Determination of the pKa of the Aci‐Nitro Intermediate in o‐Nitrobenzyl Systems

The structural volume changes following photoexcitation of o‐nitrobenzyl systems are used to estimate the excited state pKa of the aci‐nitro intermediate in aqueous solutions. The rather large contractions induced in solution by UV excitation are due to the rapid deprotonation of the short‐lived aci‐nitro intermediate, which leads to the formation of two charged species. The magnitude of the measured contraction as a function of the pH in acidic solutions follows a sigmoidal curve, from which it is possible to extract the pKa of the aci‐nitro intermediate. This method is generally applicable to short‐lived intermediates with stronger acid character than the parent compound, provided they undergo irreversible chemical transformations to a product that cannot rebind the photodetached proton. The reaction volume for water formation from its ionic constituents at basic pH allows the determination of the deprotonation quantum yields.