Heat of formation and strain energy

After all the parameters are obtained, we can now do a molecular mechanics calculation. What is the result A structure for the molecule is obtained by minimizing the total energy. This is a straightforward task in principle. Since Eq. 2.40 constitutes an analytical expression relating energy and geometry, we can use the derivatives of this equation to assist us in geometry optimization. 

The other outcome from a molecular mechanics calculation is a value for ,o, (Eq. 2.40). However, t , is not a particularly useful quantity. It is just a number, obtained by adding up a collection of equations. We want to minimize i t to obtain the best possible geometry, but the actual value of the number does not directly relate to any experimental quantity. As such. 

There are some instances in which Etot is useful. If we are comparing stereoisomers, values of Etot provide useful relative energies. That is because stereoisomers will always have identical contributors to Etot, both in terms of the equations and the parameters involved. Since all structures along a torsional path are stereoisomers (conformers), Etot can be used to determine rotation barriers. Note that E,ot cannot be used for constitutional isomers, such as n-butane vs. isobutane. That is because different parameters are likely involved, such as a CH3-CHR2 k and r in isobutane vs. the CH3-CH2R k and r for w-butane. Only after E, t values for these two structures are converted to heats of formation can energy comparisons be made. 

In general, then, the molecular mechanics method produces AH values. In principle, the information to derive AS ° is embedded in the method, but in practice the method is not nearly accurate enough to produce meaningful AS° values. 

That is, nothing that we know about chemistry justifies dissection of the total energy of a molecule into separable components as implied by Eq. 2.40. The only justification for the method is that it works— not always, but often. 

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HEAT OF FORMATION AND STRAIN ENERGY CALCULATIONS (UNIT=KCAUMOLE)... 

Figure 15.1. Heats of formation and strain energies of cyclic hydrocarbons in kilocalories per mole. The values given to 0.1 kcal/mol are experimental values (Refs. 2, 3, 17) and have an uncertainty of 0.5-1.0 kcal/mol. The values given as integers are calculated values (B3LYP/6-311+G ) and have an uncertainty of 3 kcal/mol.

Table 4 Calculated Heats of Formation and Strain Energies for Simple Saturated Thiins <75JA5167>...

Whereas semiempirical MO calculations do not provide reliable estimates of cycloproparene geometries, they do give reliable heats of formation and strain energies (SE) in comparison with both ab initio methods and experiment. The strain energies of 1 and 11... 

TABLE 1. Calculated (MM2) and experimental heats of formation and strain energy in some spiroannulated cyclopropanes... 

The heat of hydrogenation of 1-azacyclopentene was determined by measuring the heat of hydrogenation of its trimer. The data give information on the heats of formation and strain energies of a number of cyclic and acyclic imines454. 

TABLE 6. Heats of formation and strain energies of cycloalkanes and cycloalkenes (kcal mol )... 

Included in reports of thermochemical studies are estimates of the heats of formation and strain energies of the azoalkenes (11) and (12) and hydrocarbon (13). The conversion of the azoalkenes into bicyclo[n, 2, Ojalkanes is now suggested to be much less exothermic than previously estimated. 

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