Heat of formation calculations

Several modifications of the Engler-Schleyer force field have appeared. White (27) added olefin parameters and used an efficient two-stage Newton-Raphson minimization modification. The accuracy of the White force field regarding heat of formation calculations is... 

Nitrosulphonic anhydride is a white, crystalline solid, of density 2-14. On heating it undergoes partial decomposition 7 it begins to melt at 217° C. to a yellow liquid and distils at about 860° C. The heat of formation, calculated from the heat of dissolution in potassium hydroxide, is 112 Calories ... 

The heat of combustion (experimental values) and the heat of formation (calculated) by Tomioka [87] and Lenze et al. [88] for nitrocelluloses of different nitrogen content are listed in Table 69. 

In Tables 10 to 12 we show the heats of formation calculated by the various methods, together with their deviation from the experimentally observed values for alkanes and cycloalkanes, alkenes and cydoalkenes, and acetylenes and aromatic compounds. Table 13 shows a comparison of heats of formation of hydrocarbon radicals calculated by the MINDO methods. Finally, in Tables 14 and 15 we show the results of MINDO/1 calculations on a selection of oxygen- and nitrogen-containing compounds. 

Table 3.3. Standard enthalphies (heats) of formation of nickel aluminides and their effective heats of formation calculated for the effective concentration at the interface corresponding to the composition (3.5 at.% Ni, 96.5 at.% Al) of the eutectic with the lowest melting point in the Ni-Al binary system.261 For all the intermetallic compounds, the limiting element is nickel...

Experimental thermochemical results were mainly required to extend the para-metrization of the current force fields to highly strained compounds. Heats of formation calculated with Allinger s MM2 force field for alkanes32) and its extension to alkylbenzenes 30) proved to be in by far the best agreement with the experimental results 27). A few examples which demonstrate the quality of this agreement are shown in Table 1. 

The number of electrons available for empirical evaluation of metal-metal bonding has been taken as the Pauling metallic valence less the number of H ions per metal. In this connection the valence numbers of Borelius (6) give somewhat better correlations—e.g., in differentiating Pd from Ag (valences 7 and 1, respectively). Heats of formation calculated from the lattice energies by the Born-Haber cycle are not yet sufficiently accurate to be useful numerically, but they provide an interesting rationalization of the formation of many hydrides. This is the principal reason for considering such a naive model. 

If AH/ (298 °K) values for all the species with one or two heavy atoms in the bond separation reaction are known and if the bond separation energy at 298 K can be predicted from theoretical calculations, the heat of formation of the three-membered ring can be estimated. In Table 6, we present heats of formation calculated in this manner from experimental heats of formation for molecules with one and two heavy atoms from Table 4 and our theoretical bond separation energies from Table 1. The... 

Heats of formation calculated in this way would refer to elements in states in which the atoms formed single bonds, as they do in the molecules P4 and Sg. Nitrogen (No) and oxygen (Oo) contain multiple bonds, and the nitrogen and oxygen molecules are more stable, by 110 kcal/mole and 48 kcal/mole. respectively, than they vould be if the molecules contained single bonds (as in P4 and S ). Hence we must correct for this extra stability, by using the equation... 

The negative of the conventional heat of formation calculated for cellulose from the heat of combustion of starch. 

We can also calculate a strainless heat of formation of anfi-butane, which is the heat of formation of an anti structure made up of hypothetical strainless bonding interactions. As shown, we add the strainless bond increments to obtain the value of -32.92 kcal/mol for the strainless bond enthalpy (SBE). This value must also be corrected for the T / R term, but the POP and TOR terms are again taken to be zero. Thus, the strainless AH°f (hfs) is the sum of —32.92 and + 2.40, which is —30.52 kcal/mol. That differs from the heat of formation calculated with normal bond increments (—30.51 kcal/mol) by 0.01 kcal/mol, which is the inherent strain energy of anti-butane. ... 

There are two other approaches to defining aromaticity that we will consider. The first is based on a comparison of the heat of formation of an organic compound with the heat of formation calculated for a nondelocalized reference system. The difference between those two values would then be the resonance energy for this system. 

Semiempirical programs usually report heats of formation calculated from the electronic energies less the calculated energies for the atoms plus the experimental heat of formations for the atoms ... 

The electron-impact AP of 12.1 eV seems therefore to indicate the energy of a certain orbital in methanol and seems in principle to have nothing to do with the dissociation limit giving CH20H. The consequence of this interpretation should therefore be that this AP should not be used for calculation of the heat of formation of CH20H . In this case, however, the AP and the dissociation limit probably agree approximately, because the heat of formation, calculated from higher alcohols, is only a little lower. " ... 

In this exercise, we will compare the energies of a pair of tautomers using the heats of formation calculated by the semiempirical AMI method. These two tautomers can be directly compared because they have the same molecular formula CjH O. Most organic textbooks discuss the relative stability of ketones and their tautomeric enol forms. For acetone, there are two tautomers in equilibrium ... 

The least-squares fitting of the data for the compounds in Table 11.1 gave us the parameter values to be used in the MM4 heat of formation calculation. They are shown in Table 11.2. These parameter values are worth some discussion. Two parameters that are obviously required are those for the C-C and C-H bonds, which are known to have values of about -105 and -90kcal/mol (bond energies). ... 

Three- and four-membered rings (cyclopropane and cyclobutane derivatives) were treated in heat of formation calculations by MM3. Each of them required its own structural parameter set and its own ring constants (R3 and R4, respectively). Four-membered rings have also been treated by MM4. These small ring compounds present no particular difficulties and are well treated in the same way as are the alkanes. 

A force field was thereby devised to fit the structures and vibrational spectra of these molecules. But, of course, if we change the zero of reference for our bending, then we have to expect that there will be required one or more parameters in the heat of formation calculations to allow for this change of reference. At a minimum, we expect that/ 3 and R parameters will be needed because the G s here are substantially different from the usual values. These parameters are composed of two parts, one of which is from the same bonding change described for R. The second part involves the strain required to deform the geometry and reset Go from tetrahedral to the small ring value. 

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