Models for understanding the structural effects on the cation-anion interaction strength of dicationic ionic liquids

Electrospray ionization mass spectrometry with variable collision-induced dissociation (CID) was used to conduct a comprehensive study of cation-anion interaction strength in dicationic ionic liquids (ILs). This investigation reports the influence of anion structure (CI-, Br-, NO3-, BF4-, and SCN-) and spacer chain length (n = 4, 6, 8, and 10) on the interaction strength of dicationic ILs derived from 1,n-bis(3-methylimidazolim-1-yl)alkane. The anion-cation interaction strength was determined from the binary mixture of Its and from the center-of mass energy (E-cm). Moreover, two novel models were proposed, based on the extended first order and zero order equations, which allow determining the order of stability from the constant dissociation (k(d)) and decay rate (k(0)), respectively. Additionally, quantum chemical calculations of cation-anion energy were done in order to support experimental results. The order of energy of the anion-cation interaction determined by both E(cm,)1/2 and K-d followed the same trend, while the order of the cation-anion interaction strength observed in the binary mixtures was in accordance with quantum chemical calculations. The model based on zero order process did not show good agreement with either the E-cm,E-1/2 or K-d, thus indicating that it is a very simplified model that does not result in an appropriate order for the cation-anion energy interaction. Orders were not comparable; however, it was the first time that binary mixture experiments and E-cm were used with the same set of ILs. (C) 2017 Elsevier B.V. All rights reserved.