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?

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 (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 (2H)J(NN) value in the HBF(4) salt of [(15)N(2)]-1,6-dimethyl-1,6-diazacyclodecane 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 (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 (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.

Symmetrization of Cationic Hydrogen Bridges of Protonated Sponges Induced by Solvent and Counteranion Interactions as Revealed by NMR Spectroscopy

“…The scenarios of Scheme 5 might also explain the results of Lloyd-Jones et al, [50] who found particularly small values of JA C H T U N G T R E N N U N G (N,N) for solutions of iodides of partially methylated protonated proton sponges in acetonitrile. Successive replacement of methyl by hydrogen reduced the values of JA C H T U N G T R E N N U N G (N,N).…”

“…[50] Scheme 3. Doubly 15 N-labeled protonated proton sponges for which coupling constants across the intramolecular 15 …”

“…On the other hand, DFT calculations showed that the NHN hydrogen-bond geometries did not substantially change when the methyl groups were replaced by hydrogen. [50] These small values can be interpreted in terms of microsolvation of the NH groups by solvent molecules, which lowers the hydrogen-bond symmetry. This is essentially the case of A C H T U N G T R E N N U N G Scheme 5a.…”

Structural studies of peri-interactions and bond formation between electron-rich atomic centres and N-phenylcarboxamides or nitroalkenyl groups

“…Lloyd-Jones has reported interesting initial investigations of estimating distances between peri-substituents using 15 N, 15 N coupling constants across hydrogen bonded amino groups, but correlation of calculated coupling constants with N,N separation was better than for the observed data. 43 While our measurements in most cases indicate a short contact between dimethylamino and electrophilic groups, they do not give direct insight into the mode of the interaction. Only in 22 is there clear evidence for bond formation.…”

Car–Parrinello and path integral molecular dynamics study of the intramolecular hydrogen bond in the novel class of anionic H-chelates: 6-Nitro-2,3-dipyrrol-2-ylquinoxaline anion

“…In the infrared spectra an intense and broad band assigned to m as (N-HÁ Á ÁN) stretching mode is observed at extremely low frequencies, in the region around 500-600 cm À1 [11]. The NMR 15 N-15 N coupling constants across the hydrogen bond ð 2 J NN Þ do not exceed value of 11 Hz [11][12][13].…”