What is a hydrogen bond? Resonance covalency in the supramolecular domain

We address the broader conceptual and pedagogical implications of recent recommendations of the International Union of Pure and Applied Chemistry (IUPAC) concerning the re-definition of hydrogen bonding, drawing upon the recommended IUPAC statistical methodology of mutually correlated experimental and theoretical descriptors to operationally address the title question. Both direct and statistical lines of evidence point to the essential resonance covalency of H-bonding interactions, rather than the statistically insignificant ‘‘dipole–dipole’’ character that is persistently advocated in current textbooks. The revised conception of H-bonding is both supported by modern quantum chemical technology and consistent with the pre-quantal insights of G. N. Lewis and other bonding pioneers. We offer specific suggestions for how relatively minor changes in the usual discussion of Lewis-structural and resonance concepts—supported by modern web-based computational modeling tools—can readily accommodate this fundamental change of perspective.