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首页> 外文期刊>Chemistry: A European journal >Scalar coupling between the N-15 centres in methylated 1,8-diaminonaphthalenes and 1,6-diazacyclodecane: To what extent is (2H)J(NN) a reliable indicator of N-N distance?
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Scalar coupling between the N-15 centres in methylated 1,8-diaminonaphthalenes and 1,6-diazacyclodecane: To what extent is (2H)J(NN) a reliable indicator of N-N distance?

机译：甲基化的1,8-二氨基萘和1,6-二氮杂环癸烷中N-15中心之间的标量耦合：（2H）J（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 centres ((2)HJ(NN)) in the free-base and protonated in the free forms of the complete series of [N-15(2)]-N-methylated 1,8-diamino naphthalenes in [D-7]DMF solution have been determined, either directly (N-15{H-1} NMR), or, indirectly (C-13{H-1} NMR and simulation of the X part of the ABX spectrum (X = C-13, A, B = N-15)). Additionally, the 1%, value in the HBF4 salt of [N-15(2)]-1,6-dimethyl-1,6-diazacyclodecane was determined, indirectly by C-13{H-1} 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 (2H)J(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 (2.9 to 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 centres. Neither the pK(a) nor the N-N distance in the protonated forms correlates with (2H)J(NN). However, the sum of the C-13 NMR shifts of the naphthalene ring C(1,8) carbons which are attached directly to the nitrogen centres correlates linearly with (2H)J(NN) and with the degree of methylation. The gas-phase computed (2)HJ(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 centres is decreased upon formation of strong charge-dispersing intermolecular hydrogen bonds of the free N-H groups with the solvent. [References: 64]
机译：氢键氮中心（（2）HJ（NN））在游离碱中的标量偶合，并以[N-15（2）]-N-甲基化1,8-二氨基的完整系列的游离形式质子化已直接（N-15 {H-1} NMR）或间接（C-13 {H-1} NMR）测定了[D-7] DMF溶液中的萘，并模拟了ABX光谱的X部分（X = C-13，A，B = N-15）。此外，间接地通过C-13 {H-1} NMR光谱法测定[N-15（2）]-1,6-二甲基-1,6-二氮杂环癸烷的HBF4盐的1％值。通过DFT计算并参考CSD证实，除了全甲基化化合物（质子海绵）的游离碱以外，萘基支架的刚性使得每个系列中的N，N距离或HN，N角度偏差都非常小。萘环严重变形以减轻应变的地方。尽管有这些限制，但随着甲基化程度的增加，（2H）J（NN）的值在质子化序列中从1.5 Hz平稳增加到8.5 Hz。由于在N，N中心之间缺乏氢键，因此在全甲基化化合物（质子海绵）中未检测到标量N，N耦合，游离碱形式的效应明显不足（2.9至3.7 Hz）。质子化形式的pK（a）或N-N距离均不与（2H）J（NN）相关。但是，直接连接到氮中心的萘环C（1,8）碳的C-13 NMR位移之和与（2H）J（NN）和甲基化程度线性相关。气相计算的（2）HJ（NN）在整个同源序列中几乎恒定，并且接近四甲基化离子的实验值。但是，在涉及每个N-H单元的微溶剂化的结构中，计算的耦合常数会衰减，然后趋势与实验匹配。这些实验和计算观察结果表明，在游离N-H基团与溶剂形成强电荷分散的分子间氢键后，两个N中心之间的费米接触减少。 [参考：64]

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