Gaseous Supramolecules of Imidazolium Ionic Liquids: “Magic” Numbers and Intrinsic Strengths of Hydrogen Bonds

Abstract

Abstract Electrospray ionization mass spectrometry (ESI‐MS) is found to gently and efficiently transfer small to large as well as singly to multiply charged [X + ] n [A − ] m supramolecules of imidazolium ion (X + ) ionic liquids to the gas phase, and to reveal “magic numbers” for their most favored assemblies. Tandem mass spectrometric experiments (ESI‐MS/MS) were then used to dissociate, via low‐energy collision activation, mixed and loosely bonded [A‐ ‐ ‐ ‐X‐ ‐ ‐ ‐A′] − and [X‐ ‐ ‐ ‐A‐ ‐ ‐ ‐X′] + gaseous supramolecules, as well as their higher homologues, and to estimate and order via Cooks' kinetic method (CKM) and B3LYP/6‐311G(d,p) calculations the intrinsic solvent‐free magnitude of hydrogen bonds. For the five anions studied, the relative order of intrinsic hydrogen‐bond strengths to the 1‐ n ‐butyl‐3‐methylimidazolium ion [X 1 ] + is: CF 3 CO 2 − (zero) > BF 4 − (−3.1) > PF 6 − (−10.0) > InCl 4 − (−16.4) and BPh 4 − (−17.6 kcal mol −1 ). The relative hydrogen‐bond strength for InCl 4 − was measured via CKM whereas those for the other anions were calculated and used as CKM references. A good correlation coefficient ( R =0.998) between fragment ion ratios and calculated hydrogen‐bond strengths and an effective temperature ( T eff ) of 430 K demonstrate the CKM reliability for measuring hydrogen‐bond strengths in gaseous ionic liquid supramolecules. Using CKM and T eff of 430 K, the intrinsic hydrogen‐bond strengths of BF 4 − for the three cations investigated is: 1‐ n ‐butyl‐3‐methyl‐imidazolium ion (0) > 1,3‐di‐[( R )‐3‐methyl‐2‐butyl]‐imidazolium ion (−2.4) > 1,3‐di‐[( R )‐α‐methylbenzyl]‐imidazolium ion (−3.0 kcal mol −1 ). As evidenced by “magic” numbers, greater stabilities are found for the [(X 1 ) 2 (BF 4 ) 3 ] − and [(X 1 ) 5 A 4 ] + supramolecules (A≠InCl 4 − ).