Exploring the Microscopic Aspects of 1-Methyl-3-octylimidazolium Tetrafluoroborate Mixtures with Formamide, N -Methylformamide, and N , N -Dimethylformamide by Multiple Spectroscopic Techniques and Computations.

The microscopic aspects of 1-methyl-3-octylimidazolium tetrafluoroborate ([MOIm][BF 4 ]) mixtures with formamide (FA), N -methylformamide (NMF), and N , N -dimethylformamide (DMF) were investigated using spectroscopic techniques of femtosecond Raman-induced Kerr effect spectroscopy (fs-RIKES), FT-IR, and NMR. Molecular dynamics simulations and quantum chemistry calculations were also performed. According to fs-RIKES, the first moment of the low-frequency spectrum bands mainly originating from the intermolecular vibrations in the [MOIm][BF 4 ]/FA and [MOIm][BF 4 ]/DMF systems changed gradually with the molecular liquid mole fraction X ML but that in the [MOIm][BF 4 ]/NMF system was constant up to X NMF = 0.7 and then gradually increased in the range of X NMF ≥ 0.7. Excluding the contribution of the 2D hydrogen-bonding network due to the presence of FA in the low-frequency spectrum band, the X ML dependence of the normalized first moment of the low-frequency band in the [MOIm][BF 4 ]/FA and [MOIm][BF 4 ]/NMF systems revealed that the normalized first moment did not remarkably change in the range of X ML < 0.7 but drastically increased in X ML ≥ 0.7. FT-IR results indicated that the amide C═O band shifted to the low-frequency side with increasing X ML for the three mixtures due to the hydrogen bonds. The imidazolium ring C-H band also showed a similar tendency to the amide C═O band. 19 F NMR probed the microenvironment of [BF 4 ] - in the mixtures. The [MOIm][BF 4 ]/NMF and [MOIm][BF 4 ]/DMF systems showed an up-field shift of the F atoms of the anion with increasing X ML , and the [MOIm][BF 4 ]/FA system exhibited a down-field shift. Steep changes in the chemical shifts were confirmed in the region of X ML > 0.8. On the basis of the quantum chemistry calculations, the observed chemical shifts with increasing X ML were mainly attributed to the many-body interactions of ions and amides for the [MOIm][BF 4 ]/FA and [MOIm][BF 4 ]/DMF systems. Meanwhile, the long distance between the cation and the anion was due to the high dielectric medium for the [MOIm][BF 4 ]/NMF system, which led to an up-field shift.