Spectroscopic and computational analysis of the molecular interactions in the ionic liquid ion pair [BMP]+[TFSI]-

1-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP](+)[TFSI](-)) ion pairs were studied using DFT at the B3LYP/6-31 + G(d) level. Nine locally stable conformations of the ion pair were located. In the most stable conformation, [TFSI](-) takes a cis conformation and lies below the pyrrolidinium ring. Atoms-in-molecules (AIM) and electron density analysis indicated the existence of nine hydrogen bonds. Interaction energies were recalculated at the Second-order Moller-Plesset (MP2) level to show the importance of dispersion interaction. Further investigation through natural bond orbital (NBO) analysis provided insight into the importance of charge transfer interactions in the ion pair. Harmonic vibrations of the ion pair were calculated and compared with vibrations of the free ions as well as the experimental infrared spectrum. Assignments and frequency shifts are discussed in light of the inter-ionic interactions. (C) 2012 Elsevier B.V. All rights reserved.