Thymine hydrate (TH) is a classic hydrate system in which water molecules are highly confined in linear one-dimensional channels. Here, we describe our efforts to tune the channel volume by preparing isomorphous mixed crystals with increasing concentrations of uracil (Ur) and 5-ethyluracil (EUr), molecular analogues with slightly smaller and larger volumes, respectively. Solutions with up to 20 mol % Ur successfully yielded isomorphous mixed crystals of TH-Ur_x in phase-pure form, though analysis of the mixed crystal compositions indicated that only ∼46% of the Ur available in the initial growth solution was proportionately included in the host lattice. The thermal stability of the mixed TH-Ur_x material was significantly altered, with a greater fraction of water loss occurring at low temperature compared to the pure hydrate phase. Isomorphous mixed crystals with EUr substitutions, TH-EUr_x, were successfully prepared from solutions with 5–10 mol % EUr, although the fraction of EUr included was much lower (∼13–36%) than Ur. As the concentration of EUr in the growth solution increased to 15–20 mol %, an unexpected mixed composition anhydrate emerged as the dominant crystallization product. Although packing fraction estimates suggested that higher EUr substitution levels in the hydrate were theoretically possible, the formation of the mixed anhydrate presented a more favorable route to a dense material phase.

中文翻译：

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通道水合物的空间扰动：同构的极限

胸腺嘧啶水合物 ( TH ) 是一种经典的水合物系统，其中水分子被高度限制在线性一维通道中。在这里，我们描述了我们通过制备具有增加浓度的尿嘧啶 (Ur) 和 5-乙基尿嘧啶 (EUr) 的同晶混合晶体来调整通道体积的努力，分子类似物的体积分别略小和较大。含有高达 20 mol % Ur 的溶液成功地产生了相纯形式的TH-Ur _x同晶混晶，尽管对混晶成分的分析表明，初始生长溶液中只有约 46% 的 Ur 按比例包含在主晶格。混合TH-Ur _x的热稳定性材料发生了显着变化，与纯水合物相相比，在低温下发生的水损失比例更大。从含有 5-10 mol% EUr 的溶液中成功制备了具有 EUr 取代基的同晶混合晶体TH-EUr _x，尽管所包含的 EUr 比例远低于 Ur（~13-36%）（~13-36%）。随着生长溶液中 EUr 的浓度增加到 15-20 mol%，一种意想不到的混合成分无水物作为主要结晶产物出现。尽管堆积分数估计表明水合物中较高的 EUr 取代水平在理论上是可能的，但混合无水物的形成提供了更有利的致密材料相途径。