Dynamical Strengthening of Covalent and Non-Covalent Molecular Interactions by Nuclear Quantum Effects at Finite Temperature

TL;DR

This paper demonstrates that nuclear quantum effects can enhance various molecular interactions at finite temperature, leading to stabilization and altered dynamics, with implications for understanding molecular behavior.

Contribution

It reveals how nuclear quantum effects can strengthen covalent and non-covalent interactions through different physical mechanisms, a novel insight into molecular stabilization.

Findings

NQE can increase orbital overlap and electrostatic interactions within molecules.

NQE can localize methyl rotors by changing bond orders.

NQE can enhance van der Waals interactions via increased polarizability.

Abstract

Nuclear quantum effects (NQE) tend to generate delocalized molecular dynamics due to the inclusion of the zero point energy and its coupling with the anharmonicities in interatomic interactions. Here, we present evidence that NQE often enhance electronic interactions and, in turn, can result in dynamical molecular stabilization at finite temperature. The underlying physical mechanism promoted by NQE depends on the particular interaction under consideration. First, the effective reduction of interatomic distances between functional groups within a molecule can enhance the $n\to\pi^*$ interaction by increasing the overlap between molecular orbitals or by strengthening electrostatic interactions between neighboring charge densities. Second, NQE can localize methyl rotors by temporarily changing molecular bond orders and leading to the emergence of localized transient rotor states. Third, for noncovalent van der Waals interactions the strengthening comes from the increase of the polarizability given the expanded average interatomic distances induced by NQE. The implications of these boosted interactions include counterintuitive hydroxyl--hydroxyl bonding, hindered methyl rotor dynamics, and molecular stiffening which generates smoother free-energy surfaces. Our findings yield new insights into the versatile role of nuclear quantum fluctuations in molecules and materials.