Structural and Conformational Analysis of 1-Butyl-3-methylimidazolium Nitrate

Autor: Dean H. Johnston, Carolanne E. Norris, Mallory Gasbarre
Rok vydání: 2021
Předmět:
Zdroj: Journal of Chemical Crystallography. 52:140-151
ISSN: 1572-8854
1074-1542
DOI: 10.1007/s10870-021-00899-w
Popis: The crystal structures of two polymorphic forms of 1-butyl-3-methylimidazolium nitrate are reported. The form observed at 100 K and 200 K (Ia and Ib) crystallizes in the $$P{\bar{1}}$$ space group and contains two independent imidazolium cations and nitrate anions. At 100 K the butyl chain of one cation adopts a TT (trans–trans) conformation and the other cation adopts a G′G′ (gauche–gauche) conformation. At 200 K, the G′G′ chain is disordered with about twelve percent of the GT conformation present. A different polymorph (Form II, also crystallizing in the $$P{\bar{1}}$$ space group with two independent ion pairs) is present at 273 K displaying significant disorder in the butyl chains with a mixture of G′T, G′G′, and GT conformers. Raman spectra were collected on samples between 100 and 350 K and show changes in band frequencies and intensities consistent with conversion between different butyl chain conformations. Hydrogen bond interactions are present between cation C–H’s and oxygen atoms of the nitrate ions, with significant lengthening observed for three of the six close contacts (and formation of one new contact) upon conversion to the higher-temperature form. The structural details revealed in this study shed light on the intermolecular forces and the conformational changes that accompany phase changes in 1-butyl-3-methylimidazolium nitrate. X-ray diffraction and Raman spectroscopy of the ionic liquid 1-butyl- 3-methylimidazolium nitrate at temperatures from 100 to 300 K show evidence of two different polymorphs and significant temperature-dependent conformational changes.
Databáze: OpenAIRE
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