Composition Dependent Stokes Shift Dynamics in Binary Mixtures of 1‑Butyl-3-methylimidazolium Tetrafluoroborate with Water and Acetonitrile: Quantitative Comparison between Theory and Complete Measurements

Here we predict, using a semimolecular theory, the Stokes shift dynamics of a dipolar solute in binary mixtures of an ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate ([Bmim]­[BF₄]), with water (H₂O) and acetonitrile (CH₃CN), and compare with the experimental results. The latter are from the recent measurements that combined broad-band fluorescence up-conversion (FLUPS) with time-correlated single photon counting (TCSPC) techniques and used coumarin 153 (C153) as a solute probe. Nine different compositions of ([Bmim]­[BF₄] + H₂O) and ([Bmim]­[BF₄] + CH₃CN) binary mixtures are considered for the extensive comparison between theory and experiments. Two separate model calculations have been performed using the available experimental frequency dependent dielectric function, ε­(ω). These calculations semiquantitatively reproduce the experimentally observed (i) IL mole fraction dependence of dynamic Stokes shifts in these mixtures, (ii) composition dependence of average fast, slow, and solvation times, (iii) viscosity dependence of slow times, and (iv) the nonlinear dependence of average solvation times on experimental inverse conductivity. Variations of the calculated dynamics on water dipole moment values (gas phase or liquid phase) and sensitivity to different measurements of ε­(ω) for ([Bmim]­[BF₄] + H₂O) mixtures are examined. In addition, the importance of the missing contribution to experimental ε­(ω) from high frequency collective solvent intermolecular modes for generating the experimentally observed sub-picosecond solvation response in these (IL + polar solvent) binary mixtures has been explored

Importance of Solvents’ Translational–Rotational Coupling for Translational Jump of a Small Hydrophobic Solute in Supercooled Water

Despite clear evidence of sudden translational jump occurrence of a solute in supercooled water, a detailed mechanism of this jump is still lacking. A previous work [Indra, S.; Daschakraborty, S. Chem. Phys. Lett. 2017, 685, 322−327] put forward a mechanism of this jump from an initial solvent cage to a final one. The proposed mechanism is astoundingly similar to that of the electron/proton transfer reaction in aqueous solution. The above study identified the spatial prearrangement (rearrangement before the jump occurrence) of cage forming water solvent molecules as the actual reaction coordinate. However, the study completely ignored the contribution of the orientational prearrangement of solvent water molecules. In this study, we have monitored both the spatial and the orientational prearrangements of water solvent molecules at subzero temperatures during the jump occurrence of the solute. We have found overwhelming contributions of both the spatial and orientational prearrangements of water, which symmetrize the hydration structure at the initial and final cage positions to facilitate the jump event. Through a systematic temperature dependence study (from <i>T</i> = 240 to 270 K), we have found clear evidence that a strong synchronization between translational and rotational prearrangements of the solvent water molecules is crucial for the solute’s jump from one solvent cage to another in supercooled water (below <i>T</i> = 252 K). The above translation–rotation synchronization is probably due to the cooperative movement of solvent water molecules forming clusters in the supercooled region. Since these cooperative dynamics are the consequence of the spatiotemporal heterogeneity in the medium, we infer that the large-amplitude translational jump of the nonpolar solute probably stems from the spatiotemporal heterogeneity of supercooled water. At temperatures above the melting point, this cooperativity is partly lost since the translational and orientational prearrangements become somewhat independent of each other

Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution I: Acid and Base Coordinate and Charge Dynamics

Protonation by carbonic acid H₂CO₃ of the strong base methylamine CH₃NH₂ in a neutral contact pair in aqueous solution is followed via Car–Parrinello molecular dynamics simulations. Proton transfer (PT) occurs to form an aqueous solvent-stabilized contact ion pair within 100 fs, a fast time scale associated with the compression of the acid–base hydrogen-bond (H-bond), a key reaction coordinate. This rapid barrierless PT is consistent with the carbonic acid-protonated base p<i>K</i>_a difference that considerably favors the PT, and supports the view of intact carbonic acid as potentially important proton donor in assorted biological and environmental contexts. The charge redistribution within the H-bonded complex during PT supports a Mulliken picture of charge transfer from the nitrogen base to carbonic acid without altering the transferring hydrogen’s charge from approximately midway between that of a hydrogen atom and that of a proton

Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution II: Solvent Coordinate-Dependent Reaction Path

The protonation of methylamine base CH₃NH₂ by carbonic acid H₂CO₃ within a hydrogen (H)-bonded complex in aqueous solution was studied via Car–Parrinello dynamics in the preceding paper (Daschakraborty, S.; Kiefer, P. M.; Miller, Y.; Motro, Y.; Pines, D.; Pines, E.; Hynes, J. T. <i>J. Phys. Chem. B</i> <b>2016</b>, DOI: 10.1021/acs.jpcb.5b12742). Here some important further details of the reaction path are presented, with specific emphasis on the water solvent’s role. The overall reaction is barrierless and very rapid, on an ∼100 fs time scale, with the proton transfer (PT) event itself being very sudden (<10 fs). This transfer is preceded by the acid–base H-bond’s compression, while the water solvent changes little until the actual PT occurrence; this results from the very strong driving force for the reaction, as indicated by the very favorable acid-protonated base Δp<i>K</i>_a difference. Further solvent rearrangement follows immediately the sudden PT’s production of an incipient contact ion pair, stabilizing it by establishment of equilibrium solvation. The solvent water’s short time scale ∼120 fs response to the incipient ion pair formation is primarily associated with librational modes and H-bond compression of water molecules around the carboxylate anion and the protonated base. This is consistent with this stabilization involving significant increase in H-bonding of hydration shell waters to the negatively charged carboxylate group oxygens’ (especially the former H₂CO₃ donor oxygen) and the nitrogen of the positively charged protonated base’s NH₃⁺