Dimers of formic acid: Structures, stability, and double proton transfer

Abstract

A stochastic search of the potential energy surface for the formic acid dimers results in 21 well-defined minima. A number of structures are reported here for the first time, others have already been experimentally detected or computationally predicted. Four types of different hydrogen bonds (HBs) are at play stabilizing the clusters: primary C=O⋯ H-O and H-O⋯ H-O and secondary C=O⋯ H-C and H-O⋯ H-C HBs corresponding to well-characterized bonding paths are identified. A novel C=O⋯ C stabilizing interaction is also reported. The double proton transfer reaction is calculated to occur in a synchronous fashion, with an energy barrier smaller than the energy needed to break up the dimers.

Partition potential for hydrogen bonding in formic acid dimers

“…These observations suggest that the stronger intermolecular bonds require larger deformation of the original wave functions of the fragments, a result that may appear obvious to many chemists, but cannot be quantified without a rigorous definition of fragments within a molecule. This is also consistent with the orbital interaction picture where it is generally thought that the gain of electron density in the region of the donor and the simultaneous loss of charge in the region associated to the O atom of the acceptor is responsible for the formation of a HB . Although fragment occupation numbers remain constant in the present PDFT implementation, fragment densities are indeed distorted; these distortions are linked to the charge transfer within fragments, provided by the orbital interactions.…”

“…Farfán et al established a hierarchy of strengths of intermolecular contacts based on a combined view that includes topological analyses of the electron densities and orbital interactions. Their results suggest that the strengths of hydrogen bonding in (FA) 2 decrease as CO⋯HO>HO⋯HO>CO⋯HC>HO⋯HC.…”

“…Their results suggest that the strengths of hydrogen bonding in (FA) 2 decrease as CO⋯HO>HO⋯HO>CO⋯HC>HO⋯HC. Table lists several chemical descriptors adapted from the work of Farfán et al supplemented with quantities derived from our own PDFT results.…”

“…Despite being the smallest carboxylic acid, the conformational space for the formic acid dimers is notoriously rich, with a considerable number of structures already experimentally detected . Farfán et al calculated a set of 21 well‐defined minima in MP2/6‐311++G ( d , p ) potential energy surface (PES) . For our work, we selected 4 lowest energy motifs from this set (shown in Figure ).…”

“…Lowest energy dimers of formic acid from the MP2/6‐311++G( d , p ) potential energy surface . Both monomers are in the anti conformation.…”