Changes in microstructure of two ammonium-based protic ionic liquids proved by in situ variable-temperature 1 H NMR spectroscopy: influence of anion.

Abstract

In this work, changes in microstructure of two protic ionic liquids (PILs), namely n-butylammonium acetate (N4Ac) and n-butylammonium nitrate (N4NO3 ), are proved by in situ variable-temperature 1 H NMR spectroscopy at the temperature range from 25 to 115°C, and the influence of the nature of anion is discussed accordingly. The results demonstrate that 1 H NMR chemical shifts of alkyl protons of both N4Ac and N4NO3 are almost not changed with the increasing of temperature, due to the absence of hydrogen bond interaction between alkyl protons with anions. Whereas those of + N-H of cation decrease linearly with the temperature increasing, indicating that the hydrogen bond interaction between + N-H and anion weakens gradually. In addition, the strength of hydrogen bond interaction between + N-H and NO3- is stronger than that between + N-H and Ac- , suggesting that anions have a significant influence on microstructure due to the acidity of a Brønsted acid. Consequently, the proton transfer from cation to anion is much easier in N4Ac compared to N4NO3 . Further analyses of 1 H NMR chemical shifts of + N-H in N4Ac at the temperature range from 100 to 115°C suggest that the splitting of + N-H peak may be attributed to obvious evidence of the existence of the proton transfer from + N-H to Ac- , which leads to dissociate the contact ion-pair in N4Ac to form the neutral ion-pair 'molecule'. The results will help us to extensively understand the behavior of proton transfer and offer us some valuable information for the design of PILs. Copyright © 2017 John Wiley & Sons, Ltd.