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

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.