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 transf er reactions in aqueous solutions. of o-nitrobenzaldehyde have been in vestigated using time-resolved photoacoustics. The solvation of the ne wly formed ions is accompanied by a contraction of the medium of about -5.2 +/- 0.2 ml/mol, At a pH above 9.5, the formation of water molecu les leads to an expansion of 24.5 +/- 0.4 ml/mol. The apparent rate co nstant for the protonation of hydroxyls is k = (4.9 +/- 0.4) X 10(10) M-1 s(-1). This is the first direct time-resolved measurement of the e xtent of the structural volume changes accompanying water formation.