Solvent-Induced Red-Shifts for the Proton Stretch Vibrational Frequency in a Hydrogen-Bonded Complex. 1. A Valence Bond-Based Theoretical Approach

A theory is presented for the proton stretch vibrational frequency ν_AH for hydrogen (H−) bonded complexes of the acid dissociation type, that is, AH···B ⇔ A^–···HB⁺(but without complete proton transfer), in both polar and nonpolar solvents, with special attention given to the variation of ν_AH with the solvent’s dielectric constant ε. The theory involves a valence bond (VB) model for the complex’s electronic structure, quantization of the complex’s proton and H-bond motions, and a solvent coordinate accounting for nonequilibrium solvation. A general prediction is that ν_AH decreases with increasing ε largely due to increased solvent stabilization of the ionic VB structure A^–···HB⁺ relative to the neutral VB structure AH···B. Theoretical ν_AH versus 1/ε slope expressions are derived; these differ for polar and nonpolar solvents and allow analysis of the solvent dependence of ν_AH. The theory predicts that both polar and nonpolar slopes are determined by (i) a structure factor reflecting the complex’s size/geometry, (ii) the complex’s dipole moment in the ground vibrational state, and (iii) the dipole moment change in the transition, which especially reflects charge transfer and the solution phase proton potential shapes. The experimental proton frequency solvent dependence for several OH···O H-bonded complexes is successfully accounted for and analyzed with the theory

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⁺

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