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?

GC Lloyd-Jones*, JN Harvey, P Hodgson, M Murray, RL Woodward

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish
Pages (from-to)4523-4535
Number of pages13
JournalChemistry - A European Journal
Volume9
Issue number18
DOIs
Publication statusPublished - 22 Sep 2003

Keywords

  • density functional calculations
  • hydrogen bonds
  • NMR spectroscopy
  • solvent effects
  • spin-spin coupling
  • CAMBRIDGE STRUCTURAL DATABASE
  • N,N-CHIRAL PROTON SPONGE
  • BARRIER HYDROGEN-BONDS
  • ACID BASE-PAIRS
  • AB-INITIO
  • ENZYMATIC CATALYSIS
  • H-N
  • CONSTANTS
  • NMR
  • CHEMISTRY

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