Formation of Alkali-Halide Cluster Ions with Aza-Crown Ethers

Abstract

Molecules often form specific structures and exhibit unique functions by assembling through covalent linkages or non-covalent interactions. Ionic clusters, composed of ions bound to each other via electrostatic interactions or van der Waals forces, hold significant importance in electrolyte systems within diverse electrochemical devices. To explore the intrinsic properties of these clusters in a solvent-free environment, research has extensively utilized gas-phase mass spectrometry (MS). More recently, ion mobility spectrometry-mass spectrometry (IMS-MS) has emerged as a promising technique to offer supplementary insights into the structural characteristics of ions. The combination of collision cross-section (CCS) values obtained from IMS-MS results and molecular modeling using Density Functional Theory (DFT) calculations is commonly employed to predict structures. Additionally, the comparative analysis of ionic bond strengths within clusters can be conducted using information about fragment ions provided by Collision Induced Dissociation (CID). In this study, we investigate the behavior of ionic clusters formed when alkali metal ions were confined in conformation and coordination by aza-crown ethers as binders. Through IMS-MS and theoretical calculations, we confirmed the existence of monomer clusters (aza-crown ether + M+), resulting from the size-dependent interaction of alkali metal ions with aza-crown ethers, as well as dimer clusters effectively entrapping various anions between them. Furthermore, based on the outcomes of bond strength comparison using low-energy CID, we substantiated the divergence in cationic characteristics depending on the binder, thereby successfully demonstrating the ability to control the binding strength between ions.