15N,15N spin-spin coupling constants across N?H?N and N?H+?N hydrogen bonds: can coupling constants provide reliable estimates of N?N distances in biomolecules?

Predictive quantum chemical methods based upon coupled cluster theory of spin–spin coupling constants offer a direct tool to explore a variety of questions concerning the relationship between coupling constants and intermolecular distances, molecular orientation, changes in hybridization and related issues. Of particular interest are 2hJ(15N,15N) couplings across hydrogen bonds. In this work we present EOM–CCSD coupling constants [2hJ(15N,15N)] for a series of cationic complexes stabilized by either traditional or proton-shared N—H+—N hydrogen bonds, and relate these to 15N,15N coupling constants in neutral complexes with N—H—N hydrogen bonds. The computed 15N,15N coupling constants in these complexes vary smoothly with N—N distance, regardless of the charge or the particular binding at the N atoms. We propose that the curves which show this dependence have sufficient generality that they should be useful for determining N—N distances from experimentally measured coupling constants.