Fig. 4: Pressure-dependent vibrational properties and hydrogen bond length of NPG.
From: Understanding colossal barocaloric effects in plastic crystals
a Raman spectroscopy measurements of NPG plastic crystal in monoclinic and cubic phases with energy range between 10 and 480 meV. The insets demonstrate the low-energy (from 20 to 140 meV) and high-energy range (from 390 to 490 meV), respectively. The pressure-dependent (b) CH3 rotation and (c) O–H stretching modes of NPG molecules in isolated, monoclinic, and cubic phases; the insets visualize the corresponding mode. Negative and positive values in the horizontal axis represent tensile and compressive pressure, respectively. d Bond length of the covalent bond O–H and hydrogen bond O–H···O of NPG molecules at different pressure, along with experimental O–H covalent bond length19. The hydrogen bond O–H···O of NPG molecules in the cubic phase is estimated by the average of all O–H···O in the unit cell. e Calculated stretching frequencies of O–H bond against O···O distances in the hydrogen bond (O–H···O) of NPG, along with experimental infrared spectroscopic results20 (diamonds: combination of acid and complementary base; circles: resonance-assisted hydrogen bond; triangles: σ-cooperative hydrogen bonds; squares: isolated hydrogen bonds).
