Cetylpyridinium picrate: Spectroscopy, conductivity and DFT investigation of the structure of a new ionic liquid

A new cetylpyridinium picrate ionic liquid has been synthesized and characterized by differential thermal analysis, XRD, FT-IR, and NMR (H-1 and C-13) spectroscopy. Differential thermal analysis indicates the congruent melting of cetylpyridinium picrate and a temperature range of liquid state is 47-267 degrees C. A solid form of the sample characterized with a polymorphic transformation at 37 degrees C, which was confirmed via the XRD analysis. The interionic charge-transfer interactions, namely the charge transfer from the picrate anion to the cetylpyridinium cation, were detected by comparison of NMR spectra of DMSO solutions of the ionic liquid and cetylpyridinium chloride. The H-NMR chemical shifts' differences of about 0.2 ppm and 0.1 ppm were observed in the case of the o-protons and the oz-methylene group protons, respectively. Additionally, the charge-transfering between the attracted ions was confirmed by DFT calculations. Based on the solid state NMR technique, the higher ions mobility was suggested for the solid sample of the ionic liquid in comparison to the solid cetylpyridinium chloride monohydrate. Moreover, the association constants K-A at 22, 30, 40, and 50 degrees C, obtained from the electrical conductivity measurements, clearly testifies much lower dissociation in the case of picrate. The K-A values of cetylpyridinium picrate and chloride in DMSO medium at 22 degrees C equal 748.7 L/mol and 525.0 L/mol, respectively. The performed DFT computations of the reduced density gradient function in four proposed structures of the new ionic liquid cation-anion pair identifies the presence of weak non-covalent C-H center dot center dot center dot O=N interactions between cetyl chain hydrogen atoms and nitro groups of picrate was explored through DFT calculations and analysis of the RDG function. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

A new cetylpyridinium picrate ionic liquid was synthesized and characterized using various spectroscopic techniques, revealing properties such as phase transition from solid to liquid, interionic charge-transfer interactions, and molecular structures.