Conformational structure of room temperature ionic liquid N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesulfonyl) imide - Raman spectroscopic study and DFT calculations

Conformational structure of the N-butyl-N-methyl-pyrrolidinium (P14+) ion within its bis(trifluoromethanesulfonyl) imide (TFSI−) and bromide salts has been studied by means of DFT calculations and Raman spectroscopy. The P14+ ion gives various types of conformations with respect to the pyrrolidinium ring and N-butyl group. DFT calculations indicate that, among others, the eq- and ax-envelope conformers with the N-butyl group at equatorial and axial positions, respectively, against planar C4 atoms of the envelope pyrrolidinium ring (-NCH2CH2CH2CH2-) are relatively stable, and the former gives the global minimum. It revealed by comparing observed and calculated Raman spectra that the P14+ ion is present mainly as the ax-envelope conformer in the P14+Br− crystal, whilst the eq- and ax-envelope conformers are present in equilibrium in the P14+TFSI− ionic liquid. The presence of conformational equilibrium is further supported by Raman spectra measured by varying temperature. It is also established that conformation of the N-butyl group is restricted to the trans-TT, in which the butyl group locates trans against its nearest carbon atom in the pyrrolidinium ring, and all carbon atoms in the butyl chain are located trans each other.