Thermochemical Quantities Research Articles

Characterization of anomeric differences of ammonium-bound monomeric and dimeric complex ions of 1α- and 1β-azido-pentofuranosyl derivates by kinetic method

Relative stabilities (Δ G c) of ammonium-bound monomers and dimers of anomeric β- d-pentofuranosyl 1α- and 1β-azide derivates are determinate using the kinetic method by measuring relative rates of competitive collision-induced dissociations of dimeric [ANH 4B] + and trimeric [A 2NH 4B] + or [ANH 4B 2] + cluster ions. Comparison between calculated ammonium affinities (AAs) and relative stabilities (Δ G c) of ammonium-bound monomers shows qualitative correlations between both thermochemical quantities, but in two examples the activation barrier differences of competitive fragmentation channels cause a large disparity between both thermochemical data. Therefore, the most stable ammonium-bound monomers of the anomeric lα- and lβ-2,3,5-tri- O-benzyl-β- d-arabino-pento-furanosyl azides possess the lowest ammonium affinities and the highest relative stabilities. Two different relative stabilities measured for the same ammonium-bound homo- or hetero-dimers indicate dissimilar activated barriers of trimers transition states for dimer formations. The activated barriers of trimers depend on the relative stabilities of ammonium-bound monomer within the trimeric cluster ions.

The ionization energy of methylene (CH2) from a rotationally resolved photoelectron spectrum and its thermochemical implications

The adiabatic ionization potential of methylene has been determined to be 83772±3 cm−1 from a rotationally resolved photoelectron spectroscopic study of the CH2+ X̃ 2A1 (0,0,0)←CH2 X̃ 3B1(0,0,0) transition. This value was used to determine thermochemical quantities such as the 0 K dissociation energy of the ketene cation in CO and CH2+ D0(CH2=CO+)=33202±7 cm−1, the 0 K dissociation energy of the methyl radical D0(CH2–H)=38179±49 cm−1, the 0 K dissociation threshold of methane in CH2 and H2 D0(CH2–H2)=38232±50 cm−1 and the 0 K enthalpy of formation of CH2 ΔfH⊖(CH2,T=0 K)=390.73±0.66 kJ mol−1.

An accurate description of the ground and excited states of SiH

The astrophysical importance of the SiH radical has motivated significant experimental and theoretical work. However, only the X 2Π and A 2Δ states of SiH have been extensively investigated experimentally, while the study of higher excited states is rather limited. From a theoretical point of view, most of the studies have been focused on spectroscopic and thermochemical quantities of the ground state. The lack of accurate spectroscopic parameters (re,De,ωe,ωexe,αe,D̄e,Te) pertaining to higher excited states was the driving force of the present work, in line with our previous study of the isovalent CH molecule [A. Kalemos, A. Mavridis, and A. Metropoulos, J. Chem. Phys. 111, 9536 (1999)]. Using the multireference configuration interaction approach coupled with very large correlation-consistent basis sets, we have constructed potential energy curves for 18 molecular states correlating to Si(3P,1D,1S,5S,3P,1P)+H(2S). At the same level, the potential energy curve of the ground SiH+ state (X 1Σ+) has also been constructed. We report total energies, dissociation energies, and the usual spectroscopic constants for Si28–1,2H and for all states studied. Most of our results are in excellent agreement with existing experimental values. In particular, we believe that our dissociation energy for the X state, De=73.28 kcal/mol, is the most reliable reported so far in the literature.

Thermodynamic Calculation of Phase Diagram in the Bi-In-Sb Ternary System

A thermodynamic description of the Bi–In–Sb ternary system of lead-free solder alloys using the CALPHAD (Calculation of Phase Diagram) method is presented. Phase equilibria information such as vertical sections, liquidus projection and thermochemical quantities were calculated and compared with the experimental data. The calculated and experimental data are in excellent agreement in most cases.

Thermodynamic calculation of the In–Sn–Zn ternary system

A thermodynamic description of the In–Sn–Zn ternary system, which is of technical importance to optimize lead-free solder alloys, is presented using the CALPHAD method. Phase equilibria, such as isothermal and vertical sections, liquidus projection and mole fractions of the phase constitution, and thermochemical quantities were calculated and compared with the experimental data. They are in excellent agreement in most cases.

Relocalization in floppy free radicals: the OCNO and OCCHO isoelectronic series.

A survey of the XCNY and XCCHY radicals with X and Y = CH(2), NH, and O has been carried out by ab initio QCISD/6-311G(d,p) calculations to assess the impact of low-lying excited electronic states on the molecular dynamics. Multiple canonical structures may be drawn for each of these structural formulas, with the principal competition for most stable configuration between a (2)A' form with four electrons in a" orbitals and a (2)A" form with five a" electrons. Other low-lying configurations may include a 5a" state with nominally pentavalent nitrogen and a 6a" state. Optimized geometries and harmonic frequencies were evaluated for the lowest-energy minima on the potential energy surfaces. Localized unpaired electron density causes the 4a" state to be the most stable for (NH)CCHO and OCCHY, whereas allylic resonance stabilization favors the 5a" state for all other radicals in the set. For five of the 18 molecules studied, secondary minima (excluding conformers) are found within 30 kJ mol(-)(1) of the most stable state at the QCISD/6-311G(d,p) level, suggesting that photolysis or pyrolysis of parent compounds may result in multiple isomers of the resulting reactive intermediates. Predicted equilibrium geometries, approximate thermochemical quantities for dissociation of the central bond, and selected spectroscopic parameters are presented for all 18 structural formulas. Convergence tests were also performed for the glyoxallyl radical (OCCHO) to resolve discrepancies between single- and multireference post-SCF results. These tests find that extension of the MCSCF methods to include sigma bonding orbitals or virtual-orbital CI brings MCSCF relative energies into agreement with results from standard single-reference CI and CC methods. Relative configurational energies evaluated at Hartree-Fock levels routinely differ from post-SCF values by 30 kJ mol(-1) or more.

Transition Metal Carbides. Dependence of some thermochemical quantities on the chemical nature of reactants and stoichiometry of the product

Linear relationship were found among the plots of ΔHTad and ΔG0Tadvs. the adiabatic reaction temperature (Tad) for transition metals carbides and silicides characterised by the same stoichiometry. The slope of the straight line depends on the product stoichiometry.

Theoretical study of cyanophosphapropyne (NCCP), isocyanophosphapropyne (CNCP) and their isomers: stability and properties

Exploration of portions of the (C2NP) potential energy surface using both B3LYP and CCSD(T) methods with the 6-311++G(d,p) basis set, indicates that cyanophosphapropyne NC–CP is the most stable isomer, followed by isocyanophosphapropyne CN–CP, the linear azaphosphadicarbon CCNP and the bent isocyanophosphavinylidene NC–PC. These higher-lying isomers are relatively stable with respect to unimolecular rearrangements and fragmentations. Their molecular properties including the geometries, rotational constants, vibrational wavenumbers, 13C and 31P NMR chemical shifts, heats of formation, excitation and ionisation energies, proton and electron affinities were determined. For the thermochemical quantities, CCSD(T) and EOM-CCSD computations with larger 6-311++G(3df,2p) and aug-cc-pVTZ basis sets were employed. It is remarkable that as a substituent, the phosphaethynyl –CP moiety exerts a remarkably strong electron donor effect which markedly enhances the electron density and basicity of the attaching moieties. Thus, the proton affinities at N in NC–CP and at C in CN–CP are significantly increased thanks to the CP effect whereas those at C of CP are, as a consequence, strongly reduced.

Thermochemical ladders: scaling the ramparts of gaseous ion energetics

The use of multiple overlapping gaseous ion–molecule equilibrium measurements to construct thermochemical ladders is reviewed. The types of relative thermochemical data, which may be obtained, are summarized and the types of measurements required to convert the relative equilibrium affinity scales to absolute thermochemical quantities are discussed. The important thermochemical scales produced include proton affinities, gas phase acidities, ionization energies, electron affinities, carbocation stabilities, hydrogen bond energies, Lewis acidities, and metal cation affinities.

Molecular Structure of the Nematic Liquid Crystals made by Hydrogen Bonded in Dimers Molecules

The dimer molecular structure of 4,n-alkyloxybenzoic acids displaying nematic and smectic C liquid crystal phases is investigated. AB INITIO molecular orbital calculations at HF/STO-3G level of theory are applied in order to calculate the molecule of in its closed dimer, open dimer and monomer states. The molecular geometry parameters (stereoparameters), the thermochemical quantities and molecular polarizability are calculated. The microscopical calculations give a base for development of a mean-field theory where the constituent molecule can not be approximated as a rigid rod.

Molecular Structure of the Nematic Liquid Crystals made by Hydrogen Bonded in Dimers Molecules

The dimer molecular structure of 4,n-alkyloxybenzoic acids displaying nematic and smectic C liquid crystal phases is investigated. AB INITIO molecular orbital calculations at HF/STO-3G level of theory are applied in order to calculate the molecule of in its closed dimer, open dimer and monomer states. The molecular geometry parameters (stereoparameters), the thermochemical quantities and molecular polarizability are calculated. The microscopical calculations give a base for development of a mean-field theory where the constituent molecule can not be approximated as a rigid rod.

Copper(II)-Assisted Enantiomeric Analysis of d,l-Amino Acids Using the Kinetic Method: Chiral Recognition and Quantification in the Gas Phase

Chiral recognition of d- and l-amino acids is achieved and mixtures of enantiomers quantified in the gas phase, using the kinetics of competitive unimolecular fragmentations of trimeric Cu(II)-bound complexes. Singly charged copper(II)−amino acid cluster ions [CuII(A)(ref*)2−H]+ (A = amino acid; ref* = chiral reference ligand, selected from among the natural α-amino acids) undergo competitive collision-induced dissociation (CID) in a quadrupole ion trap to form the dimeric complexes [CuII(A)(ref*)−H]+ and [CuII(ref*)2−H]+. The abundance ratio of these fragment ions depends strongly on the stereochemistry of the ligands in the precursor [CuII(A)(ref*)2−H]+ complex ion and specifically on the chirality of the analyte amino acid. The chiral selectivity, the ratio of the two fragment ion abundances for the complex containing one enantiomer of analyte expressed relative to that for the fragments of the corresponding complex containing the other enantiomer, ranges from 0.47 to 11. An energy quantity, Δ(ΔCuIIBDE), is predicted and shown to serve as a thermochemical indicator of chiral discrimination; its value is calculated from the fragment ion abundance ratios using the kinetic method of estimating thermochemical quantities from the kinetics of cluster ion dissociation. Large chiral distinctions are observed with all of the natural chiral α-amino acids, except cysteine and arginine, by appropriate choice of the reference ligand. The Δ(ΔCuIIBDE) values range from −2.2 to 6.9 kJ/mol. Amino acids with aromatic substituents display the largest chiral distinction, which is consistent with ligand exchange chromatographic results for analogous systems. The structures of the fragment Cu(II) complexes are discussed in the light of the CID behavior of related compounds. The interactions within these ions that might contribute to chiral recognition are rationalized to account for the observed chiral effects. The sensitive nature of the methodology and the linear relationship between the logarithm of the fragment ion abundance ratio and the optical purity, which is intrinsic to the kinetic method, allows mixtures to be analyzed for small enantiomeric excess (ee) by simply recording ratios of fragment ion abundances in a mass spectrum.

Prediction of the carrier-mediated cation flux through polymer inclusion membranes via fundamental thermodynamic quantities: complexation study of bis(dodecyloxy)calix[4]arene-crown-6 with alkali metal cations

In a systematic study of alkali metal cation transport through a polymer inclusion membrane (PIM) with calixcrown carriers, a model that postulates diffusion-limited flux successfully describes PIM transport behavior. The model developed herein is based on an ion-pair extraction equilibrium at the PIM interface and provides a convenient tool for the quantitative understanding and interpretation of the transport data. Cation permeability coefficients can be easily determined and used for the quantitative correlation of fluxes employing a range of aqueous and PIM compositions. The described approach can be readily extended to competitive-transport experiments. Transport of a single cation as well as several cations in a competitive experiment was related to the stability constants of the carrier-metal complexes and the Gibbs energies of ion partitioning between source and membrane phases. The expected dependence of Cs+ ion transport on the Gibbs energy of anion partitioning was validated in a series of 8 univalent anions. Accordingly, the permeability coefficient once determined for a given metal salt, carrier, and PIM provides the basis for the prediction of the transport fluxes under different initial conditions if the thermochemical quantities which govern the complexation and distribution of the metal species into the membrane phase are known or can be estimated. In support of these efforts, a calorimetric study was carried out to obtain thermodynamic parameters for the complexation of bis(dodecyloxy)calix[4]arene-crown-6 with alkali metal ions in acetonitrile.

The temperature dependence of absolute gas phase acidities

Using both ab initio and semiempirical calculations, the temperature dependence of the thermochemical cycle relating gas phase enthalpies of acidity to electron affinities and bond dissociation energies is examined. In almost all cases, the effect of temperature is less than the uncertainties in the thermochemical quantities themselves.

Rearrangement and fragmentation pathways of [C 3H 7Z] + ions (Z = NH and S): are ion–neutral complexes important?

High level ab initio calculations at the G2(ZPE = MP2) level have been used to characterize the potential energy surfaces for rearrangement/fragmentation of various [C 3H 8N] + and [C 3H 7S] + isomers. In contrast to the behavior in the corresponding [C 3H 7O] + system, it is found that ion–neutral complexes are only of minor importance in determining the fragmentation characteristics. Either dissociation of such complexes occurs too fast due to a large barrier to their formation ([C 3H 8N] + system), or alternative lower-energy rearrangement routes that do not involve ion–neutral complexes are available ([C 3H 7S] + system). Calculated thermochemical quantities such as heats of formation and reaction barriers are found to be in reasonable agreement with experimental results. Metastable ion product abundances and results of both deuterium- and 13C-labeling experiments are rationalized in terms of the calculated potential energy surfaces and rate constants obtained using Rice-Ramsperger-Kassel-Marcus theory.

Structures, thermochemistry, and electron affinities of the germanium fluorides, GeFn/GeFn−(n=1–5)

Four different density functional methods have been employed to study the molecular structures, electron affinities, and first dissociation energies of the GeFn/GeFn−(n=1–5) molecules. The three types of electron affinities reported in this work are the adiabatic electron affinity (EAad), the vertical electron affinity (EAvert), and the vertical detachment energy (VDE). The first Ge–F dissociation energies De(Fn−1Ge–F), De(Fn−1Ge−–F), and De(Fn−1Ge–F−) of the GeFn/GeFn− species are also reported. The basis set used in this work is of double-ζ plus polarization quality with additional s- and p-type diffuse functions, labeled as DZP++. Among the four density functionals used in this work, the BHLYP (which includes 50% exact exchange) method determines the molecular structures in best agreement with experiment, while other methods generally overestimated bond lengths. The theoretical Ge–F bond distances for the GeFn−(n=1–4) anions are predicted about 0.1 Å longer than their corresponding neutral counterparts. No significantly bound minimum was found for the neutral GeF5 molecule, while a D3h structure was confirmed to be a genuine minimum for ionic GeF5−. Based on the precise experimental result of EAad(GeF), the adiabatic electron affinities obtained at the DZP++ BHLYP level of theory are again most reliable, with the BLYP method being next. The DZP++ BHLYP adiabatic electron affinities are 1.02, 0.85, 3.72, and 1.46 eV for GeF, GeF2, GeF3, and GeF4, respectively. The vertical detachment energy of GeF5− is predicted to be very large. The substantial value (1.46 eV) of the EA for GeF4 is especially interesting, in that the valence isoelectronic species SiF4 does not bind an electron. A number of experimental electron affinities and experimental thermochemical quantities appear to be error.

Thermochemical Characterization of Seaborgium Compounds in Gas Adsorption Chromatography

The chemical characterization of the element seaborgium (Z = 106) requires fast experiments such as gas adsorption chromatographic separations in quartz columns with single atoms in a clearly defined chemical state. The maximum separation time for such experiments of only about 10 s is defined by the relatively short half-lives of the currently known longest-lived isotopes 265Sg and 266Sg. To establish optimum experimental parameters, the required thermochemical quantities for chlorides, oxychlorides, and oxides of seaborgium were estimated by extrapolation. On the basis of these results, the stability and volatility of these compounds could be calculated. By use of empirical correlations, the thermochemical constants of the adsorption processes were evaluated and the retention times calculated, taking into account the composition of the reactive carrier gas. The dioxydichloride of seaborgium proved to be the most suitable chemical state regarding its stability, volatility, and retention. Its standard sublimation enthalpy is expected to be between 125 and 144 kJ/mol (larger than that of WO2Cl2), resulting in an adsorption enthalpy between −97 and −108 kJ/mol. If the chemistry of seaborgium significantly deviates from the above predicted behavior, then this could be attributed to the possible influence of relativistic effects.

Combustion effects on turbulence in a partially premixed supersonic diffusion flame

Effects of chemical heat release on turbulence in a partially premixed diffusion flame are investigated using direct numerical simulation (DNS). The full three-dimensional time-dependent compressible Navier-Stokes equations are employed to simulate the coupling between supersonic turbulence and heat release from a one-step chemical reaction governed by the Arrhenius kinetics. Four reacting cases with increasing heat release and one nonreacting case have been studied. Combustion is found to produce strong coupling among fluctuations in velocity, pressure, density, and other thermochemical quantities. In the Reynolds stress budget, the pressure–strain term becomes dominant as the heat release increases. Despite its wave behavior and its alternate roles as a source and a sink, the pressure–strain serves to promote energy transfer from the streamwise to the transverse and spanwise directions, and from the diagonal to the off-diagonal components of the Reynolds stresses, thus reducing anisotropy. Moreover, the pressure–strain, which in the case of the turbulent kinetic energy budget can be split up into a mean pressure work and a pressure–dilatation, helps to convert chemical energy from combustion into turbulence energy, leading to the phenomenon of “combustion-generated turbulence.” Both the solenoidal dissipation and the dilatational dissipation in particular increase as the heat release increases, but their effects are secondary to those of the pressure–strain within the main combustion period. The mixing layer growth rate can be predicted directly through the integrated Reynolds stress generation, with the accuracy of prediction depending on the heat release rate and the Reynolds number. A simple mechanism for the interactions of combustion and turbulence is proposed, while the modeling difficulties involved are also outlined. Finally, the differences and similarities of heat release effects and compressibility effects are clarified.

Assessment of the Re–Ta binary system

Abstract Using the CALPHAD method, the thermodynamic properties of each phase of the Re–Ta system were optimized using the experimental information on the phase diagram and the thermochemical quantities. A consistent set of parameters is presented. The calculated phase diagram is compared with experimental data, and the agreement is satisfactory in most cases.

Use of calorimetric titration to determine thermochemical data for interaction of cations with mercapto-modified silica gel

Interactions occurring between mercapto-modified silica gel and silver, mercury, copper and zinc ions in ethanolic solutions have been investigated by using calorimetric titrations with direct determination of the reaction extension. A calorimetric technique is described for the simultaneous determination of thermal effects and quantities of cations that interact at each titration point. From this procedure, values of maximum capacity of interaction ( N s), differential interaction enthalpies at different coverages of the surface (Δ int(i) H m), integral energy of interaction ( Q mon) and molar integral enthalpy of interaction for a monolayer (Δ mon H m) have been obtained. The maximum capacity of interaction values are compared with those previously published for analogous interaction processes in aqueous solutions. Thermochemical quantities are discussed in terms of evaluation criterion of the acid–base interactions, and generally support the conclusion that the Pearson's hypothesis of hard and soft acids and bases is appropriated to describe the behavior of the interaction of the above-mentioned cations.