NMR studies and DFT calculations of the symmetric intramolecular NHN-hydrogen bond of bis-(2-pyridyl)-acetonitrile: Isotope labeling strategy for the indirect 13C-detection of 15N15N couplings

The 1H, 15N, and 13C NMR spectra of partially 15N labeled bis-(2-pyridyl)-acetonitrile (1) dissolved in CDCl3 and CD2Cl2 have been measured in order to characterize its intramolecular NHN hydrogen bond. A fast proton tautomerism renders the molecule symmetric within the NMR timescale which complicates the determination of the scalar coupling constant 2hJ(15N,15N) ≡ J(N,N) across the intramolecular NHN-hydrogen bond. It is shown that an isotopic labeling scheme where experiments are performed on a mixture of 1-14N14N, 1-14N15N, and 1-15N15N facilitates the direct determination of J(N,N) from the non-decoupled 15N NMR spectra as well as the indirect detection via 13C NMR. Thus, a value of J(N,N) = 10.3 ± 0.5 Hz is obtained, which is similar to the corresponding value of 10.6 ± 0.5 Hz found previously for the seven-membered H-chelate N,N′-diphenyl-6-aminopentafulvene-1-aldimine-15N2 (2). By contrast, the crystallographic N…N distances and hydrogen bond angles of both compounds are very different, i.e. 2.65 Å and about 140° in the case of 1 and 2.79 Å and about 160° in the case of 2. However, the sum of the calculated NH and H…N distances is the same for both compounds, i.e. 2.75 Å. This finding supports the previous proposition that the values of J(A,B) of a hydrogen bond AHB are correlated with the sum of the two hydrogen – heavy atom distances rather than with the heavy atom distance.