posted on 2021-02-24, 17:04authored byO. Palumbo, F. Trequattrini, A. Cimini, A. Tsurumaki, M. A. Navarra, A. Paolone
The
intra- and intermolecular interactions in ether-functionalized
ionic liquids (ILs) are studied by means of infrared (IR) spectroscopy
measurements of N-ethoxyethyl-N-methylpiperidiniumbis(fluorosulfonyl)imide
(P1,2O2-FSI) and N-ethoxyethyl-N-methylmorpholiniumbis(fluorosulfonyl)imide (M1,2O2-FSI). The temperature dependence of the spectra in the medium IR
range allows the study of the anion conformer distribution and its
variation during phase transitions. In particular, it is found that
for both ILs the trans conformer of FSI is more stable than the cis
conformer, and the enthalpy differences between them are calculated
and are found to decrease upon the addition of a Li salt. The results
obtained in the far IR range, combined with ab initio calculation
of the ionic couple performed using the B3LYP-D functional and considering
both empirical dispersion corrections and the presence of a polar
solvent, provide evidence for the occurrence of a hydrogen bonding
between the O atom of the anion and its closest H atoms directly linked
to a C atom of the cation. The comparison with samples having the
same cations but with bis(trifluoromethanesulfonyl)imide (TFSI) as
an anion, that is, M1,2O2-TFSI and P1,2O2-TFSI,
as well as with samples having cations without the ether-functionalization
neither in the ring nor in the side chain, such as N-propyl-N-methylpyrrolidinium-FSI (PYR13-FSI) and 1-butyl-1-methylpyrrolidinium-TFSI (PYR14-TFSI),
indicates that the occurrence of such highly directional interaction
between anion and cation is better observable in the ether-functionalized
samples, in particular in those containing FSI as an anion.