Rotational Isomerism and Structure of the 1-Butyl-3-methylimidazolium Cation in the Ionic Liquid State

Abstract

The crystal structures of 1-butyl-3-methylimidazolium chloride [bmim]Cl and 1-butyl-3-methylimidazolium bromide [bmim]Br show that two rotational isomers, the TT form and the GT form, of the [bmim]+ cation exist in the crystalline state. A vibrational analysis based on a DFT calculation indicates that two characterstic Raman bands of crystalline [bmim]Cl and three of crystalline [bmim]Br can be used as marker bands of the rotational isomerism around the C7–C8 bond of the n-butyl group. The Raman spectra of liquid [bmim]BF4, in which both sets of marker bands are simultaneously observed, then prove that at least two rotational isomers of the [bmim]+ cation coexist in the ionic liquid state.

Raman Spectroscopy and Ab-Initio Model Calculations on Ionic Liquids

“…To conclude the situation for [C 4 mim] þ , Hamaguchi and Ozawa [108] and Ozawa et al [128] have discovered by the combined use of X-ray crystal- þ cation are present in both melts, as also found e.g., by Hamaguchi and Ozawa [108] …”

“…In this review we discussed in detail some examples of the conformational equilibria, e.g., those discovered by Ozawa et al [128] in liquids containing the 1-butyl-3-methylimidazolium cation. Also, we examined in some detail liquids containing the bis(trifluoromethanesulfonyl)imide anion, as described above.…”

“…6 can be ascribed to the [C 4 mim] þ cation. To pursue this question further, Ozawa et al [128] have performed density functional calculations (DFT) with Gaussian98 at the B3LYP=6-31þG ÃÃ level.…”

“…7, Hamaguchi et al [108,116,118,122,128] concluded that the two rotational isomers AA and GA must coexist in the ionic liquid state (AA and GA were called TT and GT by Hamaguchi et al). According to the Raman spectra of all the liquids in Fig.…”

Reactions of excited-state benzophenone ketyl radical in a room-temperature ionic liquid

“…For example, Hamaguchi's group used Raman spectral analysis to show for the first time that two distinct conformational structures of the imidazolium cation exist simultaneously in the IL state. 20,21 These investigators also obtained evidence of mesoscopic local structure in ILs by means of coherent anti-Stokes Raman scattering measurements. 22 Iwata et al investigated the importance of local structures in reaction dynamics by measuring the vibrational cooling rates of the excited state of trans-stilbene in ILs by picosecond time-resolved Raman spectroscopy.…”