Double Proton Transfer Across a Table: The Formic Acid Dimer–Fluorobenzene Complex

• Published in: Angewandte Chemie International Edition Vol. 60; no. 49; pp. 25674 - 25679
• Main Authors: Li, Weixing, Tikhonov, Denis S., Schnell, Melanie
• Format: Journal Article
• Language: English
• Published: Weinheim Wiley Subscription Services, Inc 01.12.2021; John Wiley and Sons Inc
• Edition: International ed. in English
• Subjects: Acids; Communication; Communications; Dimers; effective Hamiltonian; Formic acid; hydrogen bonds; proton transfer; Protons; quantum tunneling; Rotational spectra; rotational spectroscopy; Spectroscopy; Splitting; Substitution reactions
• ISSN: 1433-7851; 1521-3773; 1521-3773
• DOI: 10.1002/anie.202108242

Proton transfer via tunneling is a fundamental quantum‐mechanical phenomenon. We report rotational spectroscopy measurements of this process in the complex of the formic acid dimer with fluorobenzene. The assignment of the spectrum indicates that this complex exists in the form of a π–π stacked structure. Each rotational transition of the parent isotopologue exhibits splitting. Isotopic substitution experiments show that the spectral splitting results from double‐proton transfer tunneling in the formic acid dimer. Presence of fluorobenzene as a neighboring molecule does not quench the double proton transfer in the formic acid dimer but decreases its tunneling splitting from 341(3) MHz to 267.608(1) MHz. Calculations suggest that the presence of the weakly bounded fluorobenzene does not influence the activation energy of the proton transfer. The fluorobenzene is reoriented with respect to the formic acid dimer during the course of the reaction, slowing down the proton transfer motion. Proton transfer via tunneling is a fundamental quantum‐mechanical phenomenon. We report rotational spectroscopy measurements of this process in the complex of the formic acid dimer with fluorobenzene. The experiment indicates that this complex exists in the form of a π–π stacked structure. The fluorobenzene environment slows down the proton transfer in this complex compared to the free formic acid dimer.