Influence of Long-Range Forces on the Transition States and Dynamics of NaCl Ion-Pair Dissociation in Water

Abstract

We study NaCl ion-pair dissociation in a dilute aqueous solution using computer simulations both for the full system with long-range Coulomb interactions and for a well-chosen reference system with short-range intermolecular interactions. Analyzing results using concepts from Local Molecular Field (LMF) theory and the recently proposed AI-based analysis tool “State predictive information bottleneck” (SPIB), we show that the system with short-range interactions can accurately reproduce the transition rate for the dissociation process, the dynamics for moving between the underlying metastable states, and the transition state ensemble. Contributions from long-range interactions can be largely neglected for these processes because long-range forces from the direct interionic Coulomb interactions are almost completely canceled (>90%) by those from solvent interactions over the length scale where the transition takes place. Thus, for this important monovalent ion-pair system, short-range forces alone are able to capture detailed consequences of the collective solvent motion, allowing the use of physically suggestive and computationally efficient short-range models for the dissociation event. We believe that the framework here should be applicable to disentangling mechanisms for more complex processes such as multivalent ion disassociation, where previous work has suggested that long-range contributions may be more important.

Local Molecular Field Theory for Coulomb Interactions in Aqueous Solutions

“…Similarly, longrange interactions are found to be unimportant for monovalent ion pairing. 38,143 Then, LMF theory and the SS model suggest that neural network potentials should be accurate for modeling solutions of monovalent ions, in agreement with recent work, 55,152 especially when complemented with theoretical methods that account for the effects of long-range interactions. 153 In contrast, LMF theory suggests that shortrange neural network models will have difficulty describing solutions containing multivalent ions, as well as interfaces, because they neglect important unbalanced forces arising from long-range interactions.…”

“…In contrast to the pairing of nucleobases, and the pairing of monovalent ions, , long-range interactions are found to be significant for the association of Ca 2+ and PO 4 3– , as illustrated by the long-range decay of the PMFs in Figure and the differences between the PMFs for the GT and Full systems. Ca 2+ ···PO 4 3– pairing is not determined by hydrogen bond breakage and formation.…”

Molecular Origin of Driving-Dependent Friction in Fluids

“…The above-mentioned finding of a velocity-dependent reordering of common ion hydration shells can be nicely illustrated by considering the probability distribution that the ions at distance x are connected via n = 0, 1, 2, 3 water molecules (Figure 3e). 52,54 In the unbiased case, we find typically zero to one bridging water molecules at the minimum (x ≈ 0.28 nm), while up to three bridging water molecules are found during dissociation or association (x ≈ 0.35 nm). For the constrained distribution, 55 we indeed observe a bias toward a higher number of connecting water molecules than present in the unbiased case, indicating altered dynamics within the hydration shells.…”

Molecular origin of driving-dependent friction in fluids

“…The above finding of a velocity-dependent reordering of common ion hydration shells can be nicely illustrated by considering the probability distribution that the ions at distance x are connected via n = 0, 1, 2, 3 water molecules (Fig. 3e) [51,53]. In the unbiased case, we find typically zero to one bridging water molecules at the minimum (x ≈ 0.28 nm), while up to three bridging waters are found during dissociation or association (x ≈ 0.35 nm).…”