Impacts of External Electric Fields on Structures and Alignments of Ring Molecules

Ring molecules, which lack free ends, exhibit unique chemical and physical properties, making them promising candidates for nanodevice applications. Unlike their linear counterparts with two free ends, the behavior of ring molecules in water under external electric fields (EF) is not well understood. In this research, we employ molecular dynamics (MD) simulations to explore the structural and alignment behavior of two ring molecules of different sizes─C₃₀H₆₀ and C₆₀H₁₂₀─in water, under 300 K, 1 bar and various EF conditions, including direct current EF (DC EF), alternating current EF (AC EF), and circular polarized EF (CP EF) at different frequencies. Our findings reveal the following: (1) both large and small rings exhibit two free energy minima. For C₆₀H₁₂₀, these correspond to collapsed and stretched configurations, while for C₃₀H₆₀, they represent open and closed configurations. (2) The applied EF can regulate the depth of these free energy minima. For C₆₀H₁₂₀, no EF, AC EF, and high-frequency CP EF favor the collapsed state, while DC EF and low-frequency CP EF promote the stretched configuration. In the case of C₃₀H₆₀, no EF and high-frequency CP EF favor the open-ring state, whereas all other EF conditions tend to close the ring. (3) Both ring molecules align with the directional EF to minimize disruption of the hydrogen-bond network, with C₆₀H₁₂₀ showing a stronger alignment effect than C₃₀H₆₀ due to its longer structure. (4) Under CP EF, ring molecules exhibit rotation driven by the rotating EF, but there is a lag in the angle between the EF vector and the molecule's elongation. Higher frequency CP EF shows less ability to capture and align the molecule. This research enhances our understanding of how ring molecules behave in water under external EF and provides a theoretical foundation for future engineering applications involving controlled manipulation of these molecules.