Pablo E. Videla

Nuclear quantum effects on the structure and the dynamics of [H2O]8 at low temperatures

We use ring-polymer-molecular-dynamics (RPMD) techniques and the semi-empirical q-TIP4P/F water model to investigate the relationship between hydrogen bond connectivity and the characteristics of nuclear position fluctuations, including explicit incorporation of quantum effects, for the energetically low lying isomers of the prototype cluster [H2O]8 at T = 50 K and at 150 K. Our results reveal that tunneling and zero-point energy effects lead to sensible increments in the magnitudes of the fluctuations of intra and intermolecular distances. The degree of proton spatial delocalization is found to map logically with the hydrogen-bond connectivity pattern of the cluster. Dangling hydrogen bonds exhibit the largest extent of spatial delocalization and participate in shorter intramolecular O-H bonds. Combined effects from quantum and polarization fluctuations on the resulting individual dipole moments are also examined. From the dynamical side, we analyze the characteristics of the infrared absorption spectrum. The incorporation of nuclear quantum fluctuations promotes red shifts and sensible broadening relative to the classical profile, bringing the simulation results in much more satisfactory agreement with direct experimental information in the mid and high frequency range of the stretching band. While RPMD predictions overestimate the peak position of the low frequency shoulder, the overall agreement with that reported using an accurate, parameterized, many-body potential is reasonable, and far superior to that one obtains by implementing a partially adiabatic centroid molecular dynamics approach. Quantum effects on the collective dynamics, as reported by instantaneous normal modes, are also discussed.

Surface Isotope Segregation as a Probe of Temperature in Water Nanoclusters.

Using ring polymer molecular dynamics simulations, we examine equilibrium and dynamical characteristics of solid-like, aqueous clusters that combine isotopic mixtures of HDO dilute in H2O, at temperatures intermediate between 50 and 175 K. In particular, we focus attention on the relative thermodynamic stabilities of the two isotopes at dangling hydrogen bond sites. The water octamer is analyzed as a reference system. For this aggregate, decreasing temperature yields a gradual stabilization of the light isotope at dangling sites in molecules acting as single-donor-double-acceptors of hydrogen bonds. At T ∼ 50 K, the imbalance between the corresponding quantum kinetic energies leads to a free energy difference between dangling and hydrogen bonded sites of the order of ∼2kBT. Similar free energy differences were found at dangling sites in Nw = 50 water clusters. The extent of the H/D segregation can be adequately monitored by modifications in the peak intensity of the high frequency shoulder of the stretching band of the infrared spectrum. These signals, in turn, represent a potential experimental signature of the elusive temperature of clusters in molecular beams.

A quantum molecular dynamics study of aqueous solvation dynamics

Ring polymer molecular dynamics experiments have been carried out to examine effects derived from nuclear quantum fluctuations at ambient conditions on equilibrium and non-equilibrium dynamical characteristics of charge solvation by a popular simple, rigid, water model, SPC/E , and for a more recent, and flexible, q-TIP4P/F model, to examine the generality of conclusions. In particular, we have recorded the relaxation of the solvent energy gap following instantaneous, ±e charge jumps in an initially uncharged Lennard-Jones-like solute. In both charge cases, quantum effects are reflected in sharper decays at the initial stages of the relaxation, which produce up to a ∼20% reduction in the characteristic timescales describing the solvation processes. For anionic solvation, the magnitude of polarization fluctuations controlling the extent of the water proton localization in the first solvation shell is somewhat more marked than for cations, bringing the quantum solvation process closer to the classical case. Effects on the solvation response from the explicit incorporation of flexibility in the water Hamiltonian are also examined. Predictions from linear response theories for the overall relaxation profile and for the corresponding characteristic timescales are reasonably accurate for the solvation of cations, whereas we find that they are much less satisfactory for the anionic case.

Isotopic Preferential Solvation of I(-) in Low-Temperature Water Nanoclusters.

We present results from ring polymer molecular dynamics experiments that provide microscopic insights into the characteristics of the isotopic stabilizations of H and D aqueous species in the first solvation shell of a halide I(-) ion in water nanoclusters at low temperatures. The analysis of the simplest I(-)·(HOD) dimer shows a clear propensity for the light isotope to lie at the non-hydrogen-bonded dangling position. Our results predict that, at T ∼ 50 K, I(-)·(DOH) isomers are three times more abundant than I(-)·(HOD) ones. The reasons for such stabilization can be traced back to differences in the nuclear kinetic energy projected along directions perpendicular to the plane of the water molecule. Dynamical implications of these imbalances are shown to be reflected in the characteristics of the corresponding bands of the infrared spectroscopic signals. A similar analysis performed in larger aggregates containing ∼20 water molecules reveals, in contrast, a stabilization of the light isotope along I(-)···HO hydrogen bonds. Effects derived from the consideration of smaller halide anions with larger electric fields at the surface are also examined.

Effects of aligned α‐helix peptide dipoles on experimental electrostatic potentials

Aligned protein α‐helix dipoles have been implicated in protein function and structure. The recent breakthroughs in high‐resolution electron microscopy (EM) of macromolecules makes it possible to explore fundamental aspects of structural biology at the detailed molecular level. The electrostatic potential (ESP) generated by aligned protein α‐helix dipole should be observable in high‐resolution EM maps despite the fact that the effect may be partially screened by induced electric fields. Here, we show that aligned backbone dipoles in protein α‐helices account for long‐range features in the protein ESP functions. Our results are consistent with experimental EM maps and density functional theory calculations, including direct Fourier summation for proper calculation of the ESP due to the nonlocal nature of the ESP function from aligned dipoles and other partial atomic charges.

Positional Isotope Exchange in HX·(H2O)n (X = F, I) Clusters at Low Temperatures

We present molecular dynamics simulation results describing proton/deuteron exchange equilibria along hydrogen bonds at the vicinity of HX acids (X = F, I) in aqueous clusters at low temperatures. To allow for an adequate description of proton transfer processes, our simulation scheme resorted on the implementation of a multistate empirical valence bond hamiltonian coupled to a path integral scheme to account for effects derived from nuclear quantum fluctuations. We focused attention on clusters comprising a number of water molecules close to the threshold values necessary to stabilize contact-ion-pairs. For X = F, our results reveal a clear propensity of the heavy isotope to lie at the bond bridging the halide to the nearest water molecule. Contrasting, for X = I, the thermodynamic stability is reversed and the former connectivity is preferentially articulated via the light isotope. These trends remain valid for undissociated and ionic descriptions of the stable valence bond states. The preferences are rationalized in terms of differences in the quantum kinetic energies of the isotopes which, in turn, reflect the extent of the local spatial confinements prevailing along the different hydrogen bonds in the clusters. In most cases, these features are also clearly reflected in the characteristics of the corresponding stretching bands of the simulated infrared spectra. This opens interesting possibilities to gauge the extent of the isotopic thermodynamic stabilizations and the strengths of the different hydrogen bonds by following the magnitudes and shifts of the spectral signals in temperature-controlled experiments, performed on mixed clusters combining H2O and HOD.

Inclusion of nuclear quantum effects for simulations of nonlinear spectroscopy

The computation and interpretation of nonlinear vibrational spectroscopy is of vital importance for understanding a wide range of dynamical processes in molecular systems. Here, we introduce an approach to evaluate multi-time response functions in terms of multi-time double symmetrized Kubo transformed thermal correlation functions. Furthermore, we introduce a multi-time extension of ring polymer molecular dynamics to evaluate these Kubo transforms. Benchmark calculations show that the approximations are useful for short times even for nonlinear operators, providing a consistent improvement over classical simulations of multi-time correlation functions. The introduced methodology thus provides a practical way of including nuclear quantum effects in multi-time response functions of non-linear optical spectroscopy.