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G. Bu 3SNH AIBN Br + + ∆ 17% 81% 2% We will see in Chapter 19 that calculations show cyclohexyl radical to be about 8 kcal/mol more stable than cyclopentylmethyl radical. Were the reaction under strict thermodynamic control, products derived from cyclopentylmethyl radical should not be observed at all. However, the transition state corresponding to radical attack on the “internal” double bond carbon (leading to cyclopentylmethyl radical) is about 3 kcal/mol lower in energy than that corresponding to radical attract on the “external” double bond carbon (leading to cyclohexyl radical).

It is the kinetically-favored pathway leading to the kinetic product. Pathway B proceeds through the higher-energy transition state, but leads to the lower-energy products. It is the thermodynamically-favored pathway leading to the thermodynamic product. By varying conditions (temperature, reaction time, solvent) chemists can affect the product distribution. , pathway B would be both the kinetic and thermodynamic pathway, and its product would be both the kinetic and thermodynamic product. energy A B reaction coordinate Here too, varying reaction conditions will affect product distribution, because the difference in activation energies will not be the same as the difference in product energies.

It should be noted that were the exact exchange/correlation functional known, then the density functional approach would be “exact”. While “better” forms of such 31 Chapter 2 31 3/21/03, 11:46 AM functionals are constantly being developed, there is (at present) no systematic way to improve the functional to achieve an arbitrary level of accuracy. Density functional models, like Hartree-Fock models are applicable to molecules of moderate size (50-100 atoms). Most modern implementations of density functional theory divide the problem into two parts.

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