Solid , Solution and Gas Phase Interactions of an Imidazolium-Based Task-Specific Ionic Liquid Derived from Natural Kojic Acid

Um líquido iônico imidazólio de função específica derivado do ácido kójico natural teve suas interações supramoleculares investigadas no estado sólido, em solução e em fase gasosa. Diferentes técnicas como difração de raios X de monocristal, espectroscopia de ressonância magnética nuclear (NMR), espectrofotometria UV-Vis, medidas de condutividade, espalhamento de raios X em baixo ângulo (SAXS), espectrometria de massas (tandem) com ionização por electrospray (ESI-MS(/MS)) e cálculos teóricos permitiram uma investigação estrutural aprofundada desse líquido iônico de função específica e suas interações supramoleculares.


NMR experiments
NMR spectra were recorded on a 7.05 T instrument using a 5-mm internal diameter probe operating at 300 MHz for 1 H and at 75 MHz for 13 C.Chemical shifts were expressed in parts per million (ppm) and referenced by the signals of the residual hydrogen atoms of deuterated benzene as the external reference (7.16 ppm and 128.39 ppm, respectively).

UV-Vis analyses
UV-Vis spectra were recorded in an apparatus with a diode array.UV-Vis analyses were performed in an Agilent Technologies 8453 spectrophotometer in 1-cm quartz cuvettes at 25 °C.

X-ray diffraction
The molecular structure of MIK.Cl was determined by X-ray diffraction.The X-ray measurements were carried out in an apparatus with an area detector diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at room temperature (T = 293 K).

MS analyses
ESI-MS and ESI-MS/MS measurements were performed in the positive-ion mode (m/z 50-2000 range) on an HDMS instrument.This instrument has a hybrid quadrupole/ion mobility/orthogonal acceleration time-offlight (oa-TOF) geometry and was used in the TOF V+ mode.All samples were dissolved and were directly infused into the ESI source at a flow rate of 10 µL min -1 after 5 min at 100 °C.ESI source conditions were as follows: capillary voltage 3.0 kV, sample cone 20 V, extraction cone 3 V.

Conductivity measurements
Conductivity measurements were performed with a Tecnal Tec-4MP with platinum electrodes at 25 °C.All analyses required 15 mL (pure solvent or mixtures, as indicated) and were carried out under stirring.KCl stock solutions were used to set the standard parameter at 146.7 mS cm -1 .

SAXS
SAXS experiments were performed on the SAXS1 beam line of the Brazilian Synchrotron Light Laboratory (LNLS), monitored with a photomultiplier, and detected on a Pilatus detector (300K Dectris), generating scattering wave vectors (q) from 0.07 to 5.5 nm -1 .The wavelength of the incident X-ray beam (λ) was 0.1488 nm.Background and parasitic scattering were determined by separate measurements on an empty holder and subtracted.The exposure time was set as 300 s for all samples.Only saturated solutions of MIK.Cl in water and methanol were used in the experiments.
The X-ray scattering is experimentally determined as a function of the scattering vector, q, whose modulus is given by: where θ is half the scattering angle (2θ).The correlation γ(r), and interface distribution functions, G(r) were generated from SAXS data to obtain the mean of distance of the scattering center.The G(r) were obtained by applying the second derivative to the Fourier transformed SAXS data using a procedure proposed in literature. 1,2

Calculations
The theoretical treatment of the system included in this work was performed using the density functional theory (DFT) 3 approach and ab initio second-order Møller-Plesset perturbation theory (MP2). 4,5Initially, we tested several basis set functions, as well as several exchange-correlation functional B3LYP, 6 M02X, 7 PBE1PBE, 8 B98 9 and the ab initio MP2 method aiming to choose a level of theory that presents a good compromise between accuracy and computational effort.For comparisons, we also tested the G2MP2 10 and CBS-QB3 11 methods, which are complex energy computations methods involving several predefined calculations on the specified molecular system (not shown).We chose to use the MP2/6-311+G* level without loss of quality and with lower computational cost.Binding energies, enthalpies for ion-pairing and Gibbs free energies using different computational methods were also performed (not shown).Comparison with two highly accurate G2MP2 and CBS-QB3 pre-defined methods allows us to again note that the MP2/6-311+G* level of theory adequately represents the thermodynamic properties of the system.We selected the MP2/6-311+G* as a level of theory for the second part of the computational work.In the second part of the computational work, the crystallographically-determined coordinates were taken frozen during all calculations which are obtained at MP2/6-311+G* level.Quantum theory of "atoms in molecules" (QTAIM) study using Bader's theory 12 has been performed, as it is very effective for evaluating topological parameters of hydrogen bonds.QTAIM calculations were performed by using AIMALL program. 13The Morokuma energy decomposition analysis (EDA) was performed using AOMix program. 14The natural bond orbital analysis (NBO) was also carried out. 15Both geometrical and electronic theoretical calculations were carried out using the Gaussian 09 program suite.