Molecule 1-Methyl-5-nitroindoline Probes the Structural Change of Liquid Water with Temperature

The molecule 1-methyl-5-nitroindoline (MNI) demonstrates to be a suitable probe to detect the change of structure of liquid water by varying temperature. The relationship of the peak wavenumber of the first absorption band of this probe with temperature clearly shows a minimum at 43 °C that indicates a change of structure in the liquid water at ambient pressure. The plot of the MNI wavenumber peak absorption against the heat capacity of liquid water shows a temperature range (35-45 °C) that resembles the crossover temperature proposed by Mallamace et al. (Faraday Discuss. 2013, 167, 95) from thermodynamic measurements (42 ± 5 °C). The crossover temperature range includes the so-called phase-transition temperature at 34.6 °C and 0.05 atm measured by Dougherty et al. (J. Phys. Chem. A 2006, 110, 7577-7580) and matches the temperature measured from solvatochromism by Catalán and Gonzalo (43 °C) (Chem. Phys. Lett., 2017, 679, 86-89) as the origin of a structural change in liquid water.