Experimental heats of formation

The experimental heats of formation for methane and formaldhyde are -16.0 and -25.0 kcal-moT, respectively (both at 0 K). 

Both values are in excellent agreement with the experimental heats of formation. Note that the experimental value for fluoromethane is quite uncertain. ... 

Experimental heats of formation, Pu intermetallics vs. corresponding heats calculated from Miedema... 

Miedema model calculated heats vs. experimental heats of formation,... 

Fig. 6. Comparison of relative experimental heats of formation with those calculated by means of MINDO/3 AAH°, w and AAH calc (kJ mol-1) for some selected carbocations...

However, in general the MINDO/3 calculations reproduce the experimental heats of formation to satisfaction, as can be seen in Fig. 6. 

Using standard enthalpies from Ref. 132) a reaction enthalpy of 92 kJ mol-1 is the result for reaction (24). This value lies near the above estimated limit and not far from the values which have been calculated from experimental heats of formation for the second and third propagation step (102 kJ mol-1). 

The standard deviation between experimental and calculated heats of reaction are between 0.5 and 1 kcal/mol for those classes of compounds where enough experimental heats of formation are available to allow a full parameterization. For those classes of compounds where insufficient heats of formation are known to allow the determination of all parameters for 1,2- and 1,3-interactions, an estimate can be given for the bond energy terms which are the dominating parameters. Even here, therefore, a reasonable value for the reaction enthalpy is available. 

Similar MNDOj calculations for N, O, F compounds using bond parameters for O—F and N—F permitted enthalpies of unknown compounds such as FbN(OF)3 to be predicted and possible syntheses evaluated (96). [The input data to this set of calculations included a wrong value for AiJf(F(g)) = 270.3 kJ mol-1. It should be noted that experimental heats of formation of atoms are needed in all these calculations.]... 

Table 54. Experimental heats of formation of isomeric compounds...

In our own validation sets, experimental heats of formation are preferentially taken from recognized standard compilations [38-40]. If there are enough experimental data for a given element, we normally only use reference values that are accurate to 2 kcal/mol. If there is a lack of reliable data, we may accept experimental heats of formation with a quoted experimental error of up to 5 kcal/mol. This choice is motivated by the target accuracy of the established semiempirical methods. If experimental data are missing for a small molecule of interest, we consider it legitimate [18] to employ computed heats of formation from high-level ab initio methods as substitutes. 

In order to illustrate these ideas, let us consider the determination of the thermochemistry of C Hg, which has five stable isomers. These isomers and their experimental heats of formation are trans-CH3CHCHCH3 (AHf (exp = - 3.0 kcal/mol), CIS-CH3CHCHCH3 (AHf (exp) =-1.9 kcal/mol), CH2CHCH2CH3 (AHf (exp) =-0.2 kcal/mol), CH2QCH3)2... 

It is likely that different quantum chemical models will perform differently in each of these situations. Processes which involve net loss or gain of an electron pair are likely to be problematic for Hartree-Fock models, which treat the electrons as essentially independent particles, but less so for density functional models and MP2 models, which attempt to account for electron correlation. Models should fare better for processes in which reactants and products are similar and benefit from cancellation of errors, than those where reactants and products are markedly different. The only exception might be for semi-empirical models, which have been explicitly parameterized to reproduce individual experimental heats of formation, and might not be expected to benefit from error cancellation. 

The results presented here show that quantum-chemistry methods, whose accuracy and sophistication continue to increase, are capable of providing thermochemical data of practical value for modehng organometallic tin chemistry. In particular, the relativistic effective core potential used here appears to provide an adequate description of the electronic structure at tin, based on the favorable comparisons between experimental heats of formation and values predicted by the ECP/BAC-MP4 method. Trends in heats of... 

Note that the construction of an isodesmic equation is something of an art, depending on chemical intuition and available experimental data. In the above situation, if an experimental heat of formation for quinoline were not available, we might decide to resort to a reaction like... 

The necessary experimental heats of formation are known to exquisite accuracy (or defined as zero, in the case of H2), and the calculations will be trivial for such small molecules, but accurately accounting for the enormous differences in the natures of the bonds on the two sides of Eq. (10.40) will require levels of electronic-structure theory nearly as high as those that would be necessary for a direct or parametric computation on 6-methylquinoline alone. The one virtue of Eq. (10.40), which is an example of a bond separation reaction , is that the total amount of unpaired electron spin on the two sides of the reaction is the same (in this case, zero) such a reaction is called isogyric . Note that atomization processes are... 

The experimental heat of formation of tetrazole of 28.11 eV, compared to values for pyrazole (39.391 eV), imidazole (39.790 eV) and 1,2,4-triazole (34.377 eV), suggests that tetrazole is less stable than the lower azoles (66JCP759). Dewar has calculated resonance energies of 26.0 kcal mol-1 for 1H-tetrazole and 29.25 kcal mol 1 for 27/-tetrazole which compare with values for pyrazole (35.5), imidazole (22.9) and Iff-1,2,3-triazole (27.9 kcal mol-1) (66JCP759). 

Oog and Hunt8711 found the heat of combustion of ethylene oxide to be 312 55 0.20 koal./mole. The heats of fusion and vaporization, determined with great precision by Giauque and Gordon,8 8 are 1236 and 6101 kcal./moie respectively. An approximate value for the strain energy of ethylene oxide has been computed by Nelson and Jeesup.mv The experimental heat of formation, derived by combustion calari-> metry, was subtracted from the calculated total bond energy, obtained... 

---