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

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 (Teff) of 430 K demonstrate the CKM reliability for measuring hydrogen‐bond strengths in gaseous ionic liquid supramolecules. Using CKM and Teff 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−).