NaCl effect on the excited-state proton dissociation reaction of naphthols: water structure in the presence of NaCl

The NaCl effect on the proton dissociation reaction in water at 300 K has been studied by means of nanosecond and picosecond spectroscopy with fluorimetry. The proton dissociation rate constant k/sub 1/ decreased with an increase of NaCl according to the equation k/sub 1/ = k/sub 0/((H/sub 2/O)/sub 4/)(1 - ..cap alpha..(NaCl)), where k/sub 0/ denotes the second-order rate constant for the proton transfer from excited naphthols to water clusters (H/sub 2/O)/sub 4/ and ..cap alpha.. is a constant value. This means that the decrease in the k/sub 1/ value is caused by NaCl-induced destruction of water clusters resulting in production of the hydrated ions (Na/sup +/)/sub hyd/ and (Cl/sup -/)/sub hyd/. The average hydration number of Na/sup +/ or Cl/sup -/ in water is obtained to be 4.4 or 6.6, respectively. The kinetic H-D isotope effect for k/sub 1/ in the presence of NaCl was also examined.