The hydrogen-bond donor strength of ureas, widely used in hydrogen-bond donor catalysis, molecular recognition, and self-assembly, can be enhanced by increasing the size of the chalcogen X in the CX bond from O to S to Se and by introducing more electron-withdrawing substituents because both modifications increase the positive charge on the NH groups which become better hydrogen-bond donors. However, in 1,3-diaryl X-ureas, a steric mechanism disrupts the positive additivity of these two tuning factors, as revealed by our quantum-chemical analyses. This leads to an enhanced hydrogen-bond donor strength, despite a lower NH acidity, for 1,3-diaryl substituted O-ureas compared to the S- and Se-urea analogs. In addition, we provide a strategy to overcome this steric limitation using a predistorted urea-type hydrogen-bond donor featuring group 14 elements in the CX bond so that the hydrogen-bond donor strength of X-urea derivatives bearing two aryl substituents can be enhanced upon varying X down group 14.

中文翻译：

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尿素氢键供体优势：越大并不总是越好


广泛用于氢键供体催化、分子识别和自组装的尿素的氢键供体强度可以通过将 C X 键中硫属元 X 的大小从 O 增加到 S 再到 Se 并引入更多的吸电子取代基来提高，因为这两种修饰都增加了 NH 基团上的正电荷，从而成为更好的氢键供体。然而，正如我们的量子化学分析所揭示的那样，在 1,3-二芳基 X-脲中，空间机制破坏了这两个调谐因子的正加性。与 S 和硒尿素类似物相比，尽管 NH 酸度较低，但这导致 1,3-二芳基取代的 O-脲的氢键供体强度增强。此外，我们提供了一种策略来克服这种空间限制，使用在 C X 键中具有第 14 族元素的预扭曲尿素型氢键供体，以便在改变 X 向下第 14 族时可以提高带有两个芳基取代基的 X-尿素衍生物的氢键供体强度。