Spin coupling constants nitrogen-proton

Crepaux, D., Lehn, J. M., Dean, R. R. Nuclear Spin-Spin Interactions. X. Signs of Geminal and Vicinal Nitrogen-Proton Coupling Constants. Stereochemistry and Medium Effects on NH Couplings. Mol. Phys. 16, 225 (1969). 

The position of /V-methyl group in C-nitro-1,2,4-triazoles influences the spin density redistribution at the nitrogen atoms of triazole cycle and is particularly displayed in the changing of proton-coupling constant [881], Nitro group passes approximately half of all spin density (Table 3.37). 

The radical ion of phenoxazine has a uniformly spaced four-line spectra with the line intensities in the ratio 1 2 2 1. The measured coupling constant was 9.83 gauss, the line width 8.19 gauss, and the g value 2.0049. This hyperfine structure can be interpreted as a basic three-line splitting by the nitrogen-14 nucleus (nuclear spin 1) and a doublet splitting with nearly identical spacing due to the attached acidic proton (nuclear spin ). 

The values of the hyperfine coupling constants are collected in Table 1. Because the hfs constants for aromatic protons are proportional to the spin densities at the adjacent carbon atoms through the McConnell relation,8 these values provide a clear-cut picture of the spin density distribution within the molecular framework of radical cation l+ The hfs constants for the nitrogen atoms should reflect, besides the spin density at the nitrogen atom itself, the spin density at the adjacent atoms as well as the nature of their bonds to the nitrogen.7... 

Left two-pulse [(a) primary ESEEM] and three-pulse [(b) stimulated echo ESEEM] sequences t is the (fixed) delay time between pulses one and two and T is a variable delay time. Right frequency domain and time domain (inset) of the two-pulse EESEM spectrum of VO - vanabin, recorded at the m = — 1 /2 line, at 77 K and a pulse width of 20 ns.P l The superhyperfine coupling constant = 4.5 MHz (obtained from the N double-quantum lines at 3.9 and 7.1 MHz) is in accord with amine nitrogen provided by lysines of the vanadium-binding protein. The spin echo due to proton coupling, at 13.7 MHz, was also observed. Reproduced from K. Eukui et al., J. Am. Chem. Soc. 125, 6352-6353. Copyright (2003), with permission from the American Chemical Society. 

It should be pointed out that the spin density on the nitrogen of this radical is about the same as in the radical PDA-, as concluded from its hyperfine coupling constant (7.9 G), a fact consistent with the preceding ascription. Also, no spin coupling with the w-protons is detected. 

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