Concerted Pair Motion Due to Double Hydrogen Bonding: The Formic Acid Dimer Case

Abstract

Formic acid dimer as the prototypical doubly hydrogenbonded
gas-phase species is discussed from the perspective of the
three translational and the three rotational degrees of freedom which are
lost when two formic acid molecules form a stable complex. The experimental
characterisation of these strongly hindered translations and
rotations is reviewed, as are attempts to describe the associated fundamental
vibrations, their combinations, and their thermal shifts by different
electronic structure calculations and vibrational models. A remarkable
match is confirmed for the combination of a CCSD(T)-level harmonic
treatment and an MP2-level anharmonic VPT2 correction. Qualitatively
correct thermal shifts of the vibrational spectra can be obtained from
classical molecular dynamics in CCSD(T)-quality force fields. A detailed
analysis suggests that this agreement between experiment and composite
theoretical treatment is not strongly affected by fortuitous error
cancellation but fully converged variational treatments of the six pair
or intermolecular modes and their overtones and combinations in this
model system would be welcome.