Electric dipole moments of nanosolvated acid molecules in water clusters

TL;DR

This study measures the electric dipole moments of nanosolvated acid molecules in water clusters, revealing insights into their microscopic structure, ionic dissociation onset, and the role of molecular motion in their polarity.

Contribution

It provides experimental measurements combined with molecular dynamics modeling to understand how nanoscale water clusters with acids exhibit enhanced and fluctuating dipole moments.

Findings

Dipole moments increase notably at cluster size n≈5-6.

Enhanced susceptibility due to acid addition indicates ionic dissociation onset.

Thermal and zero-point motions significantly influence cluster polarity.

Abstract

The electric dipole moments of $(H_{2}O)_{n}DCl$ ($n=3-9$) clusters have been measured by the beam deflection method. Reflecting the (dynamical) charge distribution within the system, the dipole moment contributes information about the microscopic structure of nanoscale solvation. The addition of a DCl molecule to a water cluster results in a strongly enhanced susceptibility. There is evidence for a noticeable rise in the dipole moment occurring at $n\approx5-6$. This size is consistent with predictions for the onset of ionic dissociation. Additionally, a molecular dynamics model suggests that even with a nominally bound impurity an enhanced dipole moment can arise due to the thermal and zero point motion of the proton and the water molecules. The experimental measurements and the calculations draw attention to the importance of fluctuations in defining the polarity of water-based nanoclusters, and generally to the essential role played by motional effects in determining the response of fluxional nanoscale systems under realistic conditions.