Value For Heat Of Formation Research Articles

Performance of Gradient-Corrected and Hybrid Density Functional Theory: Role of the Underlying Local Density Approximation and the Gradient Correction.

We have analyzed the performance of several widely used density functional theory procedures, namely B-P86, B-PW91, B-LYP, B3-P86, B3-PW91, and B3-LYP, for the E3 set of thermochemical properties. Each of these procedures employs a local density approximation (LDA) functional and a gradient correction for the correlation energy. We find that the VWN3 LDA functional in B-P86, B-PW91, B3-P86, and B3-PW91 leads to extremely large deviations from benchmark values for heats of formation (as large as -455.6 kJ mol(-1) for the B-PW91(VWN3) value for azulene!) and that VWN3 also gives significant errors in the calculated ionization energies and electron affinities. The PW91 gradient correction generally performs much better than P86 for heats of formation, and we propose that this is because P86 severely violates a uniform scaling condition that PW91 almost satisfies. Thus, of the procedures that we have examined, we recommend the use of the VWN5 or PW92 forms of LDA, preferably in combination with the PW91 gradient correction. Our results confirm previous findings that VWN3 is a more suitable LDA than VWN5 for B3-LYP, and we attribute this to fortuitous error cancellation between understabilization of molecules by LYP and overstabilization by VWN3.

Why does the B3LYP hybrid functional fail for metals?

The B3LYP hybrid functional has shown to successfully predict a wide range of molecular properties. For periodic systems, however, the failure to attain the exact homogeneous electron gas limit as well as the semiempirical construction turns out to be a major drawback of the functional. We rigorously assess the B3LYP functional for solids through calculations of lattice parameters, bulk moduli, and thermochemical properties (atomization energies and reaction energies). The theoretical lattice constants overestimate the experimental ones by approximately 1%, and hence behave similarly to the PBE gradient-corrected exchange-correlation functional. B3LYP atomization energies of solids are drastically worse than those of nonempirical hybrid Hartree-Fock/density functionals (HF/DFT) such as PBE0 and HSE03. These large errors can be traced back to the lack of a proper description of "free-electron-like" systems with a significant itinerant character (metals and small gap semiconductors). Similar calculations using the popular semiempirical B3PW91 hybrid functional, which fulfills the uniform electron gas limit, show a clear improvement over B3LYP regarding atomization energies. Finally, theoretical values for heats of formation for both the B3LYP as well as the B3PW91 functionals are presented. These document a most likely fortuitously good agreement with experiment for the B3LYP hybrid functional.

Modified tight-binding model for fast and accurate estimation of thermochemistry and molecular structure. Parameters and results for hydrocarbons

A modified tight-binding method, MTB, based on the orthogonal scheme, is described. The parameters for C and H atoms are reported. The MTB values for heats of formation are surprisingly close to the experimental data. The mean absolute error (MAE) of 2.4 kcal/mol is considerably smaller than that of B3LYP and MAEs of the standard semiempirical calculations. The MTB method provides accurate molecular geometries; MAEs of the calculated bond lengths and bond angles are 0.011 Å and 1.6°, respectively. Also, activation barriers and vibrational frequencies are well reproduced. The scheme is very promising for atomic simulations of carbon nanostructures and hydrocarbon systems.

The Heat of Combustion of Tobacco and Carbon Oxide Formation

Abstract Recent studies demonstrated a relationship between mass burn rates of straight-grade cigarettes and heats of combustion of the tobacco materials. In the present work, relationships between measured heats of combustion and elemental composition of the tobacco materials were further analyzed. Heats of combustion measured in oxygen were directly correlated with the carbon and hydrogen content of the tobacco materials tested. Ash content of the materials was inversely related to the heats of combustion. The water insoluble residues from exhaustively extracted tobacco materials showed higher heats of combustion and higher carbon content than the non-extracted materials, confirming a direct relationship between carbon content and heat of combustion. A value for the heat of formation of tobacco was estimated (1175 cal/g) from the heat of combustion data and elemental analysis results. The estimated value for heat of formation of tobacco appears to be constant regardless of the material type. Heat values measured in air were uniformly lower than the combustion heats in oxygen, suggesting formation of CO and other reaction products. Gases produced during bomb calorimetry experiments with five tobacco materials were analyzed for CO and CO2 content. When the materials were burned in oxygen, no CO was found in the gases produced. Measured heats of combustion matched estimates based on CO2 found in the gas and conversion of the sample hydrogen content to water. Materials burned in air produced CO2 (56% to 77% of the sample carbon content) and appreciable amounts of CO (7% to 16% of the sample carbon content). Unburned residue containing carbon and hydrogen was found in the air combustion experiments. Estimated heat values based on amounts of CO and CO2 found in the gas and water formed from the hydrogen lost during combustion in air were higher than the measured values. These observations indicate formation of products containing hydrogen when the materials were burned in air. CO and CO2 formation during combustion in air were related to the composition of the tobacco materials. Materials with high carbon and low ash content showed evidence of higher CO2 formation. Amounts of unburned residue also varied with material composition. Thus, energy released during tobacco combustion in air is related to material-dependent formation of reaction products in addition to the carbon oxides and to the quantity of unburned material.

Collisions of slow electrons with methane: ionisation, fragmentation and resonances

Ionisation efficiency and appearance potential functions for CH4 +, CH3 +, CH2 + and CH+ have been measured using a monoenergetic electron beam colliding with methane. The upper limit for the first ionisation potential of CH4 is determined to be 12.71 eV. Appearance energies for the ionic fragments have yielded values for heats of formation which indicate a large reverse activation energy in the case of CH+. Structure in the ion yield curves for CH4 +, CH3 + and CH2 + in the energy region 19-22 eV is in qualitative agreement with the results of Marmet and Binette (1978) and may be due to highly excited states of methane. The first excited state of CH+ as well as four excited states of CH2 + are observed. Other structure reported in various RPD measurements is not reproduced. The electron transmission function of CH4, measured on two spectrometers over 1-20 eV, reveals a broad minimum at 7.8 eV and a resonance at 18.5 eV.

Gibbs energy of aluminous minerals

The discrepancy between the tabulated Gibbs Energies of Formation for Al2SiO5 and corundum relative to muscovite and kaolinite is considered to lie principally with the latter two minerals. New values for heat of formation of gibbsite [Gbs] will affect the tabulated ΔHf0, ΔGf(298,1)0for the other aluminous minerals which are referred to gibbsite as calorimetric aluminum reference. Gibbs Energy Difference Functions, calculated from phase equilibria in the system CaO-Al2O3-SiO2-(H2O-CO2), can be used to estimate consistent ΔHf0, ΔGf(298,1)0values for aluminous minerals. A self consistent data set is presented referred to ΔGf(298,1)0[Corundum]=−378.08 kcal mol−1. Two independent values for ΔGf(298,1)0[Anorthite]=−961.52 and −960.29 kcal, from a recalculation of the ΔHf0[Anor] based upon the revised ΔHf(298,1)0[Gbs]=−309.325 kcal mol−1 and from measurement of silica activity on the anorthite-saturated part of the CaO-Al2O3-SiO2 liquidus, respectively, are considered to show the magnitude of the discrepancy and are used in the calculations.

Ground states of σ-bonded molecules—VIII : MINDO Calculations for species involved in nitration by acetyl nitrate

It is shown that the MINDO semiempirical SCF MO method gives good values for heats of formation of inorganic species and ions as well as neutral molecules. Calculations are reported for the various species that could be involved in nitrations by nitric acid in acetic anhydride; it is concluded that the active agent is protonatedacetyl nitr acetyl nitrate protonated at the ether-type oxygen.

Thermochemical studies on phosphorus iodides

New values for the standard heats of formation of phosphorus tri-iodide and diphosphorus tetraiodide [ΔHf°(c)=–59·3 ± 2·5; –113·1 ± 5·0 kJ mole–1 respectively] are reported. The mean bond dissociation energy P – I is calculated to be 210 kJ mole–1, and the bond energy EP – P= 232 kJ mole–1.

A calorimetric study of periodate species in aqueous solution

Heats of several reactions involving sodium periodate, NaIO4(c), have been measured in an isothermal jacketed calorimeter. The apparent acid ionization constant of periodic acid was redetermined at an ionic strength of 0.13. From these data the following values for heats of formation at 25.00° have been calculated: NaIO4(c), −105.15 ± 0.30; IO4−(aq), −38.8 ± 0.4; H5IO6(aq), −188.9 ± 1.0; H4IO6−(aq), −186.3 ± 1.0 kcal/mole.

The thermochemistry of sulphur-containing molecules and radicals—II : The dissociation energies of bonds involving sulphur: the heats of formation of sulphur-containing radicals

The values derived from pyrolysis rate and electron impact studies for the dissociation energies of carbon-sulphur bonds in organic thiols, sulphides and sulphones are critically discussed. Once these have been settled, they may be used, in conjunction with the heat of formation data given in Part I and other relevant thermochemical data, to derive values for the gas-phase heats of formation of certain sulphur-containing free radicals of key importance. These in turn, may be used to derive several whole new series of carbon-sulphur bond dissociation energies in molecules and radicals. Inspection of such series casts doubts upon the currently accepted values for the heats of formation of the n-butyl and t-butyl radicals and alternative values are suggested. The carbon-sulphur bond dissociation energies are much greater in RS radicals than in RSO 2 radicals. Values for the mean carbon-sulphur bond dissociation energies in sulphoxides are reported. The CS bond dissociation energies in carbon disulphide and carbon oxysulphide are discussed. New series of sulphur-sulphur, hydrogen-sulphur and sulphur-oxygen bond dissociation energies are derived and tabulated. It is shown that, ceteris paribus, the dissociation energy of the sulphur-oxygen bond in sulphoxides is about 22 kcal mole −1 lower than the dissociation energy of the first sulphur-oxygen bond in the corresponding sulphone. The variation of the sulphur-oxygen bond dissociation energy in sulphones, R 2SO 2 , with variation in the nature of the R groups is demostrated and discussed. This variation has not yet been given a satisfactory interpretation.

A SYSTEM OF MOLECULAR THERMOCHEMISTRY FOR ORGANIC GASES AND LIQUIDS: PART II. EXTENSION TO COMPOUNDS CONTAINING SULPHUR AND OXYGEN

The system of bond and group energies described in Part I has been extended and applied to certain compounds containing sulphur and oxygen. The substances treated are the alkanethiols, the thiaalkanes, and the dithiaalkanes. The bond additivity scheme enables reliable values for heats of formation, combustion, atomization, and vaporization to be calculated for these compounds.A similar treatment has been applied to the data for alcohols, ethers, and alkyl peroxides.

Thermochemistry and structure of atoms

1. The heat of hydrolysis of gaseous boron chloride has been determined. 2. Applying the concepts of electronegativity, of degree of ionicity of bonds, and of the function (see our previous communication) it has been shown that inversion cannot take place in the heats of formation of the series of chlorides B, Al, Se, Y, and La, but that the rule of thermochemical logarithmics should be observed here. This allows of the derivation of the following values for heat of formation of BCl3 (keaI per male). $$B_{(cryst.)} + {3 \mathord{\left/ {\vphantom {3 2}} \right. \kern-\nulldelimiterspace} 2}Cl_{2_{(gas)} } = BCl_{3_{(cryst.)} } ,\Delta \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{H} _{2S3} = - 114 \pm 3$$ or $$B_{(cryst.)} + {3 \mathord{\left/ {\vphantom {3 2}} \right. \kern-\nulldelimiterspace} 2}Cl_{2_{(gas)} } = BCl_{2_{(gaseous)} } ,\Delta \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{H} _{2S3} = - 108 \pm 3.$$ 3. The experimental values found for the heat of hydrolysis of BCl3 show that the correct for its best of formation is in accordance with the expression $$2B_{(cryst.)} + {3 \mathord{\left/ {\vphantom {3 2}} \right. \kern-\nulldelimiterspace} 2}O_{2_{(gas)} } = B_2 O_{3_{(cryst.)} } ,\Delta \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{H} _{2S3} = - 341 \pm 3.$$ This confirms the work of Reth and Todd and Miller, but not Eggersgluess, Monroe, and Parker, and also shows that the results calculated indirectly by Johnston, Hersch, and Kerr for the heat of formation of B2O3 are wrong. 4. Combining the measurements of specific heat (entropy) give by these authors with the results presented in the present paper, the following figure can be derived for the free energy of formation of boron oxide (keal per moles $$2B_{(cryst.)} + {3 \mathord{\left/ {\vphantom {3 2}} \right. \kern-\nulldelimiterspace} 2}O_{2_{(gas)} } = B_2 O_{3_{(cryst.)} } ,\Delta \underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{F} _{2S3} = - 322 \pm 3$$ 5. The rule of thermochemical logarithmics is followed without exception by the chlorides and oxides of the elements of Group III of the periodic system.