Theoretical And Experimental Study On ~1H NMR Chemical Shifts Of Ionic Liquids

In recent years,ionic liquids that due to their unique physical and chemical properties,such as low flammability,low viscosity and low melting point,have attracted extensive attention of chemical researchers.The study of the properties of ionic liquids depends heavily on nuclear magnetic resonance?NMR?techniques.However,it is believed that the interaction between anions and cations of ionic liquids is extremely complicated.The traditional experimental methods are difficult to obtain an accurate NMR spectrum,in addition,an one-to-one assignment of observed ¹H chemical shifts for ionic liquids is also often very challenging.These difficulties made the mechanism of some reactions hard to clarify.Therefore,it is urgent to study ¹H or ¹³C NMR spectra of ionic liquids by means of theoretical calculations.At present,the theoretical calculation about quantum chemistry of ¹H NMR of organic compounds has achieved remarkable results,and the calculated results of chemical shifts are relatively reliable.In this paper,the predictive calculation of ¹H NMR chemical shift of ionic liquids was studied.Firstly,the TMS standard,MSTD standard and linear regression method were used to calculate the ¹H NMR chemical shifts of ionic liquids.The relative error of the TMS standard is smaller.Although these three methods are more accurate for the calculation of ionic liquids containing weakly coordinating anions,the calculations for those ionic liquids containing strongly coordinated anions have large errors.In order to be able to further accurately calculate the NMR chemical shift of ionic liquids,a method of relative reference standard?RRS?is proposed in this paper.The RRS method used ionic liquid or organics as reference standard that are similar to the studied ionic liquid to calculate ¹H NMR chemical shift.Because of the anion type has larger impact on the chemical shift of the single ion pair.Using the RRS method,the calculated results agree well with the experimental values on account of elimination of errors caused by the anion.While the RRS uses experimental ¹H NMR data of the reference ionic liquids,it is found that the RRS method can effectively predict ¹H NMR chemical shifts at different theoretical calculation levels.In order to further study the chemical reaction of ionic liquids absorbing SO₂,the ¹H NMR chemical shifts of ionic liquid before and after SO₂ absorption was calculated by RRS method.The reaction mechanism was deduced by comparing the calculated data with experimental values.