Controlling Selectivity by Controlling Energy Partitioning in a Thermal Reaction in Solution

Area of Science:

• Chemical kinetics
• Physical chemistry
• Reaction dynamics

Background:

• Kinetic isotope effects (KIEs) are sensitive probes of reaction mechanisms.
• Statistical rate theories, like RRKM theory, assume energy randomization within a molecule.
• The applicability of these theories to radical reactions, particularly α-cleavage, requires careful examination.

Purpose of the Study:

• To assess the validity of statistical rate theories for predicting KIEs in alkoxy radical α-cleavage.
• To investigate the influence of substituent size on energy distribution and KIEs.
• To determine the conditions under which cleavage proceeds non-statistically.

Main Methods:

• Comparison of experimental KIEs with predictions from statistical rate theories.
• Systematic variation of substituent size in alkoxy radicals.
• Analysis of energy imparted to radicals and its distribution.

Main Results:

• Cleavage behavior (statistical vs. non-statistical) depends on the cleavage barrier and radical internal energy.
• Large alkyl substituents increase vibrational energy but lead to non-statistical distribution.
• This non-statistical energy distribution results in KIEs lower than those predicted by purely statistical models.

Conclusions:

• Statistical rate theories are not universally applicable to alkoxy radical α-cleavage.
• Substituent effects play a crucial role in determining the statistical nature of the reaction.
• A semistatistical localized RRKM model can approximate the observed KIEs under specific conditions.