Dynamic differential calorimetry of intermetallic compounds: II.Heats of formation,heats and entropies of fusion of rare earths-lead (REPb3) compounds

Dynamic differential calorimetry has been employed to evaluate the heats of formation, heats and entropies of fusion of REPb₃ compounds, where RE=La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm and Yb. The results obtained have been estimated to be correct to within ±5–6‰ The general trend is a decrease in the heat of formation from La to Tm, which is correlated with the magnitude of the lanthanide contraction in these compounds.     

First-principles studies of Al-Ni intermetallic compounds

The structural properties, heats of formation, elastic properties, and electronic structures of Al-Ni intermetallic compounds are analyzed here in detail by using density functional theory. Higher calculated absolute values of heats of formation indicate a very strong chemical interaction between Al and Ni for all Al-Ni intermetallic compounds. According to the computational single crystal elastic constants, all the Al-Ni intermetallic compounds considered here are mechanically stable. The polycrystalline elastic modulus and Poisson's ratio have been deduced by using Voigt, Reuss, and Hill (VRH) approximations, and the calculated ratio of shear modulus to bulk modulus indicated that AlNi, Al₃Ni, AlNi₃ and Al₃Ni₅ compounds are ductile materials, but Al₄Ni₃ and Al₃Ni₂ are brittle materials. With increasing Ni concentration, the bulk modulus of Al-Ni intermetallic compounds increases in a linear manner. The electronic energy band structures confirm that all Al-Ni intermetallic compounds are conductors.     

New parametrization for mindo/3 calculations of the structure of compounds with N-N Bonds

Conclusions A new set of _NN and b_nn parameters has been proposed for the MINDO/3 method. The use of these parameters permits an improvement in the accuracy of the calculation of the heats of formation, dipole moments, and first ionisation potentials for nitrogen-containing compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2373–2375, October, 1987.     

Dynamic differential calorimetry of intermetallic compounds I. Heat of formation,heat and entropy of fusion of rare earth—tin compounds

Dynamic differential calorimetry has been employed to evaluate the heats of formation, heats and entropies of fusion of RESn₂ compounds, where rate earths (RE) La, Ce, Pr, Nd, Sm, Gd, Eu, Yb and Ca. The results obtained have been estimated to be correct to within ±5–6%. The agreement between these values and those obtained from other authors can be taken as an indication of the reliability of the method.     

Evaluation of MINDO/3 calculated structures. II. Branching errors in alkanes and cycloalkanes

MINDO/3 calculations have been carried out for a series of branched chain alkanes in order to assess effects of branching on calculated geometries and heats of formation (ΔH_f). With vicinal branching, MINDO/3 calculates the central C? C bond to be too long. Bond angles are also found to be distorted. Errors in calculated heats of formation are large when geminal branching is present and significant with vicinal branching. Branching error corrections for ΔH_f have been derived and applied to a separate series of branched acyclic and cyclic compounds. For the test sample, application of the branching error corrections gave calculated structures of acyclic branched hydrocarbons with heats of formation having an average absolute error of 1.3 kcal/mole rather than 17.3 kcal/mole before correction. Cyclic branched hydrocarbons are shown to be less well corrected. Calculations of heats of reaction have also been carried out for some isomerization and cyclization reactions using the MINDO/3 and MNDO methods. It is clear from the comparisons that MNDO calculations give less severe errors for highly branched compounds but the errors are still substantial. For prediction of heats of reaction, the error-corrected calculations are shown to be superior to the “raw” calculations obtained by MINDO/3 or MNDO.     

MASSENSPEKTROMETRISCHE UNTERSUCHUNG ORGANISCHER STICKSTOFFVERBINDUNGEN XVIIIMECGABUSNEB;ndash DER NH3- ELIMINIERUNG AUS 3-PHENYL-PROPYL- UND 5-PHENYLPENTYLAMIN

The mechanisms for ammonia elimination from the title compounds have been studied using ²H labelled compounds, heats of formation data derived from appearance potential measurements of the [M ? NH₃]⁺· ions and by comparison of the collisional activation spectra of these ions with those of the corresponding phenylalkenes. Possible ion structures are discussed as a function of the ion life time.     

New potential high energy density compounds: Oxadiaziridine derivatives

The -CN, -N₃, -NF₂, -NH₂, -NHNO₂, -NO₂, and -ONO₂ derivatives of oxadiaziridine were studied using B3LYP/6-311G** level of density functional theory. The gas phase heats of formation of oxadiaziridine derivatives were calculated by isodesmic reaction. All these compounds have high and positive heats of formation due to strain energies of small ring. Detonation properties were calculated via Kamlet-Jacobes equations and specific impulse. The effects of substituent groups on detonation performance were discussed. The impact sensitivity was estimated according to the “available free space per molecule in unit cell” and “energy gaps” methods. The similar conclusions were given by two different methods. The effects of substituents on impact sensitivity were discussed. According to the given estimations of detonation performance and sensitivity, some oxadiaziridine derivatives may be considered promising high energies materials.     

Optimization and application of lithium parameters for PM3

Lithium parameters have been optimized for Stewart's standard PM3 method. The average deviation of the heats of formation calculated for 18 reference compounds is 6.2 kcal/mol from the experimental or high-level ab initio data; the average deviation with Li/MNDO is 18.9 kcal/mol. The average error in bond lengths is also reduced by a factor of two to three. Ionization potentials and dipole moments are reproduced with comparable accuracy than Li/MNDO. However, the mean deviation for the heats of formation of both methods increases when being applied to other systems, especially to small inorgnic molecules. The applicability of the new parameter set is demonstrated further for various compounds not included in the reference set, for the calculation of the activation barriers of several lithiation reactions, as well as for the estimation of oligomerization energies of methyl lithium (including the tetramer). Li/PM3 gives reliable results even for large dimeric complexes, like [{4-(CH₃CR)C₅H₄N}Li]₂, containing TMEDA or THF as coligands and reproduces the haptotropic interaction between Li⁺ and π-systems (e.g., in benzyl lithium) as well as the relative energies and structural features of compounds with “hypervalent” atoms (e.g., in lithiated sulfones). © John Wiley & Sons, Inc.     

Pushing and pulling electrons: the effect on the heat of formation of trifluoromethyl compounds

The effect on heats of formation, of conjoined or proximate functional groups which can interact via polar or resonance effects, is examined using the –CF₃ group as a standard. Two metrics are applied: the difference in heat of formation of G–CF₃ and G–CH₃, where –G is a wide range of functional groups, and also the deviation of the heat of formation of G–CF₃ from the average of the heats of formation of G–G and CF₃–CF₃. This latter metric reveals both stabilizing and destabilizing effects on the heat of formation, of up to 60 kcal/mol, depending on the polar and resonance nature of the –G structure. The possibility of using such metrics as a correction the group additivity values is examined.     

Molecular mechanics (MM3) calculations on azoxy compounds

The MM3 force field has been extended to include azoxy compounds and also the related amine oxides, both aliphatic and aromatic. The structures of nine molecules were all well fit. The heats of formation for the aliphatic compounds were also well fit, and the vibrational spectra of eight compounds were also fit to the accuracy expected for such calculations. Because many of the experimental data needed to derive the force field were either lacking or were inadequate, ab initio calculations on structures, optimized at the MP2/6-31G* level, were used as needed. © 1994 by John Wiley & Sons, Inc.     

Theoretical thermochemistry of some LiXHn and BeXHn compounds

Summary In this theoretical work, we consider the geometrical, electronic and energetic properties of some lithium and beryllium derivatives. The standard heats of formation of these compounds have been calculated at the MP4=SDTQ/6-31+G(2df, p)//MP2=FULL/6-31G(d, p) level. The values obtained at this level of the theory are also compared with the heats of formation deduced from a composite procedure in which it is assumed that some corrections can be treated separately and combined in an additive manner. We find that the values determined with the complete 6-31+G(2df, p) basis set are the more accurate.     

Quantum chemical studies on the structures, properties and decomposition of the azido derivatives of trinitrobenzenes

The geometries,heats of formation and electronic structures of 15 azido-derivatives of 1,2,3-TNB (Ⅰ),1,2,4-TNB (Ⅱ) and 1,3,5-TNB (Ⅲ) have been studied using quantum chemical AMI method at HF level.The effect of azido substitution on the structures and properties of TNBs has been discussed and the relative stability of the title compounds has been established.The processes of the decomposition of the title compounds by breaking C-NO2,C-N3 and CN-N2 bonds are investigated at UHF-AM1 level.It is shown that the decomposition of the title compounds may be initiated by the cleavage of both C-NO2 and N-N2 bonds.     

Application of SINDO1 to chlorine and sodium compounds

SINDO1 calculations are presented for ground state geometries, heats of formation, ionization potentials and dipole moments of chlorine and sodium compounds. These calculations are based on a new parametrization of SINDO1 for second-row elements which features inclusion of 3d orbitals and zero point energies. The comparison shows a substantial improvement over MNDO in geometries and heats of formation of hypervalent compounds and ionization potentials, whereas other properties are of similar quality.     

New thermochemical reference system expressing the relative energy of organic compounds

To facilitate the comparison of the energies of nonisomeric organic compounds, we introduce a new thermochemical reference system. The reference substances, instead of elements, are series of compounds: unbranched alkanes and their selected derivatives (halogenides, ethers, sulfides, and tertiary amines). The relative enthalpies calculated from the heats of formation directly reflect the relative energy of both isomeric and nonisomeric compounds, and they can be used in the calculation of the heats of reaction. The relative enthalpies of the elements (in kJ/mol) are C, –22.846; H₂, 43.426; O₂, 251.66; N₂, –178.62; S, –41.62; F₂, 396.12; C1₂, 94.22; Br₂, 8.16; and I₂, –107.64.     

Theoretical predictions on pentaerythritol tetranitrate-based high energy density compounds

A novel family of pentaerythritol tetranitrate (PETN) derivatives based parent PETN skeleton were designed by introducing two energetic groups –NF₂ and –NO₂. Their electronic structure, heats of formation, detonation properties, impact sensitivity, and thermal stability were investigated by using density functional theory. The findings reveal that most of the title compounds have good detonation performance. The –NF₂ group played an important role in improving the densities, heats of detonation, and detonation properties of the designed molecules. The values of h₅₀ for almost all the PETN derivatives are higher than that of octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. An analysis of bond dissociation energy suggests that the N-NO₂ bond tends to be a trigger bond in thermal decomposition. Taking both detonation properties and thermal stabilities into consideration, the three compounds may be selected as potential high-energy-density compounds.     

Phase formation and thermal stability of the compounds in the Bi2O3-PbO system

The scientific interest for the Bi₂O₃-PbO system has increased due to the importance of the PbO in the high-T _c superconducting phase formation in the Bi₂O₃-SrO-CaO-CuO system. Also Bi₂O₃-PbO system contains compounds with some specific semiconductor and dielectric properties and Bi₂O₃-based solid solutions are well known as high oxygen ion conductors.Previously, several low melting defined compounds have been identified in the system: 6Bi₂O₃·PbO; 3Bi₂O₃·2PbO; 4Bi₂O₃·5PbO; 4Bi₂O₃·6PbO and Bi₂O₃·3PbO.This work deals with the phase formation and thermal stability of these compounds. Under non-isothermal conditions, in all mixtures regardless of the Bi₂O₃/PbO ratio, the compound 6Bi₂O₃·PbO is preferentially formed, followed by the compound 4Bi₂O₃·5PbO. The formation of the compound 4Bi₂O₃·6PbO was not confirmed while the formation of the compound Bi₂O₃ 3PbO occurs through a complex mechanism which includes an intermediate step in which a solid solution with the litharge structure was identified. Under isothermal conditions in the same temperature range the tendency to form the stoichiometric compounds increases. All compounds form, decompose and melt at temperatures between 530–780°C.     

First-principle studies of Ca-X (X=Si,Ge,Sn,Pb) intermetallic compounds

The structural properties, elastic properties, heats of formation, electronic structures, and densities of states of 20 intermetallic compounds in the Ca-X (X=Si, Ge, Sn, Pb) systems have been systematically investigated by using first-principle calculations. Our computational results indicated that with increasing atomic weight of X, the bulk modulus of Ca-X intermetallic compounds decreases gradually. It was also found that Ca₃₆Sn₂₃ and CaPb are mechanically unstable phases. Results on the electronic energy band and densities of states also indicated that Ca₃Si₄ is an indirect band gap semiconductor with a band gap of 0.598 eV, and Ca₂Si, Ca₂Ge, Ca₂Sn, and Ca₂Pb are direct band gap semiconductors with band gaps of 0.324, 0.265, 0.06, and 0.07 eV, respectively. In addition, it is found that the absolute values of heats of formation for all Ca-X intermetallics are larger than 30 kJ/mol atom.     

Heavy fermion behavior in Cu3Au-structure compounds: CePb3 and U(In,Sn)3

The recent discovery of anomalous properties associated with extremely narrow f-band character in the electronic density of states in the vicinity of the Fermi energy has stimulated considerable interest in these systems. Some particularly exciting recent discoveries have emerged from studies of the magnetic and electronic properties of cerium- and uranium-based Cu₃Au intermetallic compounds which form with group IIIB and group IVB elements. It has been reported that CePb₃ is a heavy fermion antiferromagnetic system with a Néel temperature of 1.1K. In addition, this system displays field-induced superconductivity for magnetic fields greater than 15 T and temperatures below 0.5K. Studies of (Ce,La)Pb₃ clearly indicate that the heavy fermion behavior reported for this system is primarily a reflection of single-ion Kondo behavior. In addition, studies of U(In,Sn)₃ and Ce(In,Sn)₃ explicitly indicate that the heavy fermion and mixed-valent behavior reported for these systems are a result of a similar hybridization-driven mechanism. The CeX₃ and UX₃ systems where X is a group IIIB or group IVB element display many of the features of current interest to magnetic moment formation and allow a unique opportunity to study these phenomena systematically.     

Molecular design of all nitrogen pentazole‐based high energy density compounds with oxygen balance equal to zero

We designed a new family of pentazole‐based high energy density compounds with oxygen balance equal to zero by introducing −NH₂, −NO₂, −N₃, −CF₂NF₂, and −C[NO₂]₃, and the properties including density, heats of formation, detonation performances, and impact sensitivity were investigated using density functional theory. The results show that half of these new energetic molecules exhibit higher densities than RDX (1.82 g/cm³), in which H5 gives the highest density of 2.09 g/cm³. Among all the 54 designed molecules, 22 compounds have higher D and P than RDX and eleven compounds have higher D and P than HMX, indicating that designing the pentazole‐based derivatives with oxygen balance equal to zero is a very effective way to obtain potential energetic compounds with outstanding detonation properties. Taking both the detonation performance and stability into consideration, nine compounds may be recognized as potential candidates of high energy density compounds. It is expected that our results will contribute to the theoretical design of new‐generation energetic explosives.     

Prototropic behavior of cyclohexane substituted urethane and urea compounds. Observation of H-bond mediated 4HJH1H3 coupling constants across urea fragments

Two 1-aryl-3-cyclohexylurea and two 1-aryl-3-cyclohexylurethane with and without alkyl tail in aryl fragments were synthesized and their variable-temperature ¹H NMR spectra in chloroform, DMSO and DMF were recorded. The temperature dependences of chemical shift of NH protons for all compounds have been observed. The type of dependence has been explained by the aggregation of molecules and formation of ionic structures. The formation of intermolecular complexes leads to possible formation of symmetrical N1 ^{… …} H ^……N3 intramolecular hydrogen bond in urea fragment. As a result,^4hJ_H1H3 trans-hydrogen bond scalar coupling constants have been observed for the first time in low molecular weight compounds using a simple one-dimensional ¹H NMR experiment. The formation of strong intramolecular H-bond leads to π-conjugation not only with spacer but with phenyl ring too. It support the Gilli conception that RAHB formation is a result of π-electron delocalization. The presence in urea fragment of such kind interaction leads to formation of ionic structure, which has been detected by NMR and UV spectroscopies. The formation of ionic structure can explain the catalytic activity of such compounds and the mechanism of transformation in organic and bioorganic reactions in which involved the urea compounds.