Confinement of 1‑butyl‑3‑methylimidazolium in cucurbiturils

Abstract

Confinement of room temperature ionic liquids (RTIL) within the macrocyclic hosts significantly alters their physicochemical properties and widens their scope of applications. In this light encapsulation of the 1‑butyl‑3‑methylimidazolium tetrafluoroborate (BmimBF4) within cucurbit[n]uril (CBn, n = 6–8) hosts has been analyzed employing the density functional theory. Theoretical calculations predict that the BmimBF4 ion pair combines with CB6 or CB7 in 1:1 proportions whilst the complexation with CB8 complex renders 1:1 as well as 1:2 stoichiometries. The lowest energy complexes show alkyl chain of the cation penetrating within CBn cavities. Moreover the CB6 complex reveals the BF4 anion is bound to methyl protons of the Bmim cation externally via hydrogen bonding. On the other hand, the CB7 and CB8 complexes facilitate interactions between BF4 anion and their methine protons. The ramifications of such noncovalent binding to vibrational and NMR spectra are further deciphered using the quantum theory of atoms in molecules (QTAIM), and noncovalent interactions reduced density gradient (NCI-RDG) methods in conjunction with natural bond orbital analyses (NBO). The methyl protons of butyl chain in BmimBF4 emerge with the up-field signals in 1H NMR spectra as opposed to the largely deshielded methine protons which can be attributed to hydrogen bonding and enhanced aromatic character of the imidazolium ring evidenced from the nucleus independent chemical shifts.