Anion of the formic acid dimer as a model for intermolecular proton transfer induced by a pi* excess electron

The neutral and anionic formic acid dimers have been studied at the second-order Moller-Plesset and coupled-cluster level of theory with single, double, and perturbative triple excitations with augmented, correlation-consistent basis sets of double- and triple-zeta quality. Scans of the potential-energy surface for the anion were performed at the density-functional level of theory with a hybrid B3LYP functional and a high-quality basis set. Our main finding is that the formic acid dimer is susceptible to intermolecular proton transfer upon an excess electron attachment. The unpaired electron occupies a pi(*) orbital, the molecular moiety that accommodates an excess electron "buckles," and a proton is transferred to the unit where the excess electron is localized. As a consequence of these geometrical transformations, the electron vertical detachment energy becomes substantial, 2.35 eV. The anion is barely adiabatically unstable with respect to the neutral at 0 K. However, at standard conditions and in terms of Gibbs free energy, the anion is more stable than the neutral by +37 meV. The neutral and anionic dimers display different IR characteristics. In summary, the formic acid dimer can exist in two quasidegenerate states (neutral and anionic), which can be viewed as "zero" and "one" in the binary system. These two states are switchable and distinguishable.