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首页> 外文期刊>Chemistry: A European journal >Scalar Coupling between the ~15N Centres in Methylated 1,8-Diaminonaphthalenes and 1,6-Diazacyclodecane:To What Extent is ~2HJ_NN a Reliable Indicator of N-N Distance?
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Scalar Coupling between the ~15N Centres in Methylated 1,8-Diaminonaphthalenes and 1,6-Diazacyclodecane:To What Extent is ~2HJ_NN a Reliable Indicator of N-N Distance?

机译：甲基化的1,8-二氨基萘的〜15N中心与1,6-二氮杂环癸烷之间的标量耦合：〜2HJ_NN在多大程度上是N-N距离的可靠指标？

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The scalar couplings between hydrogen bonded nitrogen centers (~2HJ_NN)in the free-base and protonated forms of the complete series of[~15N-2]-N-methylated 1,8-diamino naphthalenes in [D_7]DMF solution have been determined,either directly(~15N{~1H}NMR),or,indirectly(~13C{~1H}NMR and simulation of the X part of the ABX spectrum(X=~13C,A,B=~15N)).Additionally,the ~2HJ_NN value in the HBF_4 salt of [~15N_2]-1,6-dimethyl-1,6-diazacyclodecan was determined,indirectly by ~13C{~1H}NMR spectroscopy.As confirmed by DFT calculations and by reference to CSD,the rigid nature of the naphthalene scaffold results in rather low deviations in N,N distance or H-N,N angle within each series,apart from the free base of the permethylated compound(proton sponge)where the naphthalene ring is severely distorted to relieve strain.Despite such restrictions,the ~2HJ_NN values increase smoothly from 1.5 to 8.5 Hz in the protonated series as the degree of methylation increases.The effect in the free-base forms is much less pronounced(2.9to 3.7 Hz)with no scalar N,N coupling detected in the permethylated compound(proton sponge)due to the lack of hydrogen bond between the N,N centers.Neither the pK_a nor the N-N distance in the protonated forms correlates with ~2HJ_NN.However,the sum of the ~13C NMR shifts of the naphthalene ring C(1,8)carbons which are attached directly to the nitrogen centers correlates linearly with ~2HJ_NN and with the degree of methylation.The gas-phase computed ~2HJ_NN is almost constant throughout the homologous series,and close to the experimental value for the tetramethylated ion.However,the computed coupling constant is attenuated in structures involving microsolvation of each N-H unit,and the trend then matches experiment.These experimental and computational observations suggest that Fermi contact between the two N centers is decreased upon formation of strong charge-dis-persing intermolecular hydrogen bonds of the free N-H groups with the solvent.
机译：确定了[D_7] DMF溶液中[〜15N-2] -N-甲基化的1,8-二氨基萘全系列的游离碱和质子化形式中氢键氮中心（〜2HJ_NN）之间的标量偶合，直接（〜15N {〜1H} NMR）或间接（〜13C {〜1H} NMR和ABX光谱X部分的模拟（X =〜13C，A，B =〜15N））。 ，通过〜13C {〜1H} NMR光谱法间接测定了[〜15N_2] -1,6-二甲基-1,6-二氮杂环癸烷的HBF_4盐中的〜2HJ_NN值。通过DFT计算和参考CSD的证实，萘骨架的刚性会导致每个系列中N，N距离或HN，N角度的偏差很小，除了全甲基化化合物（质子海绵）的游离碱（萘环严重变形以减轻应变）之外尽管有这些限制，但随着甲基化程度的增加，质子化序列中〜2HJ_NN值从1.5 Hz平稳增加到8.5 Hz。在游离碱形式下的影响是在全甲基化化合物（质子海绵）中未检测到标量N，N耦合，因此不明显（2.9至3.7 Hz），因为N，N中心之间没有氢键。质子化形式的pK_a和NN距离都没有与〜2HJ_NN相关。但是，直接连接到氮中心的萘环C（1,8）碳的〜13C NMR位移之和与〜2HJ_NN和甲基化程度呈线性关系。计算出的〜2HJ_NN在整个同源序列中几乎是恒定的，接近四甲基化离子的实验值。但是，在涉及每个NH单元的微溶剂化的结构中，计算出的耦合常数会衰减，然后趋势与实验匹配。这些实验和计算结果观察结果表明，在自由的NH基团与溶剂形成强分散电荷的分子间氢键后，两个N中心之间的费米接触减少。

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