Dynamical strengthening of covalent and non-covalent molecular interactions by nuclear quantum effects at finite temperature

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 → π* 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.

Machine Learning Force Fields

“…Even relatively small molecules were shown to display nontrivial electronic effects, influencing their dynamics and allowing a better understanding of experimental observations. 161 Many other unknown chemical effects potentially wait to be discovered by studies now possible with ML-FFs. At the speed at which improvements to existing ML-FFs are published, it is not unreasonable to expect significant advances that will make similar studies possible for larger systems and help realize many more “dreams” in the near future.…”

“…(mentioned earlier in the paragraph on electronic effects) investigated NQEs in small organic molecules. 161 The study revealed that NQEs can dynamically strengthen molecular interactions by enhancing n → π* donation through increasing orbital overlap, or by strengthening electrostatic interactions between neighboring charge densities (see Figure 19 C:3). Another interesting observed effect is a temporary change of bond orders, which can lead to emerging localized transient states of methyl rotors.…”

“…Notably, recent developments could show that machine learning can provide novel insight beyond providing efficient shortcuts of complex physical computations. 33 , 55 , 59 , 62 , 70 , 105 , 160 , 161 …”

“…Copyright 2018 Chmiela et al). (3) Schematic description of the enhancement in intra- and intermolecular interactions due to NQEs (adapted with permission from ref ( 161 ). Copyright 2020 Sauceda et al).…”

“…Here, including NQEs broadens the radial distribution function significantly, which also increases the molecular polarizability. 161 A change in the distribution of interatomic distances also influences electronic effects: A recent study of ref ( 70 ). (mentioned earlier in the paragraph on electronic effects) investigated NQEs in small organic molecules.…”