Dominik Marx

The nature of the hydrated excess proton in water

Explanations for the anomalously high mobility of protons in liquid water began with Grotthusss idea, of ‘structural diffusion’ nearly two centuries ago. Subsequent explanations have refined this concept by invoking thermal hopping, , proton tunnelling, or solvation effects. More recently, two main structural models have emerged for the hydrated proton. Eigen, proposed the formation of an H9O4+ complex in which an H3O+ core is strongly hydrogen-bonded to three H2O molecules. Zundel, , meanwhile, supported the notion of an H5O2+ complex in which the proton isshared between two H2O molecules. Here we use ab initio path integral simulations to address this question. These simulations include time-independent equilibrium thermal and quantum fluctuations of all nuclei, and determine interatomic interactions from the electronic structure. We find that the hydrated proton forms a fluxional defect in the hydrogen-bonded network, with both H9O4+ and H5O2+ occurring only in thesense of ‘limiting’ or ‘ideal’ structures. The defect can become delocalized over several hydrogen bonds owing to quantum fluctuations. Solvent polarization induces a small barrier to proton transfer, which is washed out by zero-point motion. The proton can consequently be considered part of a ‘low-barrier hydrogen bond’, , in which tunnelling is negligible and the simplest concepts of transition-state theory do not apply. The rate of proton diffusion is determined by thermally induced hydrogen-bond breaking in the second solvation shell.

The nature and transport mechanism of hydrated hydroxide ions in aqueous solution

Compared to other ions, protons (H+) and hydroxide ions (OH-) exhibit anomalously high mobilities in aqueous solutions. On a qualitative level, this behaviour has long been explained by ‘structural diffusion’—the continuous interconversion between hydration complexes driven by fluctuations in the solvation shell of the hydrated ions. Detailed investigations have led to a clear understanding of the proton transport mechanism at the molecular level. In contrast, hydroxide ion mobility in basic solutions has received far less attention, even though bases and base catalysis play important roles in many organic and biochemical reactions and in the chemical industry. The reason for this may be attributed to the century-old notion that a hydrated OH- can be regarded as a water molecule missing a proton, and that the transport mechanism of such a ‘proton hole’ can be inferred from that of an excess proton by simply reversing hydrogen bond polarities. However, recent studies have identified OH- hydration complexes that bear little structural similarity to proton hydration complexes. Here we report the solution structures and transport mechanisms of hydrated hydroxide, which we obtained from first-principles computer simulations that explicitly treat quantum and thermal fluctuations of all nuclei. We find that the transport mechanism, which differs significantly from the proton hole picture, involves an interplay between the previously identified hydration complexes and is strongly influenced by nuclear quantum effects.

Density-functional study of the structure and stability of ZnO surfaces

An extensive theoretical investigation of the nonpolar (1010) and (1120) surfaces as well as the polar zinc-terminated (0001)-Zn and oxygen-terminated (0001)-O surfaces of ZnO is presented. Particular attention is given to the convergence properties of various parameters such as basis set, k-point mesh, slab thickness, or relaxation constraints within local-density and generalized-gradient approximation pseudopotential calculations using both plane-wave and mixed-basis sets. The pros and cons of different approaches to deal with the stability problem of the polar surfaces are discussed. Reliable results for the structural relaxations and the energetics of these surfaces are presented and compared to previous theoretical and experimental data, which are also concisely reviewed and commented.

Ab initio path integral molecular dynamics: Basic ideas

The essential ideas underlying ab initio path integral molecular dynamics and its efficient numerical implementation are discussed. In this approach the nuclei are treated as quantum particles within the path integral formulation of quantum statistical mechanics. The electronic degrees of freedom are treated explicitly based on state‐of‐the‐art electronic structure theory. This renders ab initio simulations of quantum systems possible without recourse to model potentials. A combined extended Lagrangian for both quantum nuclei and electrons defines a dynamical system and yields molecular dynamics trajectories that can be analyzed to obtain quantum statistical expectation values of time‐independent operators. The methodology can be applied to a wide range of fields addressing problems in molecular and condensed matter chemistry and physics.

Aqueous basic solutions: hydroxide solvation, structural diffusion, and comparison to the hydrated proton

Many hydrogen-bonded liquids, molecular solids, and lowdimensional systems support anomalous diffusion mechanisms of topological charge defects created by the addition or removal of protons. The most familiar examples are the “classic” cases of aqueous acidic and basic solutions,1 where the defects appear in the form of hydrated hydronium (H3O) and hydroxide (OH-) ions, denoted as H+(aq) and OH-(aq), respectively.2 While anomalous charge migration has important consequences in chemical,1,3,4 biological,5-8 and technological9,10 applications, Vide infra, providing a molecular-level, mechanistic understanding of the fascinating physical principles underlying the charge transport process is a challenging, yet fundamental, problem in physical chemistry.11

Efficient and general algorithms for path integral Car–Parrinello molecular dynamics

In path integral molecular dynamics, efficient sampling of the phase space is not guaranteed due to the stiff harmonic part of the action arising from the quantum kinetic energy. This problem has been eliminated by incorporating a sufficient number of thermostats into the dynamical scheme and by introducing a transformation of the path ‘‘bead’’ variables. In this paper, an efficient Car–Parrinello path integral molecular dynamics algorithm, sufficiently general to include the use of ultrasoft pseudopotentials is introduced. Difficulties encountered when combining thermostats and transformations of the Cartesian ‘‘bead’’ coordinates with the generalized orthonormality condition are circumvented by employing a constrained nonorthogonal orbital method.

Tunnelling and zero-point motion in high-pressure ice

The microscopic structure of ice poses a long-standing challenge to theory. Because of their low mass, the protons in the hydrogen bonds that define the structures of crystalline ice are susceptible to quantum-mechanical effects such as tunnelling,. High pressure provides a means of controlling the length of the hydrogen bonds in order to investigate such effects. In particular, Holzapfel predicted 26 years ago that, under pressure, hydrogen bonds might be transformed from the highly asymmetric O–H···O configuration to a symmetric state in which the proton lies midway between the two oxygens, leading to a non-molecular symmetric phase of ice, now denoted as ice ‘X’. The existence of this phase has been inferred from spectroscopy, but has still not been observed directly. Here we investigate the role of quantum effects in proton ordering and hydrogen-bond symmetrization within ice at high pressure by using a simulation technique that treats both electrons and nuclei quantum-mechanically. We find that the proton-ordered structure at low pressure, with asymmetric hydrogen bonds (ice VIII), transforms on increasing pressure to a proton-disordered asymmetric phase (ice VII) owing to translational proton tunnelling. On further compression, the zero-point fluctuations lead to strongly delocalized protons and hydrogen-bond symmetrization, even though the underlying character of the proton-transfer potential remains a double well. Only at still higher pressures does the double-well potential become transformed into a single well, whereupon the protons again become increasingly localized.

Dissecting the THz spectrum of liquid water from first principles via correlations in time and space

Solvation of molecules in water is at the heart of a myriad of molecular phenomena and of crucial importance to understanding such diverse issues as chemical reactivity or biomolecular function. Complementing well-established approaches, it has been shown that laser spectroscopy in the THz frequency domain offers new insights into hydration from small solutes to proteins. Upon introducing spatially-resolved analyses of the absorption cross section by simulations, the sensitivity of THz spectroscopy is traced back to characteristic distance-dependent modulations of absorption intensities for bulk water. The prominent peak at ≈200 cm-1 is dominated by first-shell dynamics, whereas a concerted motion involving the second solvation shell contributes most significantly to the absorption at about 80 cm-1 ≈2.4 THz. The latter can be understood in terms of an umbrella-like motion of two hydrogen-bonded tetrahedra along the connecting hydrogen bond axis. Thus, a modification of the hydrogen bond network, e.g., due to the presence of a solute, is expected to affect vibrational motion and THz absorption intensity at least on a length scale that corresponds to two layers of solvating water molecules. This result provides a molecular mechanism explaining the experimentally determined sensitivity of absorption changes in the THz domain in terms of distinct, solute-induced dynamical properties in solvation shells of (bio)molecules—even in the absence of well-defined resonances.

Covalent mechanochemistry: theoretical concepts and computational tools with applications to molecular nanomechanics.

1. What is “Covalent Mechanochemistry” ? 5412 2. From Macroscopic Milling to Bond-Selective Manipulation 5415 2.

MOLECULAR DYNAMICS IN LOW-SPIN EXCITED STATES

A Kohn–Sham-like formalism is introduced for the treatment of excited singlet states. Motivated by ideas of Ziegler’s sum method and of restricted open-shell Hartree–Fock theory, a self-consistent scheme is developed that allows the efficient and accurate calculation of excited state geometries. Vertical as well as adiabatic excitation energies for the n→π* transitions of several small molecules are obtained with reasonable accuracy. As is demonstrated for the cis-trans isomerization of formaldimine, our scheme is suited to perform molecular dynamics in the excited singlet state. This represents a first step towards the simulation of photochemical reactions of large systems.