article describing the history of the element strontium from its discovery in Strontian, Scotland, to its early uses in English medicine. Maps with directions and photographs of pertinent locations are included. digital.library.unt.edu/ark:/67531/metadc111186/
Jahn - Teller Distortion in the Phosphorescent Excited State of Three-Coordinate Au(I) Phosphine Complexes
This article discusses three-coordinate Au(I) phosphine complexes. Abstract: DFT calculations were used to optimize the phosphorescent excited state of three-coordinate [Au(PR3)3]+ complexes. The results indicate that the complexes rearrange from their singlet ground-state trigonal planar geometry to a T-shape in the lowest triplet luminescent excited state. The optimized structure of the exciton contradicts the structure predicted based on the AuP bonding properties of the ground-state HOMO and LUMO. The rearrangement to T-shape is a Jahn-Teller distortion because an electron is taken from the degenerate e' (5dxy, 5dx2-y2) orbital upon photoexcitation of the ground-state D3h complex. The calculated UV absorption and visible emission energies are consistent with the experimental data and explain the large Stokes' shifts while such correlations are not possible in optimized models that constrained the exciton to the ground-state trigonal geometry. digital.library.unt.edu/ark:/67531/metadc77134/
Rediscovery of the Elements: Gallium
Article describing the discovery of the element gallium. The authors travel to sites pertinent to the history of gallium, including the home and laboratory of its discoverer and the location from which it was procured. digital.library.unt.edu/ark:/67531/metadc111187/
Function of Conserved Residues of Human Glutathione Synthetase: Implications for the ATP-grasp Enzymes
This article discusses human glutathione synthetase. Abstract: Glutathione synthetase is an enzyme that belongs to the glutathione synthetase ATP-binding domain-like superfamily. It catalyzes the second step in the biosynthesis of glutathione from y-glutamylcysteine and glycine in an ATP-dependent manner. Glutathione synthetase has been purified and sequenced from a variety of biological sources; still, its exact mechanism is not fully understood. A variety of structural alignment methods were applied and four highly conserved residues of human glutathione synthetase (Glu-144, Asn-146, Lys-305, and Lys-364) were identified in the binding site. The function of these was studied by experimental and computational site-directed mutagenesis. The three-dimensional coordinates for several human glutathione synthetase mutant enzymes were obtained using molecular mechanics and molecular dynamics simulation techniques, starting from the reported crystal structure of human glutathione synthetase. Consistent with circular dichroism spectroscopy, the authors' results showed no major changes to overall enzyme structure upon residue mutation. However, semiempirical calculations revealed that ligand binding is affected by these mutations. The key interactions between conserved residues and ligands were detected and found to be essential for enzymatic activity. Particularly, the negatively charged Glu-144 residue plays a major role in catalysis. digital.library.unt.edu/ark:/67531/metadc75414/
Synthesis and Reactivity of a Coordinatively Unsaturated Ruthenium(II) Parent Amido Complex: Studies of X-H Activation (X = H or C)
This article discusses synthesis and reactivity of a coordinatively unsaturated Ruthenium(II) parent amido complex. The five-coordinate parent amido complex (PCP)Ru(CO)(NH2) (2) (PCP = 2,6-(CH2P-tBu2)2C6H3) has been prepared by two independent routes that involve deprotonation of Ru(II) ammine complexes. Complex 2 reacts with phenylacetylene to yield the Ru(II) acetylide complex (PCP)Ru(CO)(C≡CPh) (5) and ammonia. In addition, complex 2 rapidly activates dihydrogen at room temperature to yield ammonia and the previously reported hydride complex (PCP)Ru(CO)(H). The ability of the amido complex 2 to cleave the H-H bond is attributed to the combination of a vacant coordination site for binding/activation of dihydrogen and a basic amido ligand. Complex 2 also undergoes an intramolecular C-H activation of a methyl group on the PCP ligand to yield ammonia and a cyclometalated complex. The reaction of (PCP)Ru(CO)(CI) with MeLi allows the isolation of (PCP)Ru(CO)(Me) (8), and complex 8 undergoes an intramolecular C-H activation analogous to the amido complex 2 to produce methane and the cyclometalated complex. Determination of activation parameters for the intramolecular C-H activation transformations of 2 and 8 reveal identical ∆Hǂ {18(1) kcal/mol} with ∆Sǂ = -23(4) eu and -18(4) eu, respectively. Density functional theory has been applied to the study of intermolecular activation of methane and dihydrogen by (PCP')Ru(CO)(NH2) to yield (PCP')Ru(CO)(NH3)(X) (X = Me or H; PCP' = 2,6-(CH2-PH2)2C6H3). The results indicate that the activation of dihydrogen is both exoergic and exothermic. In contrast, the addition of a C-H bond of methane across the Ru-NH2 bond has been calculated to be endoergic and endothermic. The surprising endoergic nature of the methane C-H activation has been attributed to a large and unfavorable change in Ru-N bond dissociation energy upon conversion from Ru-amido to Ru-ammine. digital.library.unt.edu/ark:/67531/metadc77183/
Ligand-field effects for the 3p photoelectron spectra of Cr2O3
This article discusses ligand-field effects for the 3p photoelectron spectra of Cr2O3. A major reason for the departure of core level X-ray photoelectron spectra (XPS) of transition metal cations in oxides from the predictions of atomic models is shown to arise from ligand field splittings in the initial state of photoemission. This splitting often leads to a change in the spatial degeneracy of the initial state but the consequences of this for XPS have not been explicitly identified in prior work. Further changes arise from ligand field splittings in the core-hole final states. Results are reported for non-empirical, cluster model many body wavefunctions for the 3p XPS of Cr2O3. The agreement of the theoretical cluster model XPS with experiment is considerably improved over the pure atomic model. Furthermore, the treatment allows screening of the core hole through changes in the covalent character of the cluster orbitals. This is quite different from the usual description of screening in oxides within the framework of charge transfer configurations and it offers new insights into the role of charge transfer for satellite structure. digital.library.unt.edu/ark:/67531/metadc78328/
Cluster Core-Level Binding-Energy Shifts: The Role of Lattice Strain
This article discusses cluster core-level binding-energy shifts. Abstract: Our combined experimental and theoretical analysis of the shifts, with particle size, of core-level binding energies (BE's) of metal nanoparticles on insulating supports, shows that these shifts have an important initial state contribution arising, in large part, because of lattice strain. This contribution of BE shifts has not been recognized previously. Lattice strain changes the chemical bonding between the metal atoms and this change induces BE shifts. digital.library.unt.edu/ark:/67531/metadc78320/
Rediscovery of the Elements: Riddarhyttan, Sweden
Article describing the authors' tour through Riddarhyttan, Sweden, to visit the areas where cerium and cobalt were discovered. Maps and photographs of pertinent locations are located as well as a history of the area and the elements discovered there. digital.library.unt.edu/ark:/67531/metadc111193/
Riddarhyttan City Motif
First page of The Hexagon of Alpha Chi Sigma. The blue cover features a black and white drawing of a medieval knight wearing mail and a tunic with a cross motif. He is holding a shield that bears the image of a stone building with flames shooting out of the roof. The word "Riddarhyttan" appears on a banner above the knight's head. Two photographs are arranged to the left of the knight, with text and an image of a computer mouse to the right. The next page includes a table of contents, an editorial, and a list of contributors. Near the bottom of this second page is a piece labeled "On the Cover," which describes the previous page. digital.library.unt.edu/ark:/67531/metadc111192/
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