Determination of Missing Crystal Structures in the 1-Alkyl-3-methylimidazolium Hexafluorophosphate Series: Implications on Structure-Property Relationships
Determination of Missing Crystal Structures in the 1-Alkyl-3-methylimidazolium Hexafluorophosphate Series: Implications on Structure-Property Relationships
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DOI：
10.1021/cg401293g
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发表时间：
2013-12-01
影响因子：
3.8
通讯作者：
Nishikawa, Keiko
中科院分区：
文献类型：
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作者：
Endo, Takatsugu;Masu, Hyuma;Nishikawa, Keiko
我们对两种1-烷基-3-甲基咪唑六氟磷酸盐（[C（n）mim] PF_6），[C（1）mim] PF_6和[C（3）mim] PF_6进行了单晶X射线衍射分析。前者的热力学亚稳结晶相（β相）是通过在毛细管中从熔体中结晶获得的，用于晶体学分析。[C（1）mim] PF 6 β相中的阳离子和阴离子都比稳定α相中的阳离子和阴离子具有更高的点群对称性。该结果显示与β多晶型物的熔点（314 K）显著低于α多晶型物的熔点（364 K）的事实一致。[C（3）mim] PF 6的晶体结构似乎是典型的咪唑基室温离子液体晶体的特征。阳离子构象介于[C（2）mim] PF 6和[C（4）mim] PF 6的晶体结构特征之间。当结合在一起时，[C（n）mim] PF 6系列（n = 1-4）的所有晶体学数据证明了这些盐中烷基链长度依赖于结构填充的影响。计算这些结构的自由体积分数，并与简单基于公式单位体积的晶格能进行比较。结果表明，咪唑阳离子的烷基链长的差异不单调地改变与晶格能的晶体结构，后者也取决于在结构中的组成离子的包装效率。
We have performed single crystal X-ray diffraction analyses of two 1-alkyl-3-methylimidazolium hexafluorophosphate ([C(n)mim]PF6) salts, [C(1)mim]PF6, and [C(3)mim]PF6. The thermo-dynamically metastable crystalline phase (beta phase) of the former was obtained by crystallization from the melt in a capillary for the crystallographic analysis. Both the cation and the anion in the beta phase of [C(1)mim]PF6 show higher point group symmetry than those in the stable alpha phase. This result is shown to be consistent with the fact that the melting point of the beta polymorph (314 K) is significantly lower than that of the alpha polymorph (364 K). The crystal structure of [C(3)mim]PF6 seems to be typical of that characteristic of the crystals of imidazolium-based room temperature ionic liquids. The cation conformation is the one in between those characteristic of the crystal structures of [C(2)mim]PF6 and [C(4)mim]PF6. When taken together, all crystallographic data for the [C(n)mim]PF6 series (n = 1-4) demonstrate the influence of the alkyl chain length dependence on structural packing in these salts. The free volume fractions for these structures are calculated and compared to the lattice energy simply based on formula unit volume. It is shown that the difference in the alkyl chain length of the imidazolium cations does not monotonically change with the lattice energy of the crystal structure, and the latter also depends on the packing efficiency of the constituent ions in the structure.