Cross-hydrogen bond ^2hJ_N,N coupling constant, ranging from +6 to +10 Hz, in Watson-Crick and Hoogsteen base pairs in RNA ( 98JACS8293, 98PNAS14147 99NAR3104, and 00JACS8095), DNA ( 99JB67, 99JACS6019, 99JACS8661, 00JB279 and 00JB269-18) and DNA-protein complex ( 00JB39) from the quantitative-J_NN 2D H(N)N-COSY experiment.

The size of ^2hJ_N,N and the chemical shift of the imino proton (d_imino) can be correlated in which stronger values correspond to downfield chemical shifts.

^2hJ_N,N = 1.27*d_imino - 9.6 (in Hz)

Recently, a new correlation experiment called H(CN)N(H) has been proposed to observe NH...N hydrogen bonds when the hydrogen bonded protons are not observables ( 99JB207 and 00NAR1585). In addition, studies have been performed to relate the extension of hydrogen bonding to the magnitude of the constant ( 00JACS10480).

This coupling has been also used to correlate remote, non-exchangeable protons in sheared G.A mismatches ( 01JB103-19) via the H2N6N3 and (H6)N6N3H2 experiments. In addition, a suite of HNN-COSY experiments (HNN-COSY, H(C)NN(H), and H(NN)H experiments) have been proposed to assign 1H-1H correlations across NH...N hydrogen bonds ( 01JB289-21 and 01MRCS166)

Theoretical studies on the mecanisms involved in these couplings has been reported ( 01JB371-19, 01JACS4014 and 02JACS1190)

Temperature dependence of ^2hJ_N,N in RNA ( 02MRC377)