CO Inversion on a NaCl(100) Surface: A Multireference Quantum Embedding Study

We develop a multireference quantum embedding model to
investigate
a recent experimental observation of the isomerization of vibrationally
excited CO molecules on a NaCl(100) surface [Science 2020, 367, 175–178]. To explore
this mechanism, we built a reduced potential energy surface of CO
interacting with NaCl(100) using a second-order multireference perturbation
theory, modeling the adsorbate–surface interaction with our
previously developed active space embedding theory (ASET). We considered
an isolated CO molecule on NaCl(100) and a high-coverage CO monolayer
(1/1), and for both we generated potential energy surfaces parametrized
by the CO stretching, adsorption, and inversion coordinates. These
surfaces are used to determine stationary points and adsorption energies
and to perform a vibrational analysis of the states relevant to the
inversion mechanism. We found that for near-equilibrium bond lengths,
CO adsorbed in the C-down configuration is lower in energy than in
the O-down configuration. Stretching of the C–O bond reverses
the energetic order of these configurations, supporting the accepted
isomerization mechanism. The vibrational constants obtained from these
potential energy surfaces show a small (< 10 cm–1) blue- and red-shift for the C-down and O-down configurations, respectively,
in agreement with experimental assignments and previous theoretical
studies. Our vibrational analysis of the monolayer case suggests that
the O-down configuration is energetically more stable than the C-down
one beyond the 16th vibrational excited state of CO, a value slightly
smaller than the one from quasi-classical trajectory simulations (22nd)
and consistent with the experiment. Our analysis suggests that CO–CO
interactions in the monolayer play an important role in stabilizing
highly vibrationally excited states in the O-down configuration and
reducing the barrier between the C-down and O-down geometries, therefore
playing a crucial role in the inversion mechanism.