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Scalar Coupling between the 15 N Centres in Methylated 1,8‐Diaminonaphthalenes and 1,6‐Diazacyclodecane: To What Extent is 2H J NN a Reliable Indicator of NN Distance?
Author(s) -
LloydJones Guy C.,
Harvey Jeremy N.,
Hodgson Paul,
Murray Martin,
Woodward Robert L.
Publication year - 2003
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.200304890
Subject(s) - chemistry , protonation , naphthalene , hydrogen bond , ring (chemistry) , crystallography , carbon 13 nmr , nuclear magnetic resonance spectroscopy , chemical shift , proton , stereochemistry , computational chemistry , medicinal chemistry , molecule , physics , organic chemistry , ion , quantum mechanics
Abstract The scalar couplings between hydrogen bonded nitrogen centres ( 2H J NN ) in the free‐base and protonated forms of the complete series of [ 15 N 2 ]‐ N ‐methylated 1,8‐diamino naphthalenes in [D 7 ]DMF solution have been determined, either directly ( 15 N{ 1 H} NMR), or, indirectly ( 13 C{ 1 H} NMR and simulation of the X part of the ABX spectrum (X= 13 C, A,B= 15 N)). Additionally, the 2H J NN value in the HBF 4 salt of [ 15 N 2 ]‐1,6‐dimethyl‐1,6‐diazacyclodecane was determined, indirectly by 13 C{ 1 H} 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 p K a nor the NN distance in the protonated forms correlates with 2H J NN . However, the sum of the 13 C 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 2H J 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.