Hydrogen-bond networks in finite ice nanotubes

Networks of Hydrogen Bond Networks in Water Clusters

Water clusters play a prominent role in atmospheric and solution chemistry. Numerous arrangements of protons, H-bond configurations or networks, shape the cluster properties. Studies of small water clusters by cryogenic scanning tunneling microscopy and high-resolution rovibrational spectroscopy have established proton rearrangement mechanisms forming pathways between H-bond networks. The mechanisms, concerted tunneling in particular, describe the local processes connecting pairs of configurations. Here, proton rearrangement networks mapping these transformations are defined and explored to provide a global view of the H-bond configurations in clusters. The networks are constructed for clusters of different sizes and structures. Their analysis reveals an odd-even effect with respect to the number of water molecules, exponential growth of the small-world character, bimodality of the degree distributions, and gapped assortativity of the networks. The last two properties signify the unexpected division of H-bond configurations into two classes according to their network connectivity. The results demonstrate qualitative differences between proton rearrangement mechanisms, suggest a strong influence of the cluster structure. The generated networks are of interest as real-world models for network rewiring; they establish an alternative platform for studies of proton rearrangements in H-bonded systems.

Statistical Equivalence of Quantum Chemical Methods for Energy Distribution in Water Clusters

Methods of quantum chemistry are instrumental in understanding molecular structures and properties. However, the results demonstrate significant variability, which is difficult to predict and rationalize. The fundamental question is whether some molecular systems exhibit properties invariant with respect to the computational method. The idea explored here is that collective properties of statistical ensembles should be more robust than characteristics of individual molecules and their arbitrary sets. This effect is demonstrated for the complete set of hydrogen-bond topologies of the dodecahedral water cluster (H₂ O)₂₀ . Non-Gaussian energy distributions produced by various methods have the same functional form despite strong differences in mean values and standard deviations. The conclusion is tested on methods of different complexity and origin employing a number of criteria. A linear mapping between the energies produced by different methods is discussed. The significance of the results is in establishing a collective equivalence property of quantum chemical methods.

Temperature dependence of the transport of single-file water molecules through a hydrophobic channel

Although great effort has been made on the transport properties of water molecules through nanometer channels, our understanding on the effect of some basic parameters are still rather poor. In this article, we use molecular dynamics simulations to study the temperature effect on the transport of single-file water molecules through a hydrophobic channel. Of particular interest is that the water flow and average translocation time both exhibit exponential relations with the temperature. Based on the continuous-time random-walk model and Arrhenius equation, we explore some new physical insights on these exponential behaviors. With the increase of temperature, the water dipoles flip more frequently, since the estimated flipping barrier is less than 2 kB T. Specifically, the flipping frequency also shows an exponential relation with the temperature. Furthermore, the water-water interaction and water occupancy demonstrate linear relations with the temperature, and the water density profiles along the channel axis can be slightly affected by the temperature. These results not only enhance our knowledge about the temperature effect on the single-file water transport, but also have potential implications for the design of controllable nanofluidic machines.

Topological crystallography of gas hydrates

A new approach to the investigation of the proton-disordered structure of clathrate hydrates is presented. This approach is based on topological crystallography. The quotient graphs were built for the unit cells of the cubic structure I and the hexagonal structure H. This is a very convenient way to represent the topology of a hydrogen-bonding network under periodic boundary conditions. The exact proton configuration statistics for the unit cells of structure I and structure H were obtained using the quotient graphs. In addition, the statistical analysis of the proton transfer along hydrogen-bonded chains was carried out.

Configurational Entropy in Ice Nanosystems: Tools for Structure Generation and Screening

Recently, a number of experimental and theoretical studies of low-temperature ice and water in nanoscale systems have emerged. Any theoretical study trying to model such systems will encounter the proton-disorder problem, i.e., there exist many configurations differing by water-molecule rotations for a fixed oxygen atom structure. An extensive search within the allowed proton-disorder space should always be perfomed to ensure a reasonable low-energy isomer and to address the effect of proton-configurational entropy that may affect experimental observables. In the present work, an efficient general-purpose program for finite, semiperiodic, and periodic systems of hydrogen-bonded molecules is presented, which can be used in searching and enumerating the proton-configurational ensemble. Benchmarking tests are performed for ice nanotubes and finite slabs. Finally, the program is applied to experimentally appropriate ice nanosystems. A boron nitride film supported ice nanodot is studied in detail. Using a systematic generation of its proton-configurational ensemble, we find an isomer that is ∼1 eV lower in total energy than one previously studied. The present isomer features a considerable dipole moment and implies that ice nanodots are inherently ferroelectric parallel to the surface. We conclude by demonstrating how the so-called hydrogen-bond connectivity parameters can be used to screen low-energy isomers.