Fig. 2: Temperature-dependent amide-I infrared spectra and derived van’t Hoff plot.

a Temperature-dependent infrared spectra of the amide-I band of deuterated N-ethylpropionamide (NEPA) in D2O from 23 to 85 °C at 10 °C interval. b The van’t Hoff plot using the equilibrium constant of the weakly hydrogen-bonded (WHB) state and strongly hydrogen-bonded (SHB) state populations as a function of temperature. See Supplementary Note 1 for error bars’ generation.