Quantum nature of the hydrogen bond

Abstract

Hydrogen bonds are weak, generally intermolecular bonds, which hold much of soft matter together as well as the condensed phases of water, network liquids, and many ferroelectric crystals. The small mass of hydrogen means that they are inherently quantum mechanical in nature, and effects such as zero-point motion and tunneling must be considered, though all too often these effects are not considered. As a prominent example, a clear picture for the impact of quantum nuclear effects on the strength of hydrogen bonds and consequently the structure of hydrogen bonded systems is still absent. Here, we report ab initio path integral molecular dynamics studies on the quantum nature of the hydrogen bond. Through a systematic examination of a wide range of hydrogen bonded systems we show that quantum nuclear effects weaken weak hydrogen bonds but strengthen relatively strong ones. This simple correlation arises from a competition between anharmonic intermolecular bond bending and intramolecular bond stretching. A simple rule of thumb is provided that enables predictions to be made for hydrogen bonded materials in general with merely classical knowledge (such as hydrogen bond strength or hydrogen bond length). Our work rationalizes the influence of quantum nuclear effects, which can result in either weakening or strengthening of the hydrogen bonds, and the corresponding structures, across a broad range of hydrogen bonded materials. Furthermore, it highlights the need to allow flexible molecules when anharmonic potentials are used in force field-based studies of quantum nuclear effects.H ydrogen bonds are essential to life on earth. They are, for example, the main intermolecular interactions responsible for binding the two strands of DNA and holding together the condensed phases of water. H-bonds are also of great contemporary importance in nanoscience, being involved in, e.g., the functionalization and patterning of surfaces with ordered molecular overlayers (1, 2). It is known that H-bonds are complex and, in particular, because of the small mass of the proton it is often not appropriate to treat the proton in H-bonded systems as a classical particle. Instead the quantum nature of the proton must be taken into account and issues such as zero-point motion, quantum delocalization, and quantum tunneling are relevant. Recent advances in experimental techniques and the development of theoretical approaches (coupled with enormous advances in computer power) mean it is now possible to explore the quantum nature of the proton in H-bonded systems in exquisite detail. The relevance of quantum nuclear effects (QNEs) to liquid water and ice (3-8), interfacial water (9), and enzyme kinetics (10, 11) has recently been demonstrated. In particular, from first principles simulations by Morrone et al. (4,12) and neutron Compton scattering measurements by Burnham et al. (13,14), a clear picture of the impact of QNEs on the proton's real space delocalization and vibrational properties has been established. Upon increasing the H-bond strengt...

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