Dissociation of a Strong Acid in Neat Solvents: Diffusion Is Observed after Reversible Proton Ejection Inside the Solvent Shell

Abstract

Strong-acid dissociation was studied in alcohols. Optical excitation of the cationic photoacid N-methyl-6-hydroxyquinolinium triggers proton transfer to the solvent, which was probed by spectral reconstruction of picosecond fluorescence traces. The process fulfills the classical Eigen-Weller mechanism in two stages: (a) solvent-controlled reversible dissociation inside the solvent shell and (b) barrierless splitting of the encounter complex. This can be appreciated only when fluorescence band integrals are used to monitor the time evolution of the reactant and product concentrations. Band integrals are insensitive to solvent dynamics and report relative concentrations directly. This was demonstrated by first measuring the fluorescence decay of the conjugate base across the full emission band, independently of the proton-transfer reaction. Multiexponential decay curves at single wavelengths result from a dynamic red shift of fluorescence in the course of solvent relaxation, whereas clean single exponential decays are obtained if the band integral is monitored instead. The extent of the shift is consistent with previously reported femtosecond transient absorption measurements, continuum theory of solvatochromism, and molecular properties derived from quantum chemical calculations. In turn, band integrals show clean biexponential decay of the photoacid and triexponential evolution of the conjugate base in the course of the proton transfer to solvent reaction. The dissociation step follows the slowest stage of solvation, which was measured here independently by picosecond fluorescence spectroscopy in five aliphatic alcohols. Also, the rate constant of the encounter-complex splitting stage is compatible with proton diffusion. Thus, for this photoacid, both stages reach the highest possible rates: solvation and diffusion control. Under these conditions, the concentration of the encounter complex is substantial during the earliest nanosecond.

The critical size of hydrogen-bonded alcohol clusters as effective Brønsted bases in solutions

“…In neat MeOH, the excitation of AH + at 375 nm produces a main band with a peak at 517 nm and a hint of another band appearing as a shoulder peak below 470 nm. 25 The population of A* rises in 240 AE 20 ps (26%) with the rest being populated within our IRF of 120-150 ps. The dominant band at around 517 nm in MeOH is assigned to the fluorescence of excited A (A*) because the peak spectral position of A* was found to be at around 505 nm in water and 518 nm in EtOH.…”

“…11,12 When ROH loses a proton as a Brønsted acid, it forms an alkoxide ion (RO À ), which is a strong base and appears in many organic syntheses and catalyses as an intermediate. 25,26 ROHs with a wide range of basicity, from MeOH to t-butanol (t-BuOH), were used as bases here. ROH 2 + is generally very unstable due to the low basicity of ROH.…”

Alcohol Dimer is Requisite to Form an Alkyl Oxonium Ion in the Proton Transfer of a Strong (Photo)Acid to Alcohol

“…Herein, we report for the first time the spectroscopic identification of the H‐bonded clustering of alcohol molecules to facilitate the formation of the alkyl oxonium ion. To model the first step in alcohol dehydration (i.e., the acid–base reaction between a strong acid and an alcohol; Scheme ), we designed the intermolecular proton‐transfer reaction of a photoacid with ethanol (EtOH) in liquid phase. The acidity of the photoacid is turned on upon electronic excitation and promoted to approximately that of sulfuric acid (p K a ≈−2).…”

“…This dynamic process accompanies the decay tail of the fluorescence of the parent photoacid, according to a power law, t −d . In this study, N ‐methyl‐7‐hydroxyquinolinium ion was carefully chosen because its deprotonated form is neutral, so there exists much weaker ion–dipole interaction to the dissociated proton with negligible (or weak) geminate recombination . This strategy allows straightforward interpretation of simpler dynamics.…”

Broadband fluorescence reveals mechanistic differences in excited-state proton transfer to protic and aprotic solvents

“…When the reaction rate approaches the time scale of the solvent dynamics, the reaction becomes solvent-controlled and is limited by the relaxation time of the solvent. [29][30][31][32][33][34][35] This complicates the analysis because the singlewavelength uorescence decays become contaminated by the spectral shis due to solvent relaxation. Therefore, the individual contributions from the spectral shis and population dynamics must be resolved separately for a detailed quantitative analysis.…”