Reaction volume of water formation detected by time-resolved photoacoustics: photoinduced proton transfer between o-nitrobenzaldehyde and hydroxyls in water

The structural volume changes accompanying light-induced proton transfer reactions in aqueous solutions of o-nitrobenzaldehyde have been investigated using time-resolved photoacoustics. The solvation of the newly formed ions is accopanied by a contraction of the medium of about −5.2 ± 0.2 ml/mol. At a pH above 9.5, the formation of water molecules leads to an expansion of 24.5 ± 0.4 ml/mol. The apparent rate constant for the protonation of hydroxyls is k = (4.9 ± 0.4) X 1010 M− s−. This is the first direct time-resolved measurement of the extent of the structual volume changes accompanying water formation.