Molecular structure, vibrational spectra, and hydrogen bonding of the ionic liquid 1-ethyl-3methyl-1H-imidazolium tetrafluoroborate

The IR and Raman spectra and conformations of the ionic liquid 1-ethyl-3-methyl-1H-imidazolium tetrafluoroborate, [EMIM][BF4] (6), were analyzed within the framework of scaled quantum mechanics (SQM). It was shown that SQM successfully reproduced the spectra of the ionic liquid. The computations revealed that normal modes of the EMIM+ (.) BF4- ion pair closely resemble those of the isolated ions EMIM+ and BF4-, except for the antisymmetric BF stretching vibrations of the anion, and the out-of-plane and stretching vibrations of the H-C(2) moiety of the cation. The most plausible explanation for the pronounced changes of the latter vibrations upon ion-pair formation is the H-bonding between H-C(2) and BF4-. However, these weak H-bonds are of minor importance compared with the Coulomb interactions between the ions that keep them closely associated even in dilute CD2Cl2 Solutions. According to the 'gas-phase' computations, in these associates, the BF4- anion is positioned over the imidazolium ring of the EMIM+ cation and has short contacts not only with the H-C(2) of the latter, but also with a proton of the Me-N(3) group.