The monoprotonated compound N,N′,N′′-tris(p-tolyl)azacalix[3](2,6)pyridine (TAPH) contains an intramolecular hydrogen bond that is formed from three N atoms in its cavity. Constrained by the macrocyclic molecular structure, the separations between the N atoms in this bifurcated hydrogen bond are about 2.6 Å, considerably shorter than those typically observed for hydrogen bonded systems in the condensed phases. As such, TAPH exhibits significantly elongated N–H lengths in its hydrogen bond and a downfield ¹H NMR chemical shift of 22.1 ppm. In this work, we carry out ab initio molecular dynamics and ab initio path integral molecular dynamics simulations of TAPH in the acetonitrile solution to reveal the geometry and proton sharing conditions of the bifurcated short hydrogen bond and uncover how the interplay of electronic and nuclear quantum effects gives rise to its far downfield ¹H chemical shift. Taking a linear short hydrogen bond as a reference, we demonstrate the distinct features of competing quantum effects and electronic shielding effects in the bifurcated hydrogen bond of TAPH. We further use the degree of deshielding on the proton as a measure of the hydrogen bonding interactions and evaluate the strength of the bifurcated short hydrogen bond as compared to its linear counterpart.

中文翻译：

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分叉的短氢键的量子效应和1H NMR化学位移。

单质子化的化合物N，N '，N ''-三（对甲苯基）氮杂aca [3]（2,6）吡啶（TAPH）在其腔中包含由三个N原子形成的分子内氢键。受大环分子结构的约束，该分叉氢键中N原子之间的间距约为2.6，比在缩合相中氢键体系通常观察到的间距短得多。因此，TAPH在其氢键中具有显着延长的N–H长度，其低场¹ H NMR化学位移为22.1 ppm。在这项工作中，我们进行从头算分子动力学和从头算在乙腈溶液中进行TAPH的路径积分分子动力学模拟，以揭示分叉的短氢键的几何结构和质子共享条件，并揭示电子和核量子效应的相互作用如何引起其远场¹ H化学位移。以线性短氢键为参考，我们证明了TAPH分叉氢键中竞争量子效应和电子屏蔽效应的独特特征。我们进一步使用质子上的去屏蔽度作为氢键相互作用的量度，并评估了与其线性对应物相比，分叉的短氢键的强度。