N-15,N-15 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 sp in-spin coupling constants offer a direct tool to explore a variety of ques tions concerning the relationship between coupling constants and intermolec ular distances, molecular orientation, changes in hybridization and related issues. Of particular interest are (2h)J(N-15,N-15) couplings across hydro gen bonds. In this work we present EOM-CCSD coupling constants [(2h)J(N-15, N-15)] for a series of cationic complexes stabilized by either traditional or proton-shared N - H+ - N hydrogen bonds, and relate these to N-15,N-15 c oupling constants in neutral complexes with N-H-N hydrogen bonds. The compu ted N-15,N-15 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 gen erality that they should be useful for determining N - N distances from exp erimentally measured coupling constants. Copyright (C) 2001 John Wiley & So ns, Ltd.