Two-Bond Spin−Spin Coupling across a Hydrogen Bond: X−Y Coupling in the Presence and Absence of the Proton

Abstract

This paper addresses the question of whether spin−spin coupling between X and Y across an X−H−Y hydrogen bond is proof that the hydrogen bond is covalent. The results of ab initio equation-of-motion coupled cluster singles and doubles (EOM-CCSD) calculations are used to compare coupling constants (2hJX-Y) for hydrogen-bonded complexes and related complexes that have the same X−Y distance but no hydrogen bond. The results show that even in the absence of a hydrogen bond, X−Y coupling can occur. The magnitude of this coupling constant may be greater or less than it is in the hydrogen-bonded complex. Thus, these data suggest that X−Y spin−spin coupling is not a proof of covalency. The role of the hydrogen bond is to allow X and Y to approach each other close enough to form a stable complex, for which an X−Y coupling constant can be measured experimentally. The presence of the proton does influence the magnitude of the coupling constant by altering the s-electron densities on X and Y.