Temperature Dependence of Excited‐State Proton Transfer and Geminate Recombination Processes in Water and in Glycerol‐Doped Ice

Abstract

The reversible proton dissociation and geminate recombination of photoacids was studied as a function of temperature in neat water, binary water mixture containing 0.6 mol% glycerol, and doped ice containing 0.6 mol% glycerol. The deuterium isotope effect on both condensed phases was also studied. 8‐hydroxypyrene‐1,3,6 trisulfonate trisodium salt was used as the electronically‐excited‐state proton emitter. The experimental data are analyzed by the Debye–Smoluchowski equation solved numerically with boundary conditions to account for the reversibility of the reaction. We propose a qualitative model to describe the unusual temperature dependence of the proton transfer rate in the liquid phase. We also propose a model for proton transfer in solid ice based on L‐defects transport as proton acceptors. While in the liquid phase at t > 10°C the proton dissociation rate constant is almost temperature independent, in glycerol‐doped ice we find a large temperature dependence.

Effect of Pressure on the Proton-Transfer Rate from a Photoacid to a Solvent. 2. DCN2 in Propanol

“…In previous studies, − we used the longest component of the dielectric relaxation time, τ D , for the solvent-coordinate preexponential factor of the rate constant, k S = b /τ D exp(−Δ G ⧧ / RT ), where b is an empirical factor. For water, we found b ≈ 6 and, for all monols studied, the value is less, 2 < b < 4.…”

“…Excited-state proton transfer (ESPT) from a photoacid molecule − was used in studies of proton-transfer reactions in liquids and solids. Recent studies on the proton-transfer rate to the solvent − emphasize the dual role played by the solvent molecule (1) as a proton acceptor and (2) as a solvating medium of both the reactant and the product. − …”

Connection between Proton Abnormal Conductivity in Water and Dielectric Relaxation Time

“…The solvent velocity, Ṡ , on the other hand, strongly depends on the temperature. In our previous papers, − we suggested that Ṡ is related to the slow components of the solvent dielectric relaxation. We infer that Ṡ = b /τ D , where τ D is the solvent dielectric relaxation time and b is an empirical factor, dependent on the specific protic solvent, and its value is between 1 and 4 for the proton-transfer reaction from a photoacid to several alcohols.…”

“…Also we found that the inverse of the proton-transfer rate constant at low temperature has a value similar to τ D . Previously, − we found that the temperature dependence of the proton-transfer rate constant from several photoacids to solvent is explained as a continuous transition from nonadiabatic to solvent-controlled limits. In the nonadiabatic limit, the rate-determining step in the proton-transfer rate is the proton tunneling from the proton donor (the acid) to the proton acceptor (a hydrogen-bonded solvent molecule).…”

Solvent‐Dependent Photoacidity State of Pyranine Monitored by Transient Mid‐Infrared Spectroscopy

“…In this picture the excited state proton transfer dynamics, controlled by barrier crossing, is described by a two-state model (Figure 1 a), where an optically excited photoacid state converts into an excited conjugated photobase upon proton transfer. [16][17][18][19][20][21][22][23][24][25][26] A second model invokes the occurrence of and the internal conversion between nearby lying electronic excited levels of the photoacid (Figure 1 b). Typically two spectroscopically accessible states can be reached for aromatic molecules upon electronic excitation, either with light polarized along the through-bond axis ( 1 L b state) or along the through-atom axis…”