Hydrogen Bond Stabilization in 1,3-Dimethylimidazolium Methyl Sulfate and 1-Butyl-3-Methylimidazolium Hexafluorophosphate Probed by High Pressure:  The Role of Charge-Enhanced C−H···O Interactions in the Room-Temperature Ionic Liquid

• Published in: The journal of physical chemistry. B 2006-02, Vol.110 (7), p.3302-3307
• Main Authors: Chang, Hai-Chou, Jiang, Jyh-Chiang, Tsai, Wei-Cheng, Chen, Guan-Ciao, Lin, Sheng Hsien
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp0560009

The hydrogen bonding structures of room-temperature ionic liquids 1,3-dimethylimidazolium methyl sulfate and 1-butyl-3-methylimidazolium hexafluorophosphate have been studied by infrared spectroscopy. High-pressure infrared spectral profiles and theoretical calculations allow us to make a vibrational assignment of these compounds. The imidazolium C−H bands of 1,3-dimethylimidazolium methyl sulfate display anomalous non-monotonic pressure-induced frequency shifts. This discontinuity in frequency shift is related to enhanced C−H···O hydrogen bonding. This behavior is in contrast with the trend of blue shifts in frequency for the methyl C−H stretching mode at ca. 2960 cm-1. Our results indicated that the imidazolium C−H groups are more favorable sites for hydrogen bonding than the methyl C−H groups in the pure 1,3-dimethylimidazolium methyl sulfate. Nevertheless, both methyl C−H and imidazolium C−H groups are favorable sites for C−H···O hydrogen bonding in a dilute 1,3-dimethylimidazolium methyl sulfate/ D2O mixture. Hydrogen bond-like C−H···F interactions were observed between PF6 - and H atoms on the alkyl side chains and imidazolium ring for 1-butyl-3-methylimidazolium hexafluorophosphate.