  We have performed ab initio molecular dynamics simulations 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,
we have used the electron localization function and the molecular orbital
isosurfaces. 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 et al (Gilli P, Bertolasi V, Ferretti V & G. Gilli, J. Am.
Chem. Soc., 116 (1994). 909-915), we show that the proton at the transition
state is connected by a 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.
Our finding on the above analysis suggest 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.
