To analyze each crystal state of the crystal polymorphism of C18GL ( ) in detail at the molecular level, Fourier transform infrared spectroscopy (FT-IR) was used to measure type I, type II, and type III crystals and a solution melted at 100°C (Figure , see also Figure in the Supporting Information). Characteristic peaks were observed for the carbonyl ester stretching peaks in each crystal form (Table ). The wavenumber of the aliphatic carbonyl stretching peak decreased as the crystal form progressed from type I to II and III. Type I had a peak at 1748-1749 cm -1 , the same as the liquid form, and no intermolecular hydrogen bonding. In types II and III, the wavenumber of the stretching peaks gradually decreased to 1744 cm -1 and 1741 cm -1 , respectively. In other words, it is believed that the intermolecular hydrogen bonding gradually limits the carbonyl ester stretching. On the other hand, γ-lactone has two carbonyl stretching peaks at 1820 cm -1 and 1809 cm -1 in the liquid state. Type I has a peak at 1810 cm -1 and no intermolecular hydrogen bonding. Compared to type I, types II and III have peaks 1820 cm -1 and 1803 cm -1 . Peak 1820 cm -1 is also in the liquid and is not used for intermolecular hydrogen bonding, while peak 1803 cm -1 appears to be used for intermolecular hydrogen bonding. Thus, as the crystal polymorphism progresses from I to II and III, the aliphatic and γ-lactone carbonyl groups become less stretched and intermolecular hydrogen bonds are formed. The Type I crystals have no intermolecular hydrogen bonds and melt at low temperatures, and the formation of intermolecular hydrogen bonds by the carbonyl groups of γ-lactone and fatty acids is thought to change the crystal form to type II and type III and increase the melting point.