Why Does 1H NMR Signal Intensity of Concentrated Aqueous Acids and Bases Decrease Anomalously?

Abstract

The signal detection of quantitative 1H nuclear magnetic resonance (1H qNMR) for the amount of water in HCl and KOH aqueous solutions at low pH (pH −1.2) and high pH (pH 16) was discussed, and an “anomaly of decreasing 1H NMR signal intensity” was observed. After adding significant amounts of acid (H+) or base (OH−) to the solution for pH regulation, the number of 1H nuclei observed by 1H NMR significantly decreased at pH ≤ 2 and ≥ 13. The mobility and the activity coefficient of some or all the water molecules hydrated to H3O+ significantly decreased; hence, they could not be detected by 1H qNMR. The remarkably strong electrostatic interactions of the solvent with H3O+ and OH− significantly reduce the activity coefficient of the solvent by restricting the solvent molecule to the vicinity of the ion. These results are comparable to those reported for vapor pressure measurements and can be attributed to a decrease in activity. H+ (H3O+), which has a relatively small ionic radius, has a significant effect on the solvation structure and hydrogen-bond network owing to strong electrostatic interactions with the solvent. Subsequently, significant reduction in the 1H NMR relaxation time of the water molecules and signal intensity, along with a low magnetic field shift in the 1H NMR signal were also observed in the strongly acidic and strongly basic regions.

Keywords

• quantitative nuclear magnetic resonance (qnmr)
• nmr relaxation
• water activity
• hydrogen-bond network