Energy-labeling and –read-out of reactive intermediates in solution enabled by energy-dependent reaction selectivity

Any long-lived chemical structure in solution is subject to statistical energy equilibration, so the history of any specific structure does not affect its subsequent reactions. This is not true for very short-lived intermediates, since energy equilibration takes time. Here, this idea is applied to achieve the energy labeling of a reactive intermediate. The selectivity of the ring-opening α-cleavage reaction of 1-methylcyclobutoxy radical is found here to vary broadly depending on how the radical was formed. Reactions that provide little excess energy to the intermediate lead to high selectivity in the subsequent cleavage (measured as a kinetic isotope effect) while reactions that provide more excess energy to the intermediate exhibit lower selectivity. Allowing for the expected excess energy allows the prediction of the observed product ratios, and in turn the product ratios can be used to obtain a read-out of the energy present in a intermediate.