title: Reaction Mechanism for Direct Proton Transfer from Carbonic Acid to a Strong Base in Aqueous Solution II: Solvent Coordinate-Dependent Reaction Path.
abstract: 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. J. Phys. Chem. B 2016, 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 ΔpKa 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(+).
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado , Boulder, Colorado 80309-0215, United States.
authors.forename: Snehasis
authors.initials: S
authors.lastname: Daschakraborty
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado , Boulder, Colorado 80309-0215, United States.
authors.forename: Philip M
authors.initials: PM
authors.lastname: Kiefer
authors.affiliation: Department of Chemistry, Ben-Gurion University of the Negev , P.O. Box 653, Beer-Sheva 84105, Israel.
authors.forename: Yifat
authors.initials: Y
authors.lastname: Miller
authors.affiliation: Department of Chemistry, Ben-Gurion University of the Negev , P.O. Box 653, Beer-Sheva 84105, Israel.
authors.forename: Yair
authors.initials: Y
authors.lastname: Motro
authors.affiliation: Department of Chemistry, Ben-Gurion University of the Negev , P.O. Box 653, Beer-Sheva 84105, Israel.
authors.forename: Dina
authors.initials: D
authors.lastname: Pines
authors.affiliation: Department of Chemistry, Ben-Gurion University of the Negev , P.O. Box 653, Beer-Sheva 84105, Israel.
authors.forename: Ehud
authors.initials: E
authors.lastname: Pines
authors.affiliation: Department of Chemistry and Biochemistry, University of Colorado , Boulder, Colorado 80309-0215, United States.|Ecole Normale Supérieure-PSL Research University, Chemistry Department, Sorbonne Universités-UPMC University Paris 06, CNRS UMR 8640 Pasteur, 24 rue Lhomond, 75005 Paris, France.
authors.forename: James T
authors.initials: JT
authors.lastname: Hynes
mesh_terms: D002255:Carbonic Acid / Q000737:chemistry*; D006860:Hydrogen Bonding; D011522:Protons; D012997:Solvents / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D011522:Protons; D012997:Solvents; D014867:Water; D002255:Carbonic Acid