Mechanistic Insights Into Luminol Chemiexcitation: The Role of the Amide Carbonyls.

Luminol chemiluminescence originates from a chemiexcitation process that occurs during the decomposition of its bicyclic endoperoxide intermediate, involving peroxide bond cleavage and molecular nitrogen release. In this work, we investigated the mechanism of this decomposition using multiconfigurational quantum chemistry and density functional theory. Calculations on a reduced model without the aniline ring and without the alkoxide groups, which come from the amide carbonyl groups of luminol, showed that the pathway initiated by O-O bond cleavage is inaccessible, whereas the route starting by nitrogen loss and the concerted decomposition display similar activation barriers. However, only the stepwise mechanism reaches regions favorable for population transfer to the emissive excited state. The concerted mechanism therefore acts as a competing channel that reduces the chemiexcitation and chemiluminescence yields. By extending the study to a larger model (including the alkoxide groups) and to the complete luminol molecule, the energetic trends change in favor of the stepwise or asynchronous decomposition with the C-N bond breaking preceding the O-O bond cleavage, which drives the system toward the chemiexcitation region. Through the analysis of natural bond orbitals, it was shown that it is the donation of the lone pairs of the alkoxide groups that weakens the C-N bond.