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Evidence of Rotational Isomerism in 1-butyl-3-methylimidazolium Halides: A Combined High-Pressure Infrared and Raman Spectroscopic Study

Evidence of Rotational Isomerism in 1-butyl-3-methylimidazolium Halides: A Combined High-Pressure Infrared and Raman Spectroscopic Study

Hai-Chou Chang, Jyh-Chiang Jiang, Jong-Chang Su, Chao-Yen Chang, Sheng Hsien Lin

J Phys Chem A. 2007 Sep 27;111(38):9201-6.

PMID: 17628046

Abstract:

High-pressure methods were applied to investigate the rotational isomerism and the hydrogen-bonding structures of 1-butyl-3-methylimidazolium bromide and 1-butyl-3-methylimidazolium chloride, respectively. Conformation changes of the butyl chain were observed above a pressure of 0.3 GPa. Under ambient pressure, Raman spectra indicate that the more thermodynamically stable butyl structure of the cations is the gauche-anti (GA) and all-anti forms for 1-butyl-3-methylimidazolium bromide and 1-butyl-3-methylimidazolium chloride, respectively. Nevertheless, the high-pressure phases arise from the perturbed GA conformer. The imidazolium C-H bands of 1-butyl-3-methylimidazolium chloride display anomalous nonmonotonic pressure-induced frequency shifts. This discontinuity in the frequency shift is related to the modification of the imidazolium C-H---Cl- contacts upon compression. The alkyl C-H---Cl- interactions are suggested to be a compensatory mechanism to provide additional stability. Density-functional-theory-calculated results also support the high-pressure results that the methyl and butyl C-H groups are suitable proton donor sites for the GA conformer.

Chemicals Related in the Paper:

Catalog Number	Product Name	Structure	CAS Number	Price
AP79917901	1-Butyl-3-methylimidazolium chloride		79917-90-1	Price

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