Protonated Water Dimer on Benzene: Standing Eigen or Crouching Zundel?

Protonated
water clusters that are hydrogen-bonded to a neutral
benzene molecule are a reductionist model for protons at hydrophobic
surfaces, which are of fundamental importance in biological energy
transduction processes. Of particular interest is the protonated water
dimer (“Zundel ion”) on benzene, whose gas-phase messenger
IR spectrum has been previously interpreted in terms of an asymmetric
binding of the protonated water dimer to the benzene ring through
a single water molecule. This “standing Eigen” isomer
has a hydronium core. We have found an alternative “crouching
Zundel” isomer, which attaches to the benzene ring symmetrically
via both of its water molecules. When Ar-tagged, it has an IR spectrum
in much better agreement with experiment than the standing Eigen isomer,
particularly at the lower frequencies. These conclusions are based
on static harmonic (and anharmonic) normal-mode analysis using density
functional theory with various (dispersion corrected) functionals
and particularly on dynamic anharmonic spectra obtained from the dipole
autocorrelation functions from classical ab initio molecular dynamics
with the BLYP, PBE, and B3LYP functionals. Possible implications to
protons on water/organic-phase interfaces are discussed.