Structural Flexibility of Water Frameworks: A Single Large Guest‐Inducing Structure‐H and Structure‐II Hydrate Structures

A unique clathrate hydrate system in which a single guest molecule, thiane, induces the formation of both structure‐II (sII) and structure‐H (sH) hydrates under different thermodynamic conditions is reported. According to the experimental phase equilibrium data, the sII hydrate is thermodynamically more stable than the sH hydrate at lower temperatures. However, only the sH hydrate forms first from CH 4 –water–thiane mixtures, revealing a discrepancy between thermodynamic stability and actual crystallization behavior. Raman spectroscopy, supported by density functional theory and energy calculations using the COMPASS III force field, confirms that thiane adopts the chair conformation in both sII and sH hydrate structures. Among the CH 4 , CO 2 , and CH 4 /CO 2 systems, only CH 4 forms an sH hydrate with thiane, indicating that the crystallization pathway is likely small guest dominant and governed by the nucleation process. Collectively, these findings highlight the thiane hydrate system as a valuable model for investigating structural adaptability and host–guest interactions across different clathrate frameworks.