The contribution of intermolecular spin interactions to the London dispersion forces between chiral molecules

TL;DR

This paper investigates how intermolecular spin interactions influence London dispersion forces in chiral molecules, revealing a chirality-sensitive correction linked to spin-orbit effects and molecular geometry, with potential implications for understanding chiral molecular systems.

Contribution

It introduces a simple physical model describing spin-charge fluctuation coupling in chiral molecules, highlighting a new spin-dependent correction to dispersion forces.

Findings

Spin interactions contribute a chirality-sensitive correction to London forces.

The correction scales with R^-6, similar to standard dispersion forces.

The model predicts a magnetic response induced by electric field fluctuations in chiral pairs.

Abstract

Dispersion interactions are one of the components of van der Waals (vdW) forces, which play a key role in the understanding of intermolecular interactions in many physical, chemical and biological processes. The theory of dispersion forces was developed by London in the early years of quantum mechanics. However, it was only in the 1960s that it was recognized that for molecules lacking an inversion center such as chiral and helical molecules, there are chirality-sensitive corrections to the dispersion forces proportional to the rotatory power known from the theory of circular dichroism and with the same distance scaling law R-6 as the London energy. The discovery of the Chirality-Induced Spin Selectivity (CISS) effect in recent years has led to an additional twist in the study of chiral molecular systems, showing a close relation between spin and molecular geometry. Motivated by it, we propose in this investigation to describe the mutual induction of charge and spin-density fluctuations in a pair A-B of chiral molecules by a simple physical model. The model assumes that the same fluctuating electric fields responsible for vdW forces can induce a magnetic response via a Rashba-like term, so that an spin-orbit field acting on molecule B is generated by the electric field arising from charge density fluctuations in molecule A (and viceversa). Within a second-order perturbative approach, these contributions manifest as an effective intermolecular exchange interaction. Although expected to be weaker than the standard London forces, these interactions display the same R$^6$ distance scaling.