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

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 et al). 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 a ~ 100 fs timescale, 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 timescale ~120 fs response to the incipient ion pair formation is primarily associated with librational modes 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(+).

publication date

  • January 1, 2016