The diffusion of protons through water is understood within the framework of the Grotthuss mechanism, which requires that they undergo structural diffusion in a stepwise manner throughout the water network. Despite long study, this picture oversimplifies and neglects the complexity of the supramolecular structure of water. We use first-principles simulations and demonstrate that the currently accepted picture of proton diffusion is in need of revision. We show that proton and hydroxide diffusion occurs through periods of intense activity involving concerted proton hopping followed by periods of rest. The picture that emerges is that proton transfer is a multiscale and multidynamical process involving a broader distribution of pathways and timescales than currently assumed. To rationalize these phenomena, we look at the 3D water network as a distribution of closed directed rings, which reveals the presence of medium-range directional correlations in the liquid. One of the natural consequences of this feature is that both the hydronium and hydroxide ion are decorated with proton wires. These wires serve as conduits for long proton jumps over several hydrogen bonds.T he mechanism by which protons move through water is at the heart of acid-base chemistry reactions. Understanding the reaction coordinates of this process has been one of the most challenging problems in physical chemistry due to the sheer complexity of water's hydrogen bond network (1-4). Developing a molecular basis for these phenomena is of great relevance in energy conversion applications such as in the design of efficient fuel cells (5). Over 200 y ago, von Grotthuss proposed a mechanism by which water would undergo electrolytic decomposition (6). He imagined that proton conduction involved the collective shuttling of hydrogen atoms along water wires. The early 20th century found many of the great scientists of the time developing conceptual models to understand the properties of water and its constituent ions (7,8). Detailed insights into the mechanisms of proton transfer (PT) came much later from a combination of both ab initio molecular dynamics (AIMD) simulations (3, 9-13) and force-field approaches based on the empirical valence bond formalism (14-16). The current textbook picture of the Grotthuss mechanism that has resulted from these studies involves a stepwise hopping of the proton from one water molecule to the next (1,17,18). This process occurs on a timescale of 1-2 ps. For a successful transfer, the model requires solvent reorganization around the proton-receiving species to develop a coordination pattern like that of the species it will convert to, a process known as presolvation. In all of these characterizations of the Grotthuss mechanism, the role of the connectivity of the water network was not brought to the forefront (3, 19).Sometimes PT has also been thought to take on coherent character involving jumps of several protons simultaneously. In this spirit, Eigen (20) suggested that the proton could delocalize over extended hydrogen...

The hydrogen bond (HB) is central to our understanding of the properties of water. However, despite intense theoretical and experimental study, it continues to hold some surprises. Here, we show from an analysis of ab initio simulations that take proper account of nuclear quantum effects that the hydrogen-bonded protons in liquid water experience significant excursions in the direction of the acceptor oxygen atoms. This generates a small but nonnegligible fraction of transient autoprotolysis events that are not seen in simulations with classical nuclei. These events are associated with major rearrangements of the electronic density, as revealed by an analysis of the computed Wannier centers and 1 H chemical shifts. We also show that the quantum fluctuations exhibit significant correlations across neighboring HBs, consistent with an ephemeral shuttling of protons along water wires. We end by suggesting possible implications for our understanding of how perturbations (solvated ions, interfaces, and confinement) might affect the HB network in water.path integral molecular dynamics | generalized Langevin equation thermostat | ab initio liquid water D espite its apparent simplicity, liquid water exhibits a number of anomalous properties, such as a decrease in density on freezing, an isobaric density maximum, and its unusually high dielectric constant and heat capacity (1). These, together with its unquestionable importance for climate and life on Earth, have made this substance a subject of intense research by both experiments and simulations.The central concept that has been used to rationalize the peculiar behavior of water is that of the hydrogen bond (HB) (2). The nature of this bond in water has been studied in depth by atomistic computer simulations, which have investigated how it is affected by ionic and electronic polarizability (3, 4), pressure and temperature (5, 6), and nuclear quantum effects (NQEs) (7-9). Furthermore, ab initio molecular dynamics (MD) simulations have been used to shed light on autoionization (10, 11), a process with profound implications for the chemistry of aqueous solutions.In this paper, we investigate the impact of NQEs on the HB in pure water, finding a qualitative increase in fluctuations that leads to a partial dissociation of the covalent O-H bond. The weakening of this covalent bond in the presence of hydrogen bonding is consistent with the red shift of the stretching mode of water upon condensation, as well as with recent experiments demonstrating that selectively exciting the O-H stretch in water leads to a pronounced delocalization of the proton toward the acceptor oxygen atom (12). However, the role of NQEs in governing the extent of this delocalization has not been investigated before now.Our analysis is based on MD simulations of water at different thermodynamic state points, with an ab initio description of the interactions among the nuclei. We also account fully for the quantum nature of the nuclear motion, using a recently developed combination of imaginary time path i...

One contribution of 8 to a Theme Issue 'Density functional theory across chemistry, physics and biology' .

IMPORTANCE Therapeutic hypothermia may increase survival with good neurologic outcome after cardiac arrest. Trans-nasal evaporative cooling is a method used to induce cooling, primarily of the brain, during cardiopulmonary resuscitation (ie, intra-arrest). OBJECTIVE To determine whether prehospital trans-nasal evaporative intra-arrest cooling improves survival with good neurologic outcome compared with cooling initiated after hospital arrival. DESIGN, SETTING, AND PARTICIPANTS The PRINCESS trial was an investigator-initiated, randomized, clinical, international multicenter study with blinded assessment of the outcome, performed by emergency medical services in 7 European countries from July 2010 to January 2018, with final follow-up on April 29, 2018. In total, 677 patients with bystander-witnessed out-of-hospital cardiac arrest were enrolled. INTERVENTIONS Patients were randomly assigned to receive trans-nasal evaporative intra-arrest cooling (n = 343) or standard care (n = 334). Patients admitted to the hospital in both groups received systemic therapeutic hypothermia at 32°C to 34°C for 24 hours. MAIN OUTCOMES AND MEASURES The primary outcome was survival with good neurologic outcome, defined as Cerebral Performance Category (CPC) 1-2, at 90 days. Secondary outcomes were survival at 90 days and time to reach core body temperature less than 34°C. RESULTS Among the 677 randomized patients (median age, 65 years; 172 [25%] women), 671 completed the trial. Median time to core temperature less than 34°C was 105 minutes in the intervention group vs 182 minutes in the control group (P < .001). The number of patients with CPC 1-2 at 90 days was 56 of 337 (16.6%) in the intervention cooling group vs 45 of 334 (13.5%) in the control group (difference, 3.1% [95% CI, −2.3% to 8.5%]; relative risk [RR], 1.23 [95% CI, 0.86-1.72]; P = .25). In the intervention group, 60 of 337 patients (17.8%) were alive at 90 days vs 52 of 334 (15.6%) in the control group (difference, 2.2% [95% CI, −3.4% to 7.9%]; RR, 1.14 [95% CI, 0.81-1.57]; P = .44). Minor nosebleed was the most common device-related adverse event, reported in 45 of 337 patients (13%) in the intervention group. The adverse event rate within 7 days was similar between groups. CONCLUSIONS AND RELEVANCE Among patients with out-of-hospital cardiac arrest, trans-nasal evaporative intra-arrest cooling compared with usual care did not result in a statistically significant improvement in survival with good neurologic outcome at 90 days.

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The combined time-resolved photoluminescence (PL) and theoretical study performed on luminescent [Mo6Br(i)8Br(a)6](2-)-based systems unambiguously shows that their NIR-luminescence is due to at least two emissive states. By quantum chemical studies, we show for the first time that important geometrical relaxations occur at the triplet states either by the outstretching of an apex away from the square plane of the Mo6 octahedron or by the elongation of one Mo-Mo bond. Experimental PL measurements demonstrate that the external environment (counter-ions, crystal packing) of the cluster has a noticeable impact on its relaxation processes. Temperature and excitation wavelength dependence of the two components of the luminescence spectra is representative of multiple competitive de-excitation processes in contradiction with Kasha's rule. Our results also demonstrate that the relaxation processes before and after emission can be tracked via fast time-resolved spectroscopy. They also show that the surroundings of the luminescent cluster unit and the excitation wavelength could be modulated for target applications.

The scope of this article is to present an overview of the Density Functional based Tight Binding (DFTB) method and its applications. The paper introduces the basics of DFTB and its standard formulation up to second order. It also addresses methodological developments such as third order expansion, inclusion of non-covalent interactions, schemes to solve the self-interaction error, implementation of long-range short-range separation, treatment of excited states via the time-dependent DFTB scheme, inclusion of DFTB in hybrid high-level/low level schemes (DFT/DFTB or DFTB/MM), fragment decomposition of large systems, large scale potential energy landscape exploration with molecular dynamics in ground or excited states, nonadiabatic dynamics. A number of applications are reviewed, focusing on -(i)-the variety of systems that have been studied such as small molecules, large molecules and biomolecules, bare or functionalized clusters, supported or embedded systems, and -(ii)-properties and processes, such as vibrational spectroscopy, collisions, fragmentation, thermodynamics or non-adiabatic dynamics. Finally outlines and perspectives are given. ARTICLE HISTORY

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful experimental tools to probe the local atomic order of a wide range of solid-state compounds. However, due to the complexity of the related spectra, in particular for amorphous materials, their interpretation in terms of structural information is often challenging. These difficulties can be overcome by combining molecular dynamics simulations to generate realistic structural models with an ab initio evaluation of the corresponding chemical shift and quadrupolar coupling tensors. However, due to computational constraints, this approach is limited to relatively small system sizes which, for amorphous materials, prevents an adequate statistical sampling of the distribution of the local environments that is required to quantitatively describe the system. In this work, we present an approach to efficiently and accurately predict the NMR parameters of very large systems. This is achieved by using a high-dimensional neural-network representation of NMR parameters that are calculated using an ab initio formalism. To illustrate the potential of this approach, we applied this neural-network NMR (NN-NMR) method on the (17)O and (29)Si quadrupolar coupling and chemical shift parameters of various crystalline silica polymorphs and silica glasses. This approach is, in principal, general and has the potential to be applied to predict the NMR properties of various materials.