Electron Flow Characterization of Charge Transfer for Carbonic Acid to Strong Base Proton Transfer in Aqueous Solution

Protonation of the strong base methylamine CH3NH2 by carbonic acid H2CO3 in aqueous solution, HOCOOH center dot center dot center dot NH2CH3 -> HOCOO- center dot center dot center dot+HNH2CH3, has been previously studied (J. Phys. Chem. B 2016, 109, 2271-2280; J. Phys. Chem. B 2016, 109, 2281-2290) via Car-Parinnello molecular dynamics. This proton transfer (PT) reaction within a hydrogen (H)-bonded complex was found to be barrierless and very rapid, with key reaction coordinates comprising the proton coordinate, the H-bond separation R-ON, and a solvent coordinate, reflecting the water solvent rearrangement involved in the neutral to ion pair conversion. In the present work, the reaction's charge flow aspects are analyzed in detail, especially a description via Mulliken charge transfer for PT (MCTPT). A natural bond orbital analysis and some extensions of them are employed for the complex's electronic structure during the reaction trajectories. Results demonstrate that consistent with the MCTPT picture, the charge transfer (CT) occurs from a methylamine base nonbonding orbital to a carbonic acid antibonding orbital. A complementary MCTPT reaction product perspective of CT from the antibonding orbital of the HN+ moiety to the nonbonding orbital of the oxygen in the H-bond complex is also presented. sigma(OH) and sigma(HN+) bond order expressions show this CT to occur within the H-bond OHN triad, an aspect key for simultaneous bond-breaking and -forming in the PT reaction.

Automated summary

The proton transfer reaction between methylamine and carbonic acid in aqueous solution is rapid and barrierless, with key reaction coordinates involving the proton, H-bond separation, and solvent rearrangement. Charge transfer is analyzed in detail through Mulliken charge transfer for proton transfer reactions, showing CT occurring between nonbonding and antibonding orbitals. The PT reaction involves simultaneous bond-breaking and -forming within the H-bond OHN triad.