Elucidating the NuclearQuantum Dynamics of Intramolecular Double Hydrogen Transfer in Porphycene

We address the double hydrogen transfer (DHT) dynamics of the porphycene molecule: A complex paradigmatic system where the making and breaking of H-bonds in a highly anharmonic potential energy surface requires a quantum mechanical treatment not only of the electrons, but also of the nuclei. We combine density-functional theory calculations, employing hybrid functionals and van der Waals corrections, with recently proposed and optimized path-integral ring-polymer methods for the approximation of quantum vibrational spectra and reaction rates. Our full-dimensional ring-polymer instanton simulations show that below 100 K the concerted DHT tunneling pathway dominates, but between 100 K and 300 K there is a competition between concerted and stepwise pathways when nuclear quantum effects are included. We obtain ground-state reaction rates of $2.19 \times 10^{11} \mathrm{s}^{-1}$ at 150 K and $0.63 \times 10^{11} \mathrm{s}^{-1}$ at 100 K, in good agreement with experiment. We also reproduce the puzzling N-H stretching band of porphycene with very good accuracy from thermostatted ring-polymer molecular dynamics simulations. The position and lineshape of this peak, centered at around 2600 cm$^{-1}$ and spanning 750 cm$^{-1}$, stems from a combination of very strong H-bonds, the coupling to low-frequency modes, and the access to $cis$-like isomeric conformations, which cannot be appropriately captured with classical-nuclei dynamics. These results verify the appropriateness of our general theoretical approach and provide a framework for a deeper physical understanding of hydrogen transfer dynamics in complex systems

Decisive role of nuclear quantum effects on surface mediated water dissociation at finite temperature

Water molecules adsorbed on inorganic substrates play an important role in several technological applications. In the presence of light atoms in adsorbates, nuclear quantum effects (NQE) influence properties of these systems. In this work, we explore the impact of NQE on the dissociation of water wires on stepped Pt(221) surfaces. By performing ab initio molecular dynamics simulations with van der Waals corrected density functional theory, we note that several competing minima for both intact and dissociated structures are accessible at finite temperatures, making it important to assess whether harmonic estimates of the quantum free energy are sufficient to determine the relative stability of the different states. We perform ab initio path integral molecular dynamics (PIMD) in order to calculate these contributions taking into account conformational entropy and anharmonicities at finite temperatures. We propose that when when adsorption is weak and NQE on the substrate are negligible, PIMD simulations can be performed through a simple partition of the system, resulting in considerable computational savings. We calculate the contribution of NQE to the free energies, including anharmonic terms. We find that they result in an increase of up to 20% of the quantum contribution to the dissociation free energy compared to harmonic estimates. We also find that the dissociation has a negligible contribution from tunneling, but is dominated by ZPE, which can enhance the rate by three orders of magnitude. Finally we highlight how both temperature and NQE indirectly impact dipoles and the redistribution of electron density, causing work function to changes of up to 0.4 eV with respect to static estimates. This quantitative determination of the change in work function provides a possible approach to determine experimentally the most stable configurations of water oligomers on the stepped surfaces

The Surface of Electrolyte Solutions is Stratified

The distribution of ions at the air/water interface plays a decisive role in many natural processes. It is generally understood that polarizable ions with low charge density are surface-active, implying they sit on top of the water surface. Here, we revise this established hypothesis by combining surface-specific heterodyne-detected vibrational sum-frequency generation with neural network-assisted ab initio molecular dynamics simulations. Our results directly demonstrate that ions in typical electrolyte solutions are, in fact, located in a subsurface region leading to a stratification of such interfaces into two distinctive water layers. The outermost surface is ion-depleted, and the sub-surface layer is ion-enriched. As a result, an effective liquid/liquid interface buried a few {\AA} inside the solution emerges, creating a second water/electrolyte interface, in addition to the outermost air/water interface

i-PI 2.0: A Universal Force Engine for Advanced Molecular Simulations

Progress in the atomic-scale modeling of matter over the past decade has been tremendous. This progress has been brought about by improvements in methods for evaluating interatomic forces that work by either solving the electronic structure problem explicitly, or by computing accurate approximations of the solution and by the development of techniques that use the Born–Oppenheimer (BO) forces to move the atoms on the BO potential energy surface. As a consequence of these developments it is now possible to identify stable or metastable states, to sample configurations consistent with the appropriate thermodynamic ensemble, and to estimate the kinetics of reactions and phase transitions. All too often, however, progress is slowed down by the bottleneck associated with implementing new optimization algorithms and/or sampling techniques into the many existing electronic-structure and empirical-potential codes. To address this problem, we are thus releasing a new version of the i-PI software. This piece of software is an easily extensible framework for implementing advanced atomistic simulation techniques using interatomic potentials and forces calculated by an external driver code. While the original version of the code (Ceriotti et al., 2014) was developed with a focus on path integral molecular dynamics techniques, this second release of i-PI not only includes several new advanced path integral methods, but also offers other classes of algorithms. In other words, i-PI is moving towards becoming a universal force engine that is both modular and tightly coupled to the driver codes that evaluate the potential energy surface and its derivatives

Quantum tunneling in real space: Tautomerization of single porphycene molecules on the (111) surface of Cu, Ag, and Au

Tautomerization in single porphycene molecules is investigated on Cu(111), Ag(111), and Au(111) surfaces by a combination of low-temperature scanning tunneling microscopy (STM) experiments and density functional theory (DFT) calculations. It is revealed that the trans configuration is the thermodynamically stable form of porphycene on Cu(111) and Ag(111), whereas the cis configuration occurs as a meta-stable form. The trans → cis or cis → trans conversion on Cu(111) can be induced in an unidirectional fashion by injecting tunneling electrons from the STM tip or heating the surface, respectively. We find that the cis → cis tautomerization on Cu(111) occurs spontaneously via tunneling, verified by the negligible temperature dependence of the tautomerization rate below ∼23 K. Van der Waals corrected DFT calculations are used to characterize the adsorption structures of porphycene and to map the potential energy surface of the tautomerization on Cu(111). The calculated barriers are too high to be thermally overcome at cryogenic temperatures used in the experiment and zero-point energy corrections do not change this picture, leaving tunneling as the most likely mechanism. On Ag(111), the reversible trans → cis conversion occurs spontaneously at 5 K and the cis → cis tautomerization rate is much higher than on Cu(111), indicating a significantly smaller tautomerization barrier on Ag(111) due to the weaker interaction between porphycene and the surface compared to Cu(111). Additionally, the STM experiments and DFT calculations reveal that tautomerization on Cu(111) and Ag(111) occurs with migration of porphycene along the surface; thus, the translational motion couples with the tautomerization coordinate. On the other hand, the trans and cis configurations are not discernible in the STM image and no tautomerization is observed for porphycene on Au(111). The weak interaction of porphycene with Au(111) is closest to the gas-phase limit and therefore the absence of trans and cis configurations in the STM images is explained either by the rapid tautomerization rate or the similar character of the molecular frontier orbitals of the trans and cis configurations

Supporting data for "Surface Stratification Determines the Interfacial Water Structure of Simple Electrolyte Solutions"

<p>Supplementary data for "Surface Stratification Determines the Interfacial Water Structure of Simple Electrolyte Solutions"<br>Yair Litman, Kuo-Yang Chiang, Takakazu Seki, Yuki Nagata, Mischa Bonn<br><br>(<a href="https://dx.doi.org/10.1038/s41557-023-01416-6" target="_blank" rel="noopener">DOI: 10.1038/s41557-023-01416-6</a>   <a href="https://www.nature.com/articles/s41557-023-01416-6" target="_blank" rel="noopener">www.nature.com/articles/s41557-023-01416-6</a>)<br>Ab initio trajectories of air/NaOH (aq) obtained with CP2K (https://www.cp2k.org/)</p&gt

Tunnel- und Nullpunktsenergie-Effekte in vieldimensionalen Wasserstofftransferreaktionen: Von der Gasphase zur Adsorption auf Metalloberflaechen.

Hydrogen transfer reactions play a significant role in many technological applications and fundamental processes in nature. Despite appearing to be simple reactions, they constitute complex processes where nuclear quantum effects (NQE) such as zero-point energy and nuclear tunneling play a decisive role even at ambient temperature. Moreover, the anharmonic coupling between different degrees of freedom that take place in realistic systems leads to hydrogen dynamics that, in many cases, are hard to interpret and understand. Systematic and quantitative ab initio studies of hydrogen dynamics were performed in systems ranging from gas phase molecules to adsorbates on metallic surfaces using state-of-the-art methodologies based on the path integral formulation of quantum mechanics in combination with the density functional approximation. In order to achieve this task, the construction of a general infrastructure that made the required ring polymer instanton simulations feasible was created, and a new approximation which considerably reduces the computational cost of including NQE on weakly bound systems was proposed and tested in the study of water dissociation at Pt(221) surface. Practical guidelines and limitations were also discussed to help the adoption of such methodologies by the community. The system of choice for most of the studies presented in this thesis was the porphycene molecule, a paradigmatic example of a molecular switch. The are a large number of experimental results in well-controlled environments available in the literature which have demonstrated the importance of NQE and multidimensional coupling for this molecule. Therefore, the porphycene molecule provides the unique possibility to theoretically address these effects and compare the theoretical predictions with experimental results in different environments. A portion of this thesis focuses on the study of porphycene molecule in the gas phase. For this purpose, the intramolecular double hydrogen transfer (DHT) rates and vibrational spectrum were calculated. The theoretical results showed a remarkable agreement with the experiments, and enabled the explanation of the unusual infrared spectra, the elucidation of the dominant DHT mechanism, and the understanding of their temperature dependence. In all the cases, the coupling between low- and high-frequency modes proved to be essential to get qualitatively correct trends. Another portion of this thesis examines molecules adsorbed on surfaces. Studies of porphycene molecules adsorbed on (111) and (110) metal surfaces showed that the stronger the surface-molecule interaction is, the more the molecule buckles upon adsorption, leading to an overall decrease of the DHT rates. The simulations identified different temperature regimes of the DHT mechanism, which was not possible by experimental measurements, and evidenced the importance of surface fluctuations on the DHT rates. In conclusion, this thesis provides a stepping stone towards the understanding of the impact of NQE, anharmonic effects, and multidimensional mode coupling on hydrogen dynamics, and also describes novel computational tools to approach their study by using first-principle calculations.Wasserstofftransferreaktionen spielen in vielen technologischen Anwendungen und grundlegenden Prozessen in der Natur eine bedeutende Rolle. Obwohl es sich um einfache Reaktionen zu handeln scheint, umfassen sie komplexe Prozesse, bei denen nukleare Quanteneffekte (NQE) wie Nullpunktsenergie und nukleares Tunneln auch bei Raumtemperatur von entscheidender Bedeutung sind. Darüber hinaus führt die anharmonische Kopplung zwischen verschiedenen Freiheitsgraden, die in realistischen Systemen stattfindet, zu einer Dynamik des Wasserstoffs, die in vielen Fällen schwer zu interpretieren und zu verstehen ist. In der vorliegenden Arbeit werden systematische und quantitative ab initio Studien zur Wasserstoffdynamik in Systemen durchgeführt, die von Molekülen in der Gasphase bis zu Adsorbaten auf metallischen Oberflächen reichen. Hierbei werden Methoden eingesetzt, die auf der Pfadintegralformulierung der Quantenmechanik in Kombination mit Dichte-Funktional-Approximationen basieren. Um diese Aufgabe zu erfüllen, wird eine Infrastruktur entwickelt, die die erforderlichen Ringpolymer-Instanton-Simulationen ermöglicht. Weiterhin wird eine neue Näherung für schwach-bindende Systeme vorgeschlagen und für die Wasserdissoziation auf Pt(221) Oberfläche getestet, welche die Rechenkosten für die Berücksichtigung von NQE deutlich reduziert. Ebenso werden praktische Richtlinien und Einschränkungen dieser Methode erörtert, die deren Verwendung in der Community erleichtern soll. Das Porphycenmolekül - ein paradigmatisches Beispiel für einen molekularen Schalter - bildet die Grundlage für die meisten in dieser Arbeit vorgestellten Untersuchungen. In der Literatur gibt es eine große Anzahl von experimentellen Ergebnissen in gut kontrollierten Umgebungen, die die Bedeutung von NQE und mehrdimensionaler Kopplung für dieses Molekül gezeigt haben. Daher bietet das Porphycenmolekül die einzigartige Möglichkeit, diese Effekte theoretisch anzugehen und die theoretischen Vorhersagen mit experimentellen Ergebnissen in verschiedenen Umgebungen zu vergleichen. Ein Teil dieser Arbeit befasst sich mit der Untersuchung von Porphycenmolekülen in der Gasphase. Zu diesem Zweck wurden die intramolekularen Doppelwasserstofftransferraten (DHT) und das Vibrationsspektrum berechnet. Die theoretischen Ergebnisse zeigen eine bemerkenswerte Übereinstimmung mit Experimenten und ermöglichen die Erklärung der ungewöhnlichen Infrarotspektren und die Aufklärung des dominanten DHT-Mechanismus, sowie das Verständnis ihrer Temperaturabhängigkeit. In allen Fällen erwies sich die Kopplung zwischen Nieder- und Hochfrequenzmoden als entscheidend um qualitativ korrekte Trends zu erhalten. Ein weiterer Teil dieser Arbeit untersucht Moleküle, die an Oberflächen adsorbiert sind. Untersuchungen von an (111)- und (110)-Metalloberflächen adsorbierten Porphycenmolekülen zeigen, dass die Wechselwirkung zwischen Oberfläche und Molekül umso stärker ist, je mehr sich das Molekül bei der Adsorption verbiegt, was insgesamt zu einer Abnahme der DHT-Raten führt. Die Simulationen identifizieren unterschiedliche Temperaturregime des DHT-Mechanismus, was bisher durch experimentelle Messungen nicht möglich war und hebt die Bedeutung von Oberflächenfluktuationen für die DHT-Raten hervor. Zusammenfassend bildet diese Arbeit die Grundlage zum Verständnis des Einfluss von NQE, anharmonischen Effekten und mehrdimensionaler Modenkopplung auf die Wasserstoffdynamik und beschreibt zudem neuartige Simulationswerkzeuge für die ab initio Untersuchungen dieser Effekte

Multidimensional Hydrogen Tunneling in Supported Molecular Switches: The Role of Surface Interactions

The nuclear tunneling crossover temperature (T-c) of hydrogen transfer reactions in supported molecular-switch architectures can lie close to room temperature. This calls for the inclusion of nuclear quantum effects (NQEs) in the calculation of reaction rates even at high temperatures. However, computations of NQEs relying on standard parametrized dimensionality-reduced models quickly become inadequate in these environments. In this Letter, we study the paradigmatic molecular switch based on porphycene molecules adsorbed on metallic surfaces with full-dimensional calculations that combine density-functional theory for the electrons with the semiclassical ring-polymer instanton approximation for the nuclei. We show that the double intramolecular hydrogen transfer (DHT) rate can be enhanced by orders of magnitude due to surface fluctuations in the deep-tunneling regime. We also explain the origin of an Arrhenius temperature dependence of the rate below T-c and why this dependence differs at different surfaces. We propose a simple model to rationalize the temperature dependence of DHT rates spanning diverse fcc [110] surfaces