Probing Molecular Interactions in Functionalized Asymmetric Quaternary Ammonium-Based Dicationic Ionic Liquids.

Abstract

Electronic structure, binding energies, and spectral characteristics of functionalized asymmetric dicationic ionic liquids (DILs) composed of quaternary ammonium cations substituted with the ethoxyethyl and allyl/3-phenylpropyl/methoxyethoxyethyl/pentyl functionalities on two different nitrogen centers of the dication and the bis(trifluoromethanesulfonyl)imide (Tf2N-) anion were derived employing the dispersion-corrected density functional theory. DILs based on methoxyethoxyethyl-substituted cation reveal stronger binding toward the Tf2N- anion. The measured glass transition temperatures are found to be strongly dependent on the cation-anion binding facilitated through noncovalent interactions with dominant contributions from the electrostatics and hydrogen bonding. The manifestations of these interactions to vibrational spectra, in particular, to SO2 and CF3 stretchings in the complexes are presented. It has been demonstrated that the frequency down (red)-shift of the SO2 stretching in these DILs with varying substituent follows the order: methoxyethoxyethyl (35 cm-1) > allyl (23 cm-1) > pentyl (20 cm-1) > 3-phenylpropyl (5 cm-1), which is consistent with the strength of cation-anion binding. The CF3 stretching of the anion exhibits the frequency shift in the opposite direction with its hierarchy being reversed to that of SO2 stretchings; the largest upshift (blue shift) of 60 cm-1 was predicted for the DILs composed of 3-phenlpropyl substituted dications. The direction of such frequency shift has been rationalized through the difference molecular electron density maps in conjunction with the electron density at the bond critical point in the quantum theory of atoms in molecules. The underlying cation-anion binding has been analyzed through charge distribution analysis characterized in terms of molecular electrostatic potential topography. Furthermore, the observed decomposition temperatures of DILs are shown to correlate well with the maximum surface electrostatic potential parameters in quantum theory of atoms in molecules.