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

• Main Authors: Philip M. Kiefer (1777345), Snehasis Daschakraborty (1849987), Dina Pines (1777348), Ehud Pines (1777342), James T. Hynes (1581865)
• Format: Other Non-Article Part of Journal/Newspaper
• Language: unknown
• Published: 2021
• Subjects: Biophysics; Biochemistry; Genetics; Marine Biology; Inorganic Chemistry; Biological Sciences not elsewhere classified; Chemical Sciences not elsewhere classified; sup >+</ sup; ion pair conversion; electron flow characterization; 2281 – 2290; 2271 − 2280; charge flow aspects; oh </ sub; </ sub >; 3 </ sub; 2 </ sub; bond ohn triad; hocooh ··· nh; aqueous solution protonation; carbonic acid h; carbonic acid; aqueous solution; r </; charge transfer; >< sub; simultaneous bond; bond separation; bond complex; → hocoo; solvent coordinate; results demonstrate; reaction ’; reaction within; reaction trajectories; proton transfer; proton coordinate; previously studied; present work; occur within; nonbonding orbital; mctpt picture; mctpt ); electronic structure; complex ’; b 2016; aspect key; antibonding orbital; also presented; Carbonic acid
• Online Access: https://doi.org/10.1021/acs.jpcb.1c05824.s001

Protonation of the strong base methylamine CH 3 NH 2 by carbonic acid H 2 CO 3 in aqueous solution, HOCOOH···NH 2 CH 3 → HOCOO – ··· + HNH 2 CH 3 , 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. σ OH and σ 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.