What is a hydrogen bond? Resonance covalency in the supramolecular domain
Frank Weinhold and Roger A. Klein
It relates to issues I have posted about before. In an earlier article Weinhold and Klein reviewed how most introductory chemistry textbooks claim that hydrogen bonding is essentially a classical electrostatic phenomena [some sort of dipole-dipole interaction], in spite of the fact that it is largely due to coherent quantum effects.
Similar electrostatics-type assumptions are deeply embedded in the empirical point-charge potentials of widely used molecular dynamics (MD) and Monte Carlo (MC) simulation methods (Leach, 2001). These methods make no pretense to describe chemical bonding and reactivity phenomena, but are widely presumed to adequately describe H-bonding phenomena. The ubiquity of such simulation potentials in many areas of materials and biochemical research tends to reinforce and perpetuate the corresponding electrostatics-type rationalizations of H-bonding in elementary textbooks. Neither the manner in which H-bonding is now taught to beginning students nor how it is “simulated” in MD/MC potentials has changed appreciably in the past half-century.Building on the recent IUPAC revised definition of the H-bond , they propose several definitions that might be appropriate for inclusion in introductory texts. Here is the most technical version:
A fractional chemical bond due to partial intermolecular A–H:B ↔ A:−H–B+ resonance delocalization (partial 3-center/4-electron proton-sharing between Lewis bases), arising most commonly from quantum mechanical nB→ σ*AH donor–acceptor interaction.
I have emphasised before there is a "smoking gun" for this quantum view of the hydrogen bond: the existence of an excited electronic state that is a superposition of the same two "resonating" basis states [diabatic states] that make up the ground state. It should be observable in quantum chemistry calculations and in UV absorption experiments.
Weinhold and Klein also made the recommendation:
Expose students ASAP to modern theoretical discovery tools
The ready web-based availability of WebMO and other resources for calculating and visualizing accurate wavefunctions places a powerful tool in the hands of chemical educators and their laptop-toting students in the modern WiFi-activated classroom. With suitable guidebooks or Youtube tutorials [e.g., Marcel Patek, Christopher C. Cummins, or other web-based tutorial materials listed here and here], students can soon be using the same powerful computational tools that are driving chemical discovery in research laboratories around the globe. With such access, the student's laptop or mobile device can serve not only as an in-class discovery tool but also as a patient tutor and pedagogical “oracle” to provide accurate answers (and vivid graphical imagery) concerning details of valency, hybridization, and bonding in chosen chemical species, long before mathematical mastery of the underling quantum theory is attained.I thank Ross van Vuuren for bringing the article to my attention.
The paper is part of a special issue on Physical Chemistry Education.
Aside: it warmed my heart that the authors referenced my post Chemistry is Quantum Science, that highlighted one of Weinhold's earlier articles.