Multiple excited states in a two-state crossing model: Predicting barrier height evolution for H plus alkene addition reactions

In order to identify the underlying factors determining barrier heights whe n hydrogen atoms add to alkenes, we present a theoretical framework isolati ng the fundamental quantum-chemical properties involved and enabling evalua tion of the relative influence of each property. This approach describes th e control of these barriers and motivates a series of experimental measurem ents as a rigorous test. A two-state avoided curve crossing model provides the essential description. but only when multiple excited states are combin ed to yield a mixed state of dual covalent-ionic character. We show that va riations in mixed-state energy drive the evolution in barrier heights, and that by selecting a set of test reactions with diverse energetic and overla p interactions, one may discover which of several excited states dominates this evolution. Results from the experimental test show conclusively that i t is variation in the lowest ionic-state energy, and not variations in eith er singlet-triplet splitting or reaction enthalpy that drive barrier height evolution over die series of H + alkene addition reactions. Combining this result with our earlier results for H-atom abstraction reactions, we have demonstrated that barrier heights of essentially all radical-molecule react ions with electrophilic radicals are controlled by the excited ionic states formed by the transfer of an electron from the molecule to the radical.