15 N, 15 N 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 2h J ( 15 N, 15 N) couplings across hydrogen bonds. In this work we present EOM–CCSD coupling constants [ 2h J ( 15 N, 15 N)] for a series of cationic complexes stabilized by either traditional or proton‐shared N—H + —N hydrogen bonds, and relate these to 15 N, 15 N coupling constants in neutral complexes with N—H—N hydrogen bonds. The computed 15 N, 15 N 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. Copyright © 2001 John Wiley & Sons, Ltd.