Understanding Cation–Anion Association in Pyrrolidinium‐Based Ionic Liquids Through DFT and Topological Analysis

Understanding the nature and strength of cation–anion interactions is essential for the rational design of ionic liquids, as these interactions govern their key physicochemical properties. In this work, the cation–anion interactions in three representative pyrrolidinium‐based ionic liquids, 1‐butyl‐1‐methylpyrrolidinium tetracyanoborate ([BMPYR][B(CN) 4 ]), bis(oxalate)borate ([BMPYR][BOB]), and dicyanamide ([BMPYR][DCA]), were systematically investigated using density functional theory. The optimized ionic‐pair structures reveal that hydrogen bonding between the heteroatoms (N or O) of the anions and the hydrogen atoms on the pyrrolidinium ring constitutes the primary driving force for ion pairing. Interaction energy calculations show that [BMPYR][DCA] exhibits the strongest cation–anion interaction, which is attributed to the smaller size, lower steric hindrance, and higher accessibility of the nitrogen lone pairs in the dicyanamide anion. Natural bond orbital, noncovalent interaction, and atoms‐in‐molecules analyses consistently demonstrate that, in addition to the dominant ring‐centered interactions, alkyl chains of the cation participate cooperatively through weaker but non‐negligible noncovalent interactions. The combined results provide a molecular‐level understanding of how anion identity and cation structure jointly determine interaction strength and topology in pyrrolidinium‐based ionic liquids, offering theoretical guidance for the design of ionic liquids with tailored properties.