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 because energy equilibration takes time. Here, this idea is applied to achieve the ‘energy labelling’ of a reactive intermediate. The selectivity of the ring-opening α-cleavage reaction of the 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 a high selectivity in the subsequent cleavage (measured as a kinetic isotope effect), whereas reactions that provide more excess energy to the intermediate exhibit a lower selectivity. Accounting for the expected excess energy allows the prediction of the observed product ratios and, in turn, the product ratios can be used to determine the energy present in an intermediate.

溶液中任何长寿命的化学结构都需要进行统计能量平衡，因此任何特定结构的历史记录都不会影响其后续反应。对于寿命很短的中间体而言，情况并非如此，因为能量平衡需要时间。在这里，这个想法被用来实现反应性中间体的“能量标记”。在此发现1-甲基环丁氧基自由基的开环α-裂解反应的选择性根据自由基的形成方式而广泛地变化。向中间体提供很少过量能量的反应在随后的裂解中导致高选择性（以动力学同位素效应测量），而向中间体提供更多过量能量的反应则显示出较低的选择性。