posted on 2019-08-13, 14:36authored byQi Yu, William B. Carpenter, Nicholas H. C. Lewis, Andrei Tokmakoff, Joel M. Bowman
The
hydrated excess proton is a common species in aqueous chemistry,
which complexes with water in a variety of structures. The infrared
spectrum of the aqueous proton is particularly sensitive to this array
of structures, which manifests as continuous IR absorption from 1000
to 3000 cm–1 known as the “proton continuum”.
Because of the extreme breadth of the continuum and strong anharmonicity
of the involved vibrational modes, this spectrum has eluded straightforward
interpretation and simulation. Using protonated water hexamer clusters
from reactive molecular dynamics trajectories, and focusing on their
central H+(H2O)2 structures’
spectral contribution, we reproduce the linear IR spectrum of the
aqueous proton with a high-level local monomer quantum method and
highly accurate many-body potential energy surface. The accuracy of
this approach is first verified in the vibrational spectra of the
two isomers of the protonated water hexamer in the gas phase. We then
apply this approach to 800 H+(H2O)6 clusters, also written as [H+(H2O)2](H2O)4, drawn from multistate empirical valence
bond simulations of the bulk liquid to calculate the infrared spectrum
of the aqueous proton complex. Incorporation of anharmonic effects
to the vibrational potential and quantum mechanical treatment of the
proton produces a better agreement to the infrared spectrum compared
to that of the double-harmonic approximation. We assess the correlation
of the proton stretching mode with different atomistic coordinates,
finding the best correlation with ⟨ROH⟩, the expectation value of the proton–oxygen distance ROH. We also decompose the IR spectrum based
on normal mode vibrations and ⟨ROH⟩ to provide insight on how different frequency regions in
the continuum report on different configurations, vibrational modes,
and mode couplings.