Solid Phase Heats of Formation

Computational procedures typically produce gas phase AHf, whereas energetic compounds are generally liquids or solids. For molecular liquids and solids, AHf can readily be obtained from the gas phase values if the heats of vaporization and sublimation, AHvap and AHsub, are known  

In a series of studies, reviewed on several occasions [13,88,89], we have found that a variety of physical properties that depend upon noncovalent interactions, including AHvap and AHsub, can be related quantitatively to certain features of the electrostatic potentials on molecular surfaces. The electrostatic potential V(r) that the electrons and nuclei of a molecule create at any point r is given by, 

We have shown that AHvap [13] and AHsub [14] can be expressed as, 

We determined the coefficients in eqs. (17) and (18) by fitting to general databases of experimental AHvap and AHsub, using Hartree-Fock electrostatic potentials [13,14] however Rice et al reparametrized these equations at the B3LYP/6-31G level in terms of data pertaining specifically to energetic compounds [85]. Their average absolute deviations for AHvap and AHsub were 1.2 and 2.7 kcal/mole, respectively. 

For compounds that are ionic rather than molecular solids, AHf can be calculated by appropriately combining the heats of formation of the gas phase ions (computed or experimental) with the lattice energy (converted to enthalpy [91]). We have developed formulas for lattice energies in terms of properties of the electrostatic potentials on the anions surfaces [92], for any of three possible cations NH4+, Na+ and K+. 

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Politzer, P., J. S. Murray, M. E. Grice, M. DeSalvo, and E. Miller. 1997. Calculation of Heats of Sublimation and Solid Phase Heats of Formation. Mol. Phys. In press. 

TABLE 31. MM2 calculated gas- and solid-phase heats of formation for several nitramines... 

Calculated gas, liquid and/or solid phase heats of formation and solid phase heat releases.3 ... 

Calculated gas, liquid and/or solid phase heats of formation and solid phase heat releases (continued). a Compound AH/298 Kt. gas AH/298 K). liquid AHfi298 K). solid O-Solid... 

Pohtzer, P., Murray, J.S., Grice, M.E., DeSalvo, M. and Miller, E. (1997) Calculation ofheats of sublimation and solid phase heats of formation. Mol. Phys., 91, 923-928. 

Solid Phase Heats of Formation Ionic Crystals. 170... 

Fig. 4 Solid phase heats of formation for high-nitrogen crystals. Experimental values are taken from [4] and [223], 1999 Atom Equiv denotes calculations using the atom equivalent method described in [68], 2006 Group Equiv denotes calculations using the group equivalent method described in [67]. G3(MP2) and G3(MP2)//B3LYP denote calculations using variants of the G3 method [78,79], respectively...

As for the neutral crystals, we are not aware of any QSPRs developed to predict the solid phase heats of formation of high nitrogen salts, probably due to the relative scarcity of such experimental data required for establishing the correlations. 

A B3PW91/6-31G computational procedure for predicting standard gas-phase heats of formation at 298.15 K and heats of vaporization and sublimation has been presented <2005IJQ341> l,3-dithiane-2-thione was studied using this procedure and the following heats of formation were predicted gas phase, 25.7 kcal mol liquid phase, 10.3 kcal mol and solid phase, 3.1 kcal mol . 

Using the Gaussian 98 code [39], the energy minimum at 0 K was determined for each atom and molecule, and then the enthalpy and free energy at 298 K and 2000 K. From the enthalpies at 298 K were found the gas phase heats of formation, as AH(298 K) for the reactions producing the molecules from their constituent elements. For the three relevant elements that are solids at 298 K (boron, carbon and aluminum), this requires knowing their experimental heats of sublimation at 298 K, which are 133.84, 171.29 and 78.8 kcal/mole, respectively [13]. 

One final method for deriving gas-phase thermochemistry of hypervalent ions does not involve gas-phase experiments at all. It requires measuring solid-state heats of formation of ionic salts and correcting to gas-phase values using calculated lattice energies for these salts. Since the lattice energies are in the vicinity of 1000 kJ/mol, 1% accuracy on these values is necessary to derive gas-phase thermochemistry with error limits of ca. 10 kJ/mol. This accuracy has yet to be met in practice. ... 

Atomization Energies, Heats of Formation, and Heats of Reaction. Gas-phase enthalpies of formation are readily calculated from atomization energies (Section 15.14), and gas-phase heats of reaction are readily found from gas-phase heats of formation of reactants and products. All three of these related quantities are considered in this subsection. Throughout this section, only gas-phase reactions are considered because this avoids the substantial contributions of intermolecular interactions in liquids and solids. To convert a calculated gas-phase reaction energy to one involving condensed phases, we use the experimental energies of condensation of substances. 

The G3(MP2), G3(MP2)//B3LYP and our atom/group equivalent method were used to calculate the gas phase heats of formation of a set of neutral high nitrogen crystals the values were added to the predicted heats of sublimation (using the GIPF method as described in Ref. [67]) to produce solid phase... 

The total emission In the commercial heat treatment of 5 to 8 hours at 170 to 160°C varied from 0.4 to 1.2% for CO2 and 0.05 to 0.2% for CO and 0.04 to 0.1% for total acids based on dry board. Some of this emission might emanate from pyrolysis of higher molecular weight material condensed and deposited on the walls of the heat treatment chamber. The heat of formation of this CO2 and CO Is about half the total heat release measured. Part of the oxidation products might remain in the solid phase within the board material, e.g. as bound carbonyl and carboxylic groups, partly followed by heat consuming dehydration reaction. 

III, F. More phase diagrams of complex fluorides have been explored using Knudsen cell mass spectrometry (13, 15), and electron impact studies have yielded enthalpies and bond energies (1,3). The heat of formation of MoFmi) has been confirmed (12). Solid state cells have been used with lanthanide trifluorides (24) and NaNiF3 (21). 

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

TN. Dymova, N.N. Mal tseva, V.N. Konoplev, A.I. Golovanova, D.P. Alexandrov, A.S. Sizareva, Solid-phase solvate-free formation of magnesium hydroaluminates Mg(AlH )2 and MgAlHj upon mechanochemical activation or heating of magnesium hydride and aluminum chloride mixtures , Russ. J. Coord. Chem. 29 (2003) 385-389. 

When the solid phase is ice at —13°, the soln. has 30 27 per cent, of Zn(C103)2, and at —9°, 26"54 per cent. F. Ephraim and A. Jahnsen have prepared the ammoni ates, zinc tetrammino-chlorate, Zn(C103)2.4NH3, in colourless crystals of sp. gr. l-84, mol. vol. 163"2, with the dissociation press. 16 mm. at 114° 32 mm. at 140° 50 mm. at 165° and 88 mm. at 177°. A. Jahnsen s value for the heat of formation is 17 5 Cals. It explodes at 205° in an open tube and at 289° in a closed tube it also detonates when struck with a hammer. On exposing the powder to air for about five days, it loses one-fourth of the combined ammonia, and the lost ammonia is readily replaced by water, forming zinc aquo-triammino-chlorate,... 

Experiments were also carried out at 80 and lOO C. According to our observations at these high temperatures, solid- phase chemical transformations may take place between certain flavor constituents and cyclodextrin hydroxyls/monoterpene alcohols and phenolic compounds appear as a result of a solid-phase transacetylation of terpeneaoetates and phenyl-acetates with the simultaneous formation of cyclodextrin-acetates/. Long term heat treatments of cyclpdextrin-flavor complexes should not be run above 6o°C in order to avoid such phenomena. 

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