Session A26: Chemical Physics of Hydrogen Bonding I

Unravelling the structure and dynamics of concentrated aqueous proton defects using simulations incorporating both nuclear and electronic quantum effects.

Aqueous proton defects give rise to a range of structural and dynamical environments that vary with concentration. These manifest as a continuum of infra-red and Raman spectral features. However, assigning spectral features to the underlying structures formed in solution and their dynamical interconversion remains an area of significant debate. In this talk I will show how path integral ab initio molecular dynamics simulations, where the electronic structure is computed on the fly using density functional theory and nuclear quantum effects are included explicitly via path integral molecular dynamics, can be used to accurately describe the spectroscopic properties of liquid water and systems with aqueous proton defects. These simulations, which have previously been computationally intractable for such large condensed phase systems, are now possible due to our recent path integral developments. I will discuss how these simulations can be used to elucidate the linear and multidimensional spectroscopy of concentrated acid systems and the dynamics and structures that give rise to them.   

Dielectric Spectroscopy of Water: From Collective Relaxation to Quantum Effects

Despite many decades of studies, understanding and modelling dynamics of bulk and confined water still remains a great challenge. We present an overview of dielectric spectroscopy studies of water in a broad temperature range, from ambient T down to Tg\textasciitilde 136K. We demonstrate [1] that the main dielectric relaxation process of water at ambient T is a collective relaxation similar to the so-called Debye process known for many mono-alcohols. The structural relaxation of water actually appears at much higher frequency. Combining neutron scattering and dielectric relaxation spectroscopy we show that quantum fluctuations play a critical role in dynamics of deeply supercooled bulk water [2,3]. Water is the lightest molecule existing in a liquid state at ambient conditions. This strongly increases probability of quantum effects and we suggest that quantum tunneling might be the origin of water's unusual low temperature behavior [2,3]. The discovered anomalously large isotope effect in Tg of water [2] is consistent with the quantum tunneling dominating structural relaxation of water at these temperatures. Based on these results we suggest that the apparent Fragile-to-Strong Crossover in water dynamics can be ascribed to crossover from classical over-barrier relaxation to tunneling [3]. At the end we emphasize that neglecting quantum effects in simulations might be the main reason of their failure in the case of water. \\ 1. J. S. Hansen, et al.$,$ \textbf{Phys. Rev. Letters 116}, 237601 (2016). \\ 2. C. Gainaru, et al$, $\textbf{PNAS 111}, N49, 17402 (2014). \\ 3. A. L. Agapov, et al., \textbf{Phys. Rev. E 91}, 022312 (2015).   

Theoretical and Computational Studies of the IR Spectra of Small Water and Protonated Water Clusters

Recent spectroscopic measurements of IR spectra of water and protonated water clusters, from the from the Havenith and Johnson groups, respectively, present major challenges to theory. These include rigorous calculations of these spectra, using high-level ab initio potential and dipole moment surface and simple models to provide insights into both the experiments and the calculations. I will present our progress on both accounts, focusing on intermolecular and bending vibrational of the water clusters and the complex proton stretch modes in protonated water clusters. For the latter, the central role of the Zundel potential will be emphasized.   

Effects of the hydrogen-bonded network on the molecular polarizability of water

Electronic polarization plays a crucial role in determining the structural and dynamical properties of water with different boundary conditions. Although it is well known that the molecular polarization in condensed phases behaves substantially differently from that in the vacuum due to the intermolecular interaction, the environmental effects have not been fully understood from first principles methods. As a result, how to rigorously define and calculate the molecular polarizability of a water molecule in different chemical environments remains an open question. A main challenge to this puzzle arises from the intrinsic non-local nature of the electronic susceptibility. Recently we developed an ab initio local dielectric response theory [Phys. Rev. B 92, 241107, 2015] that partitions dielectric response in real space based on a Wannier representation. We apply this method to compute the molecular polarizability of water in the condensed phase, and analyze the effects of the hydrogen-bonded network resulting from the crystal field, spatial confinement, and charge transfer.   

The Molecular Structural and Electronic Properties of Liquid Water by {\it ab initio} Molecular Dynamics Simulation Based on SCAN Density Functional

We performed the {\it ab initio} molecular dynamics (AIMD) simulation of liquid water based on the recently developed SCAN meta-GGA functional. It is well-known that bulk water simulation at GGA-level DFT has several drawbacks including over-structured H-bond, and slow diffusivity etc. With SCAN functional, the intermediate range many-body van der Waals effect is captured with a more accurate description of covalent bond at the same time. Based on the above improved functional description, we found that the liquid water structure is also improved by a less over-structured H-bond structure and faster diffusive property towards the experimental direction. At the same time, the electronic property including the dipole moment and band gaps are also improved compared to GGA DFT. Our work shows that SCAN is a promising candidate for the {\it ab initio} simulation of liquid water in the future.   

Quantum nature of protons in water probed by scanning tunneling microscopy and spectroscopy

The complexity of hydrogen-bonding interaction largely arises from the quantum nature of light hydrogen nuclei, which has remained elusive for decades. Here we report the direct assessment of nuclear quantum effects on the strength of a single hydrogen bond formed at a water-salt interface, using tip-enhanced inelastic electron tunneling spectroscopy (IETS) based on a low-temperature scanning tunneling microscope (STM) [1]. The IETS signals are resonantly enhanced by “gating” the frontier orbitals of water via a chlorine-terminated STM tip, such that the hydrogen-bonding strength can be determined with unprecedentedly high accuracy from the redshift in the O-H stretching frequency of water. Isotopic substitution experiments combined with quantum simulations reveal that the anharmonic quantum fluctuations of hydrogen nuclei weaken the weak hydrogen bonds and strengthen the relatively strong ones. However, this trend can be completely reversed when the hydrogen bond is strongly coupled to the polar atomic sites of the surface. References: [1] J. Guo \textit{et al}. Science. 352£¬321 (2016).   

Why does hydronium diffuse much faster than hydroxide in liquid water?

Proton transfer, by solvated hydronium and hydroxide in water, is a fundamental process behind numerous acid-base chemical reactions. Over centuries, Grotthuss mechanism has successfully drawn our attention to the molecular structural changes during proton transfer. Yet, the faster diffusion of hydronium than that of hydroxide has not been fully clarified. We show that an accurate prediction of hydrogen-bond network in liquid water, which includes van der Waals interaction and hybrid functional, is essential to address this issue. Our ab initio molecular dynamics presents that proton transfer by hydronium is consistent with the recently proposed picture of concerted proton transfer. Furthermore, the hypercoordinated structure of hydroxide is significantly favored in the simulation with van der Waals interaction and hybrid functional. The difference in the correlated motions between these two ions naturally ensures the faster migration of hydronium than hydroxide in water.   

Discovering local order parameters in liquid water using machine learning

The local arrangement of water molecules in liquid phase is still being discussed and questioned. The prevailing view is that water is composed of a mixture of two structurally different liquids. One of the main challenges has been to find order parameters that are able to discriminate the complex structures of these distinct molecular environments. Several local order parameters have been proposed and studied in all sorts of atomistic simulations of liquid water but, to date, none has been able to capture the predicted dual character. This presents an ideal problem to treat with methods capable of unveiling information from complex data. In this talk we will discuss how local order parameters can be constructed from molecular dynamics trajectories by using machine learning and other related techniques.   

Conformation-Specific Infrared and Ultraviolet Spectroscopy of Cold [YAPAA$+$H]$^{\mathrm{+}}$ and [YGPAA$+$H]$^{\mathrm{+}}$ Ions

Incorporation of the unnatural D-proline stereoisomer into a peptide sequence is a typical strategy to synthesize model $\beta $-hairpin loops. Using conformation-specific IR and UV spectroscopy of cold ($\approx $ 10 K) gas-phase ions, we unravel the inherent conformational preferences of the $^{\mathrm{D}}$P and $^{\mathrm{L}}$P diastereomers in the protonated peptide [YAPAA$+$H]$^{\mathrm{+}}$ because only intramolecular interactions are possible in this isolated regime. Consistent with the solution phase studies, one of the conformers of [YA$^{\mathrm{D}}$PAA$+$H]$^{\mathrm{+}}$ is folded into a $\beta $-hairpin turn. However, a second predominant $\gamma $-turn conformer family is identified. The [YA$^{\mathrm{L}}$PAA$+$H]$^{\mathrm{+}}$ stereoisomer discourages $\beta $-hairpin formation. We show that the \textit{trans }($^{\mathrm{D}}$P)$\to $\textit{cis} ($^{\mathrm{L}}$P) isomerization is sterically driven and can be reversed by substituting [YG$^{\mathrm{L}}$PAA$+$H]$^{\mathrm{+}}$ for [YA$^{\mathrm{L}}$PAA$+$H]$^{\mathrm{+}}$. Therefore, we provide a basis for understanding residue-specific alterations in the potential energy surface and reveal new insights into the origin of $\beta $-hairpin formation from the bottom-up.