Synthesis, molecular structure, and properties of DABCO bromide based ionic liquid combining spectroscopic studies with DFT calculations

Abstract

Through N-alkylation of 1,4-diazabicyclo [2.2.2] octane with 1-bromooctane a new ionic liquid, namely 1‐octyl‐1,4‐diazabicyclo [2.2.2] octan‐1‐ium Bromide [C8DABCO+][Br−] has been synthesized and characterized by FT-IR, FT-Raman and by 1H and 13C-NMR spectroscopies. Three ionic C-H•••Br bonds were predicted by natural bond orbital (NBO), atoms in molecules (AIM) calculations and bond orders but a monodentate coordination between cation and anion is suggested because only one of them has higher energy and lower distance. The effect of Br on properties of IL is clearly observed by the change in orientation and direction of dipole moment vector in the IL, as compared with the cation. NBO studies reveal that the Br anion strongly stabilizes to cation to form the IL. The complete vibrational assignments of 129 and 132 vibration modes expected for the cation and its IL were performed by using two harmonic force fields with the SQMFF methodology and transferable scaling factors. The scaled harmonic force constants were also reported for both species. The Br confers a higher reactivity to [C8DABCO+][Br−], as suggested by analyses of frontier orbitals. Reasonable correlations were found between the experimental IR, Raman and NMR spectra and the corresponding theoretical ones.