A Comparative Study of the Rotational Dynamics of PF6鈥?/sup> Anions in the Crystals and Liquid States of 1-Butyl-3-methylimidazolium Hexafluorophosphate: Results from 31P NM

详细信息    查看全文

• 作者：Takatsugu Endo ; Hiroki Murata ; Mamoru Imanari ; Noriko Mizushima ; Hiroko Seki ; Sabyasachi Sen ; Keiko Nishikawa
• 刊名：The Journal of Physical Chemistry B
• 出版年：2013
• 出版时间：January 10, 2013
• 年：2013
• 卷：117
• 期：1
• 页码：326-332
• 全文大小：333K
• 年卷期：v.117,no.1(January 10, 2013)
• ISSN：1520-5207

文摘

The rotational dynamics of the hexafluorophosphate anion (PF₆^{鈥?/sup>) in the crystalline and liquid states of the archetypal room temperature ionic liquid (RTIL) 1-butyl-3-methylimidazolium hexafluorophosphate ([C₄mim]PF₆) are investigated using 31P NMR spectroscopy line shape analyses and spin鈥搇attice relaxation time measurements. The PF₆鈥?/sup> anion performs isotropic rotation in all three polymorphic crystals phases 伪, 尾, and 纬 as well as in the liquid state with a characteristic time scale that ranges from a few ps to a few hundred ps over a temperature range of 180鈥?80 K. The rotational correlation time 蟿_c for PF₆鈥?/sup> rotation follows the sequence 纬-phase < 伪-phase 鈮?liquid < 尾-phase. On the other hand, in the liquid state, all local motions in the cation as well as its global rotational reorientation are characterized by time scales that are slower compared to that for the PF₆鈥?/sup> anion rotation. The time scale 蟿_c and the activation energy of PF₆鈥?/sup> rotation in this RTIL are found to be comparable with those observed in ordinary alkali and ammonium salts despite the large counterion size and low melting point of the former. The high sphericity of the PF₆鈥?/sup> ion is hypothesized to play an important role in the decoupling of its rotational dynamics that appear to be practically independent of the averaged cation鈥揳nion interaction.}