The largest 15N-15N coupling constant across an NHN hydrogen bond.
The largest 15N-15N coupling constant across an NHN hydrogen bond.
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NHN 氢键上最大的 15N-15N 耦合常数。
DOI：
10.1002/anie.200704411
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
Limbach,Hans-Heinrich
中科院分区：
文献类型：
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作者：
Pietrzak,Mariusz;Try,AndrewC;Andrioletti,Bruno;Sessler,JonathanL;AnzenbacherJr,Pavel;Limbach,Hans-Heinrich
自从含有自旋为1/2的原子核的氢键A±H···B的核磁共振偶联常数被发现以来，如A，B=19F，15N，[1-3]，已经证实这些核磁共振参数不仅可以用来检测生物分子中的氢桥[4]，还可以用来确定溶液中强氢键的几何构型。[5]实验和从头计算[1]表明，当A···B距离最小时，标量双键耦合常数2JAB达到最大值。虽然已经确定了FHF和NHF氢键的最大值，但在NHN桥的情况下，相应的最大值仍然未知。到目前为止，只在核酸碱基对、[3]质子化海绵、[6，7]、六-[8]和七元[9]H-螯合物中检测到小于11赫兹的2J15N15N。相反，密度泛函计算给出了最大耦合常数2JNN=25 Hz，[5d]，对应的最短可能N···N距离约为2.5。[10]更可靠的小模型体系的高能级耦合团簇EOM计算预测了更大的耦合常数。这些阴离子是通过2，3-二吡咯基-2-基喹恶啉(DPQs，方案1)去质子化得到的。DPQ已被合成并作为带电物种(如氟化物)的比色阴离子受体进行研究。[13]然而，其不寻常的几何结构使得它们的单去质子化形式可能具有异常短的NHN氢键，从而具有异常大的15N-15N耦合常数。这一预期已经实现，我们在这里介绍了各种核磁共振光谱实验和从头算密度泛函计算的结果，这些结果表征了这些阴离子的分子内NHN氢键的几何结构。DPQ前驱体在5：1的Cd_2Cl2/[D6]DMSO混合溶液中加入少量固体NaH，生成DPQ1-3的NHN阴离子。生成了微量的二氢，以及相应的单阴离子作为主要的有机产物(见实验部分)。加入[D6]DMSO来溶剂化Na+反离子，而使用CD2Cl2作为主要溶剂，因为它提供了在较低温度下(发现质子交换缓慢的情况下)进行各种核磁共振光谱分析所需的还原粘度。图1a显示了在233K下记录在CD2Cl2/[D6]DMSO中的标记为H1和H17的吡咯酸质子的信号。在这种情况下，观察到两个信号，即11.81和11.93ppm。每个信号被分成一个二元组，其耦合常数为1J15N1H=±97。6和±97。7赫兹，
Since the discovery of NMR coupling constants across hydrogen bonds AÀH··· B containing nuclei with spin 1/2, such as A, B= 19F, 15N,[1–3] it has been established that these NMR parameters can not only be used to detect hydrogen bridges in biomolecules [4] but also to determine the geometries of strong hydrogen bonds in solution.[5] It has been shown experimentally and by abinitio calculations [1] that scalar two-bond coupling constants 2JAB attain maximum values when the A··· B distances are at a minimum. Whereas the maximum values have been established for FHF and NHF hydrogen bonds, the corresponding maxima are still unknown in the case of NHN bridges. To date, only 2J15N15N values less than 11 Hz have been detected in nucleic acid base pairs,[3] protonated sponges,[6, 7] and six-[8] and seven-membered [9] H-chelates. In contrast, DFT calculations gave maximum coupling constants 2JNN= 25 Hz,[5d] corresponding to the shortest possible N··· N distance of about 2.5.[10] More reliable high-level coupled-cluster EOM calculations of small model systems predicted even larger coupling constants.[11, 12] Herein we describe a novel class of anionic H-chelates with 15N–15N coupling constants of more than 16 Hz. These anions were obtained by deprotonation of 2, 3-dipyrrol-2-ylquinoxalines (DPQs, Scheme 1). DPQs have been synthesized and studied as colorimetric anion receptors for chargedense species, such as fluoride.[13] However, their unusual geometry makes it likely that their monodeprotonated forms would have unusually short NHN hydrogen bonds and thus unusually large 15N–15N coupling constants. This expectation has been realized, and we present herein the results of various NMR spectroscopy experiments and ab initio DFT calculations that characterize the geometries of the intramolecular NHN hydrogen bonds of these anions. The NHN anions of the DPQs 1–3 were generated by treatment of solutions of the DPQ precursors in 5: 1 CD2Cl2/[D6] DMSO mixtures to which small amounts of solid NaH were added. Minute quantities of dihydrogen were produced, along with the corresponding monoanion as the major organic product (see Experimental Section).[D6] DMSO was added to solvate the Na+ counterions, while CD2Cl2 was used as the primary solvent because it provided the reduced viscosity needed to carry out various NMR spectroscopic analyses at lower temperatures (conditions under which proton exchange was found to be slow).Figure1a shows the signals of the pyrrolic protons, labeled H1 and H17, of [15N2] 2 recorded at 233 K in CD2Cl2/[D6] DMSO. Under these conditions, two signals are observed, namely at 11.81 and 11.93 ppm. Each signal is split into a doublet, with coupling constants 1J15N1H= À97. 6 and À97. 7 Hz,