Sterling Chemistry Laboratory, Yale University, New Haven, Connecticut 06520, USA.
J Chem Phys. 2013 Dec 14;139(22):224305. doi: 10.1063/1.4838475.
To clarify the intramolecular distortions exhibited by the complementary ions in the archetypal ionic liquid 1-ethyl-3-methylimidazolium tetrafluoroborate [EMIM][BF4], we report the vibrational spectra of the isolated ionic constituents and small aggregates cooled to about 10 K. Deuteration of bare EMIM(+) at the C(2) position, the putative hydrogen bond donating group, establishes that the observed bulk red shift is too small (<10 cm(-1)) for hydrogen bonding to be a dominant structural feature. We then analyze how the vibrational patterns evolve with increasing size to identify the spectral signatures of well-defined structural motifs in the growing assembly. Surprisingly, the main features of the bulk spectrum are already developed in the cluster with a single BF4 (-) anion sandwiched between just two EMIM(+) cations. We suggest that this local motif, while not strongly hydrogen bonded, nonetheless induces considerable intensity in the C(2)H stretches and is a robust feature in the local molecular structure of the liquid.
为了阐明典型离子液体 1-乙基-3-甲基咪唑四氟硼酸盐 [EMIM][BF4]中互补离子所表现出的分子内扭曲，我们报告了冷却至约 10 K 的孤立离子成分和小聚集体的振动光谱。在假定的氢键供体 C(2)位置对裸露的 EMIM(+)进行氘代，确定了观察到的体相红移太小（<10 cm(-1))，氢键不是主要的结构特征。然后，我们分析了随着尺寸的增加振动模式如何演变，以确定在不断增长的组装体中明确结构基元的光谱特征。令人惊讶的是，在仅由两个 EMIM(+)阳离子夹在中间的单个 BF4(-)阴离子的簇中，已经发展出了体相光谱的主要特征。我们认为，尽管这种局部基元没有强烈的氢键，但它会在 C(2)H 伸缩中引起相当大的强度，并且是液体中局部分子结构的一个稳健特征。
