Two-Bond Spin–Spin Coupling Constants (2hJX[sbnd]Y) Across X[sbnd]H[sbnd]Y Hydrogen Bonds: Some Fundamental Questions

Abstract

Investigations of spin–spin coupling constants (JXY2h) across XHY hydrogen bonds constitute a relatively new and exciting area of theoretical and experimental research. Despite the tremendous progress that has been made in a short period of time, there remain many fundamental questions that need to be addressed. Among these are the following: (1) Is it possible to predict the signs of two-bond spin–spin coupling constants (JXY2h) across XHY hydrogen bonds? (2) What determines the sign of JXY2h? (3) Does the measurement of a two-bond coupling constant prove that the hydrogen bond is covalent? (4) What role does the proton play in coupling across hydrogen bonds? Systematic studies of two-bond coupling constants for series of complexes stabilized by CHN, NHN, OHN, FHN, CHO, NHO, OHO, and FHO hydrogen bonds have been carried out using the ab initio equation-of-motion coupled cluster singles and doubles (EOM–CCSD) method. The results of these studies are used to provide some answers to these questions.