Global Minima of Protonated Water Clusters (H2O)20H+ Revisited

Characterizing the local solvation environment of OH(-) in water clusters with AIMD

In this work, we use ab initio molecular dynamics coupled with metadynamics to explore and characterize the glassy potential energy landscape of the OH(-) in a 20 and 48 water cluster. The structural, energetic, and topological properties of OH(-) are characterized for both clusters and the molecular origins of the IR signatures are examined. We find that in both the small and large clusters, the OH(-) can donate or accept a varying number of hydrogen bonds confirming that the amphiphilic character does not depend on cluster size. However, we highlight some important differences found between the energetic and topological properties of both families of clusters which may have implications on understanding the changes in the solvation structure of OH(-) between bulk and interfacial environments. By studying the IR spectra of smaller subsets of molecules within the 20 water molecule cluster, we find that the IR spectrum of the bare OH(-) as well as the water molecule donating a strong hydrogen bond to it exhibits characteristic absorption along the amphiphilic band between 1500 and 3000 cm(-1) at positions very similar to those found for the entire hydroxide cluster. The results presented here will be useful in the calibration and improvement of both ab initio and semi-empirical methods to model this complex anion.

Diphenylalanine self assembly: novel ion mobility methods showing the essential role of water

The mechanism and driving forces behind the formation of diphenylalanine (FF) nanotubes have attracted much attention in the past decades. The hollow structure of the nanotubes suggests a role for water during the self-assembly process. Here, we use novel ion-mobility mass spectrometry methods to probe the early oligomers formed by diphenylalanine peptides. Interestingly, water-bound oligomers are observed in nano-electrospray ionization (ESI) mass spectra in the absence of bulk solvent. In addition, ligated water clusters transit the ion mobility cell but (often) dissociate before detection. These water molecules are shown to be essential for the formation of diphenylalanine oligomers larger than the dimer. The ligated water molecules exist in the solvent free environment either as neutral water or as protonated water clusters, depending on the composition of solvent from which they are sprayed. Water adduction helps stabilize conformers that are otherwise energetically unstable ultimately leading to the assembly of FF nanotubes.

Unveiling the Janus-Like Properties of OH(-)

Using ab initio simulations, we explore the glassy landscape of the OH(-)(H2O)20 cluster and its infrared spectrum. We show that the OH(-) has an amphiphilic Janus-type behavior like the hydronium ion induced by the ability of its O-H bond to be buried inside of the cluster or exposed at the surface with different coordination numbers. Recent infrared experiments of aqueous NaOH have found two pronounced peaks at 2000 and 2850 cm(-1) [Mandal, A.; J. Chem. Phys. 2014, 140, 1-12]. The microscopic origins of these spectral features remain elusive. Herein, we disentangle the contribution of the spectra between 1700 and 3000 cm(-1) in terms of the microscopic solvation structure of OH(-) and dub this as the amphiphilic band. The delocalized nature of OH(-) results in a red shift to the O-H stretch, which mixes with bend-vibrations, the extent to which is tuned by the local coordination number. These results have important bearing on understanding the spectroscopic signatures of OH(-) in environments like the air-water interface.

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.