ΔHf Values Research Articles

Accurate Gas Phase Formation Enthalpies of Alloys and Refractories Decomposition Products.

Accurate gas phase formation enthalpies, ΔHf, of metal oxides and halides are critical for the prediction of the stability of high temperature materials used in the aerospace and nuclear industries. Unfortunately, the experimental ΔHf values of these compounds in the most used databases, such as the NIST-JANAF database, are often reported with large inaccuracy, while some other ΔHf values clearly differ from the value predicted by CCSD(T) methods. To address this point, in this work we systematically predicted the ΔHf values of a series of these compounds having a group 4, 6, or 14 metal. The ΔHf values in question were derived within a composite Feller-Dixon-Peterson (FDP) scheme based protocol that combines the DLPNO-CCSD(T) enthalpy of ad hoc designed reactions and the experimental ΔHf values of few reference complexes. In agreement with other theoretical studies, we predict the ΔHf values for TiOCl2, TiOF2, GeF2, and SnF4 to be significantly different from the values tabulated in NIST-JANAF and other sources, which suggests that the tabulated experimental values are inaccurate. Similarly, the predicted ΔHf values for HfCl2, HfBr2, HfI2, MoOF4, MoCl6, WOF4, WOCl4, GeO2, SnO2, PbBr4, PbI4, and PbO2 also clearly differ from the tabulated experimental values, again suggesting large inaccuracy in the experimental values. In the case when largely different experimental values are available, we point to the value that is in better agreement with our results. We expect the ΔHf values reported in this work to be quite accurate, and thus, they might be used in thermodynamic calculations, because the effects from core correlation, relativistic effects, and basis set incompleteness were included in the DLPNO-CCSD(T) calculations. T1 and T2 values were thoroughly monitored as indicators of the quality of the reference Hartree-Fock orbitals (T1) and potential multireference character of the systems (T2).

Bile salt induced solubilization of methylene blue: Study on methylene blue fluorescence properties and molecular mechanics calculation

Methylene blue (MB) is a hydrophobic drug molecule, having importance both as a staining reagent and pharmaceutical agent. MB is strongly fluorescent, with an emission peak at 686nm (λex 665nm). In the study, the possibility of MB as an extrinsic fluorophore to study the micellization behavior of bile salts (BSs) was carried out. Since BSs are drug delivery systems, the solubilization of hydrophobic MB drug molecule by BSs was achieved and the nature of association of MB with BS media, namely sodium cholate (NaC) and sodium deoxycholate (NaDC) was evaluated. Change in the photophysical properties of MB is monitored through fluorescence intensity and fluorescence anisotropy at emission peak, 686nm of MB. Molecular mechanics calculations were carried out to evaluate the MB–BS association. The estimated heat of formation, ΔHf values are –625.19kcal/mol for MB–NaC and –757.48kcal/mol for MB–NaDC. The photophysical study also revealed that MB reports the step-wise aggregation pattern of BSs media, as an extrinsic fluorescence probe.

Provenance, age constraints and metamorphism of Ediacaran metasedimentary rocks from the El Triunfo Complex (SE Chiapas, México): evidence for Rodinia breakup and Iapetus active margin

ABSTRACTMetasedimentary rocks from the El Triunfo Complex (Jocote Unit) in the southern Chiapas Massif (SE México) are constituted mainly by sillimanite-rich micaschist, locally intercalated with marble and calc-silicate rocks. Mafic rocks (now amphibolite) intruded the sequence prior to deformation and folding. Peak metamorphic conditions are estimated by geothermobaromerty at ~6.0 kbar and ~650ºC. The timing of the metamorphic event is dated by LA-MC-ICPMS analysis on zircon rims at 438+23/–12 Ma. Furthermore, detrital zircon grains yield mainly Stenian–Tonian and minor early Mesoproterozoic ages, indicating provenance from Grenville-type orogens (such as Oaxaquia) and some older cratonic sources. The 87Sr/86Sr values of 0.70775–0.70777 and the δ13C values from +1.9‰ to +2.7‰ in associated calcite marble define the time of deposition between 600 and 580 Ma. Geochemical markers from metapelite samples (such as La/Th > 3.94, La/Sc > 3.72, Th/U > 8.19, Th/Co > 0.42 and CIA = 74 to 83), as well as Sm–Nd isotope data (εNdi = −8.1 to −4.0, TDM(Nd) = 1.65–1.32 Ga) suggest weathering of Mesoproterozoic felsic rocks during temperate to warm climate. Furthermore, Zr/Sc values (9.1–21.0), chondrite-normalized REE patterns [La/Yb]N = 10.3–23.3, Eu/Eu* < 0.64), and ΔHf values (1.98–10.02) are indicative of pelagic and zircon-depleted sediments of a passive margin. The results suggest that the Jocote Unit was deposited during the opening of the Eastern Iapetus Ocean in the Ediacaran Period. This is the first evidence for Rodinia breakup in southern México. Besides that, the Ordovician tectonothermal event is probably related to compression during subduction and accretion along the western margin of Gondwana.

Molecular Orbital Based Design Guideline for Hypergolic Ionic Liquids

AbstractCurrently, monomethyl hydrazine is the most widely used hypergolic rocket fuel. Due to its high toxic vapor, there is a thrust towards developing low‐toxic hypergolic fuels. Ultra‐low vapor pressure ionic liquids are one such potential category of fuels. However, designing ionic liquid with ignition delay comparable to monomethyl hydrazine is a challenge, because fundamental understanding of the hypergolic nature of ionic liquids is far from clear. This work used the computed energy gap values between the highest occupied molecular orbitals (HOMO) of the anions for a series of ionic liquids and the lowest occupied molecular orbital (LUMO) of HNO3, and variation in the computed relative heats of formation, ΔHf, of these anions to develop correlations to predict hypergol activity between an ionic liquid fuel and nitric acid as the oxidizer. The observed trends in HOMOLUMO energy gap and ΔHf values can be used successfully to verify not only hypergolicity of known systems but also the lack of this phenomenon in OH− and BF4− based ionic liquids. It was shown that through suitable substitution of electron withdrawing or electron donating groups in the anion, the energy gap and the ΔHf values could be tailored into an optimal range that would have a high probability for the new system to exhibit hypergolic reactivity. To validate our method, we suggest herein new ionic liquid structures for synthesis and experimental screening.

Defect Chemistry of Singly and Doubly Doped Ceria: Correlation between Ion Transport and Energetics

Earlier studies have shown a strong correlation between the enthalpy of formation, ΔHf,ox, and the ionic conductivity, σi, near room temperature in doped ceria systems, which are promising solid electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The present work demonstrates that this correlation holds at the operating temperature of IT-SOFCs, 600–700 °C. Solid solutions of Ce1−xNdxO2−0.5x, Ce1−xSmxO2−0.5x, and Ce1−xSm0.5xNd0.5xO2−0.5x are studied. The ΔHf,ox at 702 °C is determined by considering the excess heat content between 25 and 702 °C combined with the value of ΔHf,ox at 25 °C. Both σi and ΔHf,ox show maxima at x=0.15 and 0.20 for the singly and doubly doped ceria, respectively, suggesting that the number of mobile oxygen vacancies in these solid solutions reaches a maximum near those compositions. An increase in temperature results in a shift of the maximum in both ΔHf,ox and σi towards higher concentrations. This shift results from a gradual increase in dissociation of the defect associates.

Defect chemistry of singly and doubly doped ceria: correlation between ion transport and energetics.

Earlier studies have shown a strong correlation between the enthalpy of formation, ΔHf,ox , and the ionic conductivity, σi , near room temperature in doped ceria systems, which are promising solid electrolytes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). The present work demonstrates that this correlation holds at the operating temperature of IT-SOFCs, 600-700 °C. Solid solutions of Ce1-x Ndx O2-0.5x , Ce1-x Smx O2-0.5x , and Ce1-x Sm0.5x Nd0.5x O2-0.5x are studied. The ΔHf,ox at 702 °C is determined by considering the excess heat content between 25 and 702 °C combined with the value of ΔHf,ox at 25 °C. Both σi and ΔHf,ox show maxima at x=0.15 and 0.20 for the singly and doubly doped ceria, respectively, suggesting that the number of mobile oxygen vacancies in these solid solutions reaches a maximum near those compositions. An increase in temperature results in a shift of the maximum in both ΔHf,ox and σi towards higher concentrations. This shift results from a gradual increase in dissociation of the defect associates.

Scheme of thermal compression of hydrogen (TCH) using MmNi4.25Al0.75 recovered with ethyl alcohol and handled under non protective atmospheres

A MmNi4.25Al0.75 intermetallic was obtained by low energy mechanical alloying and low temperature heating at 600 °C for 24 h under Ar. The intermetallic was recovered from milling chamber using ethyl alcohol, dried, stored and handled under air at room conditions. Structure was characterized by XRD. A maximum stability temperature of 160 °C was obtained from non-isothermal DSC measurement under air. The kinetics of oxidation at 200 °C was analyzed. A maximum reaction degree (α = 0.1) was obtained after 2500 s of treatment. The hydrogen sorption properties of samples were studied by volumetric measurements. Hydrogen maximum mass percent capacity (mass %) was reached in less than 300 s. The thermodynamic sorption properties were measured. Values of ΔHf = −29 ± 2 kJ mol−1 and ΔSf = 197 ± 10 J mol−1 K−1 were obtained for absorption process and ΔHd = 28 ± 2 kJ mol−1 and ΔSd = 189 + 10 J mol−1 K−1 were obtained for desorption process. From these results, a one-stage of thermal compression of hydrogen is proposed with a standard compression ratio (Rc) of 5.71 in the 25–80 °C range.

The application of Pettifor structure maps to binary metal hydrides

The aim of this paper is the application of Pettifor stability structure maps to MHx binary metal (M) hydrides in accordance with their hydrogenation properties. The formation enthalpy (ΔHf) of selected hydrides, their stoichiometry (MHx) and hydrogen capacity (Hcap) were extracted from the literature, where these values were either measured or calculated using various models. A comparison of the enthalpy values obtained from experiments or calculations based on the Miedema, Born–Haber and Energetic models is also presented. An assessment of the effect of the ΔHf values obtained by various methods on the structure stability maps was performed by plotting the enthalpy vs. the Mendeleev number M and the value of the chemical scale, χ. Moreover, based on two- and three-dimensional Pettifor plots, the enthalpy of hydride formation and hydrogen capacity of selected hydrides are presented with respect to the hydride stoichiometry, Hx, and the Mendeleev number M. Higher values of M indicate less hydride stability and, thus, larger stability variations. The metal hydrides with only one hydrogen atom (x = 1) and M from 0 to 21 exhibit stability that is adequate for practical applications. For M from 21 to 53, all hydrides exhibit high stability regardless of Hx. For M from 53 to 102, all hydrides exhibit large variations in stability. Thus, the hydrides with x = 1–3 exhibit either high stability or stability that is adequate for practical applications, and those with x = 4 are highly unstable hydrides.

Comparative stability of isomeric (565)macrotricyclic chelates of 3d-elements formed in the systems M(II)-thiosemicarbazide-formaldehyde according to DFT B3LYP data

Comparative stability of three types of (565)macrotricyclic chelates of Mn(II), Fe(II), Co(II), Ni(II), Cu(II), and Zn(II) with chelate nodes MN2S2 and MN4 have been analyzed by means of hybrid density functional method B3LYP with the 6-31G(d) basis set using GAUSSIAN-09 software. These chelates may potentially be formed in template interaction between gelatin-immobilized hexacyanoferrate(II) of respective metal ions M(II), thiosemicarbazide H2N-HN-C(=S)-NH2, and formaldehyde H2C=O. It has been demonstrated that for all M(II) considered, the complex with chelate nodes MN2S2 is more stable. Key structural parameters of the complexes (bond lengths, bond and torsion angles) are presented. It is noted that pseudotetrahedral coordination of ligand donor centers around M(II) is typical for Mn(II), Co(II), Cu(II), and Zn(II), whereas for Fe(II) and Ni(II) it is almost planar. Values of standard enthalpy ΔHf,2980 and standard Gibbs energy ΔGf,2980 are positive for nearly all the complexes studied.

A DSC study of some biomaterials relevant to pharmaceutical industry

The present investigation concerns with thermal analysis of biomaterials such as Chitosan, Xanthan gum, Guar gum Pectin, Karaya gum, Sodium alginate, and Psyllium husk mucilage. The DSC curve of the natural polymers showed its specific endothermic peaks and ΔHf values, and these will be helpful in determining the fitness of the polymers with active drug. The drug and natural polymer compatibility study is now one of the recognized methods for preformulation step in pharmaceutical drug development. All the natural polymers showed endothermic peak below 150 °C, and this make them suitable with a wide class of therapeutic drugs.

Hydrogen absorption in CexGd1−x alloys

The effect of alloying on the thermodynamics of hydrogen absorption was studied for CexGd1−x alloys (0≤x≤1) at temperatures between 850K and 1050K in the 1–10−4Torr pressure range. The temperature-dependent hydrogen solubilities and plateau pressures for hydride formation were obtained from hydrogen absorption isotherms. The terminal hydrogen solubility (THS) at a given temperature changes in a monotonic way as a function of x. It is approximately three times higher in Gd, than in Ce, throughout the investigated temperature range. This monotonic behavior is opposed to that of many other substitutional alloys, for which the hydrogen terminal solubility increases with increasing solute concentrations. The enthalpies, ΔHf, and the entropies, ΔSf, of the dihydride formation exhibit a pronounced and broad negative minimum starting at x≈0.15, yielding the most negative ΔHf values ever found for metal hydrides. On the other hand, the enthalpies and entropies of ideal solution display a positive trend at x=0.15 and x=0.3. Both behaviors are considered in view of a reported distinct variation of the CexGd1−x hardness as a function of x. The particular compositional variations of the entropies of solution and formation as a function of x reflect most likely the vibrational properties of the hydrogen atoms in the metal matrices.

Application of quantum chemical descriptors into quantitative structure-property relationship models for prediction of the photolysis half-life of PCBs in water

Quantitative structure-property relationship (QSPR) models were developed for prediction of photolysis half-life (t1/2) of polychlorinated biphenyls (PCBs) in water under ultraviolet (UV) radiation. Quantum chemical descriptors computed by the PM3 Hamiltonian software were used as independent variables. The cross-validated Qcum2 value for the optimal QSPR model is 0.966, indicating good prediction capability for lg t1/2 values of PCBs in water. The QSPR results show that the largest negative atomic charge on a carbon atom (QC−) and the standard heat of formation (ΔHf) have a dominant effect on t1/2 values of PCBs. Higher QC− values or lower ΔHf values of the PCBs leads to higher lg t1/2 values. In addition, the lg t1/2 values of PCBs increase with the increase in the energy of the highest occupied molecular orbital values. Increasing the largest positive atomic charge on a chlorine atom and the most positive net atomic charge on a hydrogen atom in PCBs leads to the decrease of lg t1/2 values.

Theoretical determination of the properties of Methanesulfinic and Methanesulfonic acids

The structural, spectroscopic and thermochemical properties of two sulfur species of atmospheric interest, methanesulfinic (CH3S(O)OH, MSIA) and methanesulfonic (CH3S(O)2OH, MSA) acids, were determined at the CCSD(T)/CBS (coupled-cluster with single and double and perturbative triple excitations/complete basis set) level of theory. Two stable conformers were found to the methanesulfinic acid, and the determined average value for the enthalpy of formation (ΔHf) was -337.2 kJ mol-1. For methanesulfonic acid, the determined enthalpy of formation was 566.2 kJ mol-1. By carefully choosing of the chemical reactions and considering the existence of two MSIA conformers, this investigation reports the most accurate ΔHf values available to date for these species

Anticipating π-Bond Dispositions in Cyclic, Even, Classical Hydrocarbons

A new technique, which employs π-bond placement coefficients, is presented. That technique, in conjunction with a few parameters that are readily available from traditional Hückel theory, permits one to systematically anticipate π-bond placements for optimized lowest-lying singlet states. One may then foresee the relative magnitudes of calculated ΔHf values for selected sets of structural isomers. Structural predictions are compared with parameterization method 3 (PM3) calculations, density functional theory calculations and experimental results. Reasonable expectations for the most stable structure narrow the choice of molecules that may then be scrutinized by more exact computations or by experiment.

ChemInform Abstract: Keto-Enol Tautomerism of Dithiomalondialdehyde and Dithioacetylacetone. A Reinvestigation Using Version 2.1 of the AM1 Method.

The keto–enol tautomerism of dithiomalondialdehyde (DTMDA) and dithioacetylacetone (DTAA), recently studied by means of the first version of the AM1 program using MNDO parameters for sulphur atoms, has been reinvestigated by version 2.1 of the same AM1 program, in which appropriate parameters for sulphur have been inserted. Bond lengths and bond angles of the minimum energy conformations of DTMDA and DTAA are comparable with the previous ones but the ΔHf values are remarkably lower. Some tautomers show an S⋯S distance very close to the value of the van der Waals radius of S, whilst the C2v hydrogen-bonded structures of both compounds (i.e. the H-centred tautomer) appear to be, strangely, the most stable ones. The hydrogen bond strength is estimated roughly to lie in the range 22–25 kJ mol–1. The new results, on the whole, appear to be worse than predicted by the old AM1 program and the hydrogen bond is overestimated with respect to the well known weakness of the S—H⋯S bridge.

Hybrid‐filler filled polypropylene/(natural rubber) composites: Effects of natural weathering on mechanical and thermal properties and morphology

AbstractComparison was made between the properties of recycled newspaper (RNP)/carbon black (CB) and recycled newspaper (RNP)/silica hybrid filled polypropylene (PP)/natural rubber (NR) composites. The properties studied were mechanical, thermal, and morphological. These composites were also subjected to natural weathering, i.e., the tropical climate in Penang, Malaysia, for 6 months. The incorporation of CB and silica at all weight ratios of RNP/CB and RNP/silica hybrid gave increases in tensile strength, elongation at break (EB), Young's modulus, melting temperature (Tm), heat of fusion of composites (ΔHf(com)), crystallinity of composites (Xcom), and the crystallinity of PP (XPP). As expected, the tensile properties (except for Young's modulus), Tm, ΔHf(com), Xcom, and XPP of the composites exhibited lower values after weathering than before weathering. The extent of chemical degradation was studied by Fourier transform infrared spectroscopy, and the results showed the formation of several functional groups, i.e., hydroxyl, hydroperoxide, vinyl, carboxylic acid, and ketone. At the same filler weight ratio, the composites filled with RNP/CB hybrid showed higher values of tensile strength and EB but lower values of Young's modulus, ΔHf(com), XPP, and XPP, as compared to those with the RNP/silica hybrid under weathering conditions. The good retention in tensile properties indicated that the replacement of RNP by CB and silica improved the weatherability performance of the PP/NR composites. J. VINYL ADDIT. TECHNOL., 2008. © 2008 Society of Plastics Engineers

Computationals-Block Thermochemistry with the Correlation Consistent Composite Approach

The correlation consistent composite approach (ccCA) is a model chemistry that has been shown to accurately compute gas-phase enthalpies of formation for alkali and alkaline earth metal oxides and hydroxides (Ho, D. S.; DeYonker, N. J.; Wilson, A. K.; Cundari, T. R. J. Phys. Chem. A 2006, 110, 9767). The ccCA results contrast to more widely used model chemistries where calculated enthalpies of formation for such species can be in error by up to 90 kcal mol-1. In this study, we have applied ccCA to a more general set of 42 s-block molecules and compared the ccCA DeltaHf values to values obtained using the G3 and G3B model chemistries. Included in this training set are water complexes such as Na(H2O)n+ where n = 1 - 4, dimers and trimers of ionic compounds such as (LiCl)2 and (LiCl)3, and the largest ccCA computation to date: Be(acac)2, BeC10H14O4. Problems with the G3 model chemistries seem to be isolated to metal-oxygen bonded systems and Be-containing systems, as G3 and G3B still perform quite well with a 2.7 and 2.6 kcal mol-1 mean absolute deviation (MAD), respectively, for gas-phase enthalpies of formation. The MAD of the ccCA is only 2.2 kcal mol-1 for enthalpies of formation (DeltaHf) for all compounds studied herein. While this MAD is roughly double that found for a ccCA study of >350 main group (i.e., p-block) compounds, it is commensurate with typical experimental uncertainties for s-block complexes. Some molecules where G3/G3B and ccCA computed DeltaHf values deviate significantly from experiment, such as (LiCl)3, NaCN, and MgF, are inviting candidates for new experimental and high-level theoretical studies.

Accurate heat of formation for fully hydridedLaNi5 via the all-electron full-potential linearized augmented plane wave approach

The heat of formation, ΔHf, for La2Ni10H14, an important property for hydrogen storage, was remarkably overestimated in calculations, and has discouraged the use of first principles total energy methods in the search for novel metal hydrides. Here, we employ the all-electron full-potential linearized augmented plane wave method within both the generalized gradient approximation (GGA) and the local density approximation (LDA), along with a unique treatment of the total energy of the H2 molecule, which plays a critical role in ΔHf determinations. The calculated electronic properties indicate that charge transfer from the interstitial region to the H atoms stabilizes LaNi5 hydride. We find the calculated ΔHf (−31.3 kJ∕mol H2) within GGA is in excellent agreement with experiment (∼−32 kJ∕mol H2), as are the predicted geometrical structures for LaNi5 and La2Ni10H14. Surprisingly, although LDA calculations underestimate the volume of LaNi5 by 10.4%, the final value of ΔHf (−31.2 kJ∕mol H2), is also in excellent agreement with experiment.

The correlation consistent composite approach (ccCA): An alternative to the Gaussian-n methods

An alternative to the Gaussian-n (G1, G2, and G3) composite methods of computing molecular energies is proposed and is named the "correlation consistent composite approach" (ccCA, ccCA-CBS-1, ccCA-CBS-2). This approach uses the correlation consistent polarized valence (cc-pVXZ) basis sets. The G2-1 test set of 48 enthalpies of formation (DeltaHf), 38 adiabatic ionization potentials (IPs), 25 adiabatic electron affinities (EAs), and 8 adiabatic proton affinities (PAs) are computed using this approach, as well as the DeltaHf values of 30 more systems. Equilibrium molecular geometries and vibrational frequencies are obtained using B3LYP density functional theory. When applying the ccCA-CBS method with the cc-pVXZ series of basis sets augmented with diffuse functions, mean absolute deviations within the G2-1 test set compared to experiment are 1.33 kcal mol(-1) for DeltaHf,0.81 kcal mol(-1) for IPs, 1.02 kcal mol(-1) for EAs, and 1.51 kcal mol(-1) for PAs, without including the "high-level correction" (HLC) contained in the original Gn methods. Whereas the HLC originated in the Gaussian-1 method as an isogyric correction, it evolved into a fitted parameter that minimized the error of the composite methods, eliminating its physical meaning. Recomputing the G1 and G3 enthalpies of formation without the HLC reveals a systematic trend where most DeltaHf values are significantly higher than experimental values. By extrapolating electronic energies to the complete basis set (CBS) limit and adding G3-like corrections for the core-valence and infinite-order electron correlation effects, ccCA-CBS-2 often underestimates the experimental DeltaHf, especially for larger systems. This is desired as inclusion of relativistic and atomic spin-orbit effects subsequently improves theoretical DeltaHf values to give a 0.81 kcal mol(-1) mean absolute deviation with ccCA-CBS-2. The ccCA-CBS method is a viable "black box" method that can be used on systems with at least 10-15 heavy atoms.

Synthesis and hydrophilicity of poly(butylene 2,6‐naphthalate)/poly(ethylene glycol) copolymers

AbstractPoly(butylene 2,6‐naphthalate) (PBN)/poly(ethylene glycol) (PEG) copolymers were synthesized by the two‐step melt copolymerization process of dimethyl‐2,6‐naphthalenedicarboxylate (2,6‐NDC) with 1,4‐butanediol (BD) and PEG. The copolymers produced had different PEG molecular weights and contents. The structures, thermal properties, and hydrophilicities of these copolymers were studied by 1H NMR, DSC, TGA, and by contact angle and moisture content measurements. In particular, the intrinsic viscosities of PBN/PEG copolymers increased with increasing PEG molecular weights, but the melting temperatures (Tm), the cold crystallization temperatures (Tcc), and the heat of fusion (ΔHf) values of PBN/PEG copolymers decreased on increasing PEG contents or molecular weights. The thermal stabilities of the copolymers were unaffected by PEG content or molecular weight. Hydrophilicities as determined by contact angle and moisture content measurements were found to be significantly increased on increasing PEG contents and molecular weights. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2677–2683, 2006