A quantitative and a qualitative study of the resonance assisted double proton transfer in formic acid dimer

Abstract

<i>Ab initio </i>molecular dynamics simulations have been performed to study the nature of the synchronous double proton transfer in formic acid dimer. In order to understand the evolution of the bonding during the double proton transfer, the electron localization function and the molecular orbital isosurfaces have been used. During the dynamics of the double proton transfer in formic acid dimer the two formic acid monomers approach each other, forming Speakman-Hadzi type of short strong hydrogen bonds at the transition state. The Speakman-Hadzi type of short strong hydrogen bonds are also shown to be polar covalent bonds. Based on the concept of resonance assisted hydrogen bond proposed by Gilli <i>et al </i>(Gilli P, Bertolasi V, Ferretti V &amp; G. Gilli, <i>J</i> <i>Am Chem Soc, </i>116 (1994) 909), it is shown that the proton at the transition state is connected by a π-conjugated O — C — O structures, indicating a resonance assisted hydrogen bond. On this basis the double proton transfer process in the formic acid dimer can be termed as a resonance assisted double proton transfer. The present work indicates that the synchronous double proton transfer in the formic acid dimer is due to the formation of (a) Speakman Hadzi type of short strong hydrogen bonds, and (b) resonance assisted hydrogen bond at the transition state.