Proton-transfer tautomerism and enthalpies of formation of some isoquinoline derivatives

Ab initio molecular orbital theory has been used to calculate enthalpies of formation of the keto tautomers of 1-hydroxyisoquinoline, 5-hydroxyisoquinoline, and 1,5-dihydroxyisoquinoline. The high-level composite method G3//B3LYP has been used for this study, and the results have been compared with available experimental values. The keto tautomer is more favourable for 1-hydroxyisoquinoline and 1,5-dihydroxyisoquinoline, and the experimental enthalpies of formation are in better agreement with the theoretical enthalpies of formation of the keto forms.     

Calculation of the enthalpies of formation and proton affinities of some isoquinoline derivatives

Ab initio molecular orbital theory has been used to calculate enthalpies of formation of isoquinoline, 1-hydroxyisoquinoline, 5-hydroxyisoquinoline, and 1,5-dihydroxyisoquinoline as well as some pyridine and quinoline derivatives. The proton affinities of the four isoquinoline derivatives were also obtained. The high-level composite methods G3(MP2), G3(MP2)//B3LYP, G3//B3LYP, and CBS-QB3 have been used for this study, and the results have been compared with available experimental values. For six of the eight studied compounds, the theoretical enthalpies of formation were very close to the experimental values (to within 4.3 kJ · mol⁻¹); where comparison was possible, the theoretical and experimental proton affinities were also in excellent agreement with one another. However, there is an extraordinary discrepancy between theory and experiment for the enthalpies of formation of 1-hydroxyisoquinoline and 1,5-dihydroxyisoquinoline, suggesting that the experimental values for these two compounds should perhaps be re-examined. We also show that popular low cost computational methods such as B3LYP and MP2 show very large deviations from the benchmark values.     

Structural studies of cyclic ureas: 3. Enthalpy of formation of barbital

A thermochemical and thermophysical study has been carried out for crystalline barbital [5,5′-diethylbarbituric acid]. The thermochemical study was made by static bomb combustion calorimetry, from which the standard () molar enthalpy of formation of the crystalline barbital, at T = 298.15 K, was derived as −(753.0 ± 1.8) kJ · mol⁻¹. The thermophysical study was made by differential scanning calorimetry over the temperature interval (265 to 470) K. A solid–solid phase transition was found at T = 413.3 K. The vapour pressures of the crystalline barbital were measured at several temperatures between T = (355 and 377) K, by the Knudsen mass-loss effusion technique, from which the standard molar enthalpy of sublimation, at T = 298.15 K was derived as (117.3 ± 0.6) kJ · mol⁻¹. The combination of the experimental results yielded the standard molar enthalpy of formation of barbital in the gaseous phase, at T = 298.15 K, as −(635.8 ± 1.9) kJ · mol⁻¹. This value is compared and discussed with our theoretical calculations by several methods (Gaussian-n theories G2 and G3, complete basis set CBS-QB3, density functional B3P86 and B3LYP) by means of atomization and isodesmic reaction schemes.     

Correlations between MO eigenvectors and enthalpies of formation for simple organic substances

Bondingness, originally used in a qualitative analysis of the barrier to rotation in ethane, has been used to model Δ_fH° (g) for simple organic substances. The model is parameterised with a set of 345 molecules including alkanes, alkenes, alkynes, alcohols, ethers, aldehydes, ketones, carboxylic acids, esters, alkenoates, amines, amides, diazenes, nitriles, nitroalkanes, nitrates, thiols and benzenoids. The model is compared with a current empirical scheme as well as a comparison of variations of the model using different simple steric potentials. Using bondingness and the most approximate quantum chemical models a model can be formulated that is comparable with empirical group methods but requiring less parameters.     

Theoretical studies on heats of formation for cubylnitrates using density functional theory B3LYP method and semiempirical MO methods

The heats of formation (HOF) have been calculated for all the 21 cubylnitrate compounds using the semiemprical molecular orbital (MO) methods (MINDO/3, MNDO, AM1, and PM3) and for 8 of 21 cubylnitrates containing 1–4 ? ONO₂ groups using the density functional theory (DFT) method at the B3LYP/6‐31G* level by means of designed isodesmic reactions. The cubane cage skeletons in cubylnitrate molecules have been kept in setting up isodesmic reactions to produce more accurate and reliable results. It is found that there are good linear relationships between the HOFs of the 8 cubylnitrates calculated using B3LYP/6‐31G* and two semiempirical MO (PM3 and AM1) methods, and the linear correlation coefficients of PM3 and AM1 methods are 0.9901 and 0.9826, respectively. Subsequently, the accurate HOFs at B3LYP/6‐31G* level of other 13 cubylnitrates containing 4–8 ? ONO₂ groups are obtained by systematically correcting their PM3‐calculated HOFs. Compared with noncaged nitrates, all the 21 cubylnitrates have high heats of formation implying that they may be very powerful energetic materials and have highly exploitable value. The relationship between the HOFs and the molecular structures of cubylnitrates has been discussed. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Theoretical calculation of bond dissociation energies and heats of formation for nitromethane and polynitromethanes with density functional theory

The C? NO₂ bond dissociation energies (BDEs) and the heats of formation (HOFs) of nitromethane and polynitromethanes (dinitromethane, trinitromethane, and tetranitromethane) system in gas phase at 298.15 K were calculated theoretically. Density functional theory (DFT) B3LYP, B3P86, B3PW91, and PBE0 methods in combination with different basis sets were employed. It was found that the C? NO₂ bond BDEs can be improved from B3LYP to B3PW91 to B3P86 or PBE0 functional. Levels of theory employing B3P86 and PBE0 functionals were found to be sufficiently reliable without the presence of diffusion functions. As the number of NO₂ groups on the same C atom increases, the PBE0 functional performs better than the B3P86 functional. Regarding the calculated HOFs, all four functionals can yield satisfactory results with deviations of <2 kcal mol^?1 from experimental ones for CH₂(NO₂)₂ and CH(NO₂)₃, when the diffusion functions are not augmented. For the C(NO₂)₄ molecule, the large basis sets augmented with polarization functions and diffusion functions are required to yield a good result. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Combination of molecular dynamics method and 3D-RISM theory for conformational sampling of large flexible molecules in solution

We have developed an algorithm for sampling the conformational space of large flexible molecules in solution, which combines the molecular dynamics (MD) method and the three-dimensional reference interaction site model (3D-RISM) theory. The solvent-induced force acting on solute atoms was evaluated as the gradient of the solvation free energy with respect to the solute-atom coordinates. To enhance the computation speed, we have applied a multiple timestep algorithm based on the RESPA (Reversible System Propagator Algorithm) to the combined MD/3D-RISM method. By virtue of the algorithm, one can choose a longer timestep for renewing the solvent-induced force compared with that of the conformational update. To illustrate the present MD/3D-RISM simulation, we applied the method to a model of acetylacetone in aqueous solution. The multiple timestep algorithm succeeded in enhancing the computation speed by 3.4 times for this model case. Acetylacetone possesses an intramolecular hydrogen-bonding capability between the hydroxyl group and the carbonyl oxygen atom, and the molecule is significantly stabilized due to this hydrogen bond, especially in gas phase. The intramolecular hydrogen bond was kept intact during almost entire course of the MD simulation in gas phase, while in the aqueous solutions the bond is disrupted in a significant number of conformations. This result qualitatively agrees with the behavior on a free energy barrier lying upon the process for rotating a torsional degree of freedom of the hydroxyl group, where it is significantly reduced in aqueous solution by a cancellation between the electrostatic interaction and the solvation free energy.     

Ideal gas thermochemical properties of silicon containing inorganic,organic compounds,radicals, and ions

The ideal gas thermochemical properties such as standard heat of formation, entropy, and heat capacities of 112 inorganic and 35 organic neutral compounds, radicals, and ions containing silicon were calculated using molecular properties obtained with the G3B3 (or G3//B3LYP) method. Among them were linear and cyclic silanes, silenes, hydrocarbonsilanes, fluorine, and oxygen containing compounds. Many of their molecular and thermodynamic properties were calculated for the first time and 16 of them had no CAS number. Additionally the thermochemical properties were presented in the NASA 7 term polynomial format for the temperature range of 200‐6000 K commonly used in chemical kinetic modeling and simulation programs. The polynomials are available in the Supporting Information supplement to this article free of charge.     

Finite group theory for large systems. 3. Symmetry-generation of reduced matrix elements for icosahedral C(20) and C(60) molecules

This paper uses symmetry-generation to simplify the determination of Hamiltonian reduced matrix elements. It is part of a series on using computers to apply finite group theory to quantum mechanical calculations on large systems. Symmetry-generation is an expression of the whole molecule as a sum of symmetry transformations on a smaller reference structure. Then on a suitably-conditioned symmetry-adapted basis, the reduced matrix elements of the Hamiltonian are averages of certain elements of the simpler reference structure matrix. The smaller the reference structure, the greater is the computational savings. Single atom reference structures are used here for the Hückel treatment of icosahedral C(20) and C(60) fullerenes. The analytical power of this approach is illustrated by determining the two bond lengths of C(60) from spectral data.     

The performance of the rapid estimation of basis set error and correlation energy from partial charges method on new molecules of the G3/99 test set

Experimental enthalpies of formation have been approximated using single-point Hartree–Fock (HF)–self-consistent-field (SCF) total energies plus the rapid estimation of basis set error and correlation energy from partial charges (REBECEP) energy corrections. The energy corrections are calculated from the HF–SCF partial atomic charges and optimized atomic energy parameters. The performance of the method was tested on 51 closed-shell neutral molecules (50 molecules from the G3/99 thermochemistry database plus urea, composed of H, C, N, O, and F atoms). The predictive force of the method is demonstrated, because these larger molecules were not used for the optimization of the atomic parameters. We used the earlier RECEP-3 [HF/6-311+G(2d,p)] and REBECEP [HF/6-31G(d)] atomic parameter sets obtained from the G2/97 thermochemistry database (containing small molecules) together with natural population analysis and Mulliken partial charges. The best results were obtained using the natural population analysis charges, although the Mulliken charges also provide useful results. The root-mean-square deviations from the experimental enthalpies of formation for the selected 51 molecules are 1.15, 3.96, and 2.92 kcal/mol for Gaussian-3, B3LYP/6-11+G(3df,2p), and REBECEP (natural population analysis) enthalpies of formation, respectively (the corresponding average absolute deviations are 0.94, 7.09, and 2.27 kcal/mol, respectively). The REBECEP method performs considerably better for the 51 test molecules with a moderate 6-31G(d) basis set than the B3LYP method with a large 6-311+G(3df,2p) basis set. Received: 10 March 2001 / Accepted: 5 July 2001 / Published online: 11 October 2001     

Determination of combustion energies and the standard enthalpies of formation for the complexes of RE(Et_2dtc)_3(phen)

Solid complexes, RE(Et2dtc)3(phen) (RE=La, Pr, Nd, Sm-Lu), were synthesized with sodium diethyldithiocarbamate (NaEt2dtc3H2O),1,10-phenanthroline (o-phen·H2O) and hydrated lanthanide chlorides in absolute ethanol. The constant-volume combustion energies of complexes,ΔcU, were determined by a precise rotating-bomb calorimeter at 298.15 K. The standard enthalpies of combustion,ΔcHmθ, and standard enthalpies of formation,ΔfHmθ, were calculated for these complexes, respectively. The experiment results showed the "tripartite effect" of rare earth.     

Preparation, characterization and the standard enthalpy of formation of La0.95MnO3+δ and Sm0.95MnO3+δ

Among the perovskites, the rare earth manganites find application in several electrochemical devices because of their enhanced thermodynamic stability. In this paper, we present the results obtained on the preparation and characterization of La_0.95MnO_3+δ and Sm_0.95MnO_3+δ which were prepared by the solid state and sol–gel methods. XRD characterization of the manganites indicated that the crystal structure depends on the method of preparation and heat treatments. The ratio of Mn³⁺ to Mn⁴⁺ in these samples also depended on the method of preparation and heat treatments, as indicated by thermogravimetric (TG) and temperature programmed reduction (TPR) studies in Ar + 5% H₂ atmosphere. The standard molar enthalpy of formation, which is a measure of the thermodynamic stability of these compounds were determined using an isoperibol calorimeter.