Equilibrium and Dynamical Characteristics of Hydrogen Bond Bifurcations in Water–Water and Water–Ammonia Dimers: A Path Integral Molecular Dynamics Study

Abstract

We present path integral molecular dynamics results that describe the effects of nuclear quantum fluctuations on equilibrium and dynamical characteristics pertaining to bifurcation pathways in hydrogen bonded dimers combining water and ammonia, at cryogenic temperatures of the order of 20 K. Along these isomerizations, the hydrogen atoms in the molecules acting as hydrogen-bond donors interchange their original dangling/connective characters. Our results reveal that the resulting quantum transition paths comprise three stages: the initial and final ones involve overall rotations during which the two protons retain their classical-like characteristics. Effects from quantum fluctuation are clearly manifested in the changes operated at the intermediate passages over transition states, as the spatial extents of the protons stretch over typical lengths comparable to the distances between connective and dangling basins of attractions. Consequently, the classical over-the-hill path is replaced by a tunneling controlled mechanism which, within the path integral perspective, can be cast in terms of concerted inter-basin migrations of polymer beads from dangling-to-connective and from connective-to-dangling, at practically no energy costs. We also estimated the characteristic timescales describing such interconversions within the approximate ring polymer rate theory. Effects derived from full and partial deuteration are also discussed.

Collective Proton Transfers in Cyclic Water–Ammonia Tetramers: A Path Integral Machine-Learning Study

“…The number of beads was set to P = 128; this number is similar to the ones adopted in several previous studies. 30,33,34,47,48 Forces and energies were computed by implementing an NN fitting algorithm. In the present context, this strategy presents two important advantages: First, it provides estimates with ab initio quality at computer costs much more modest than those obtained from alternative schemes that require onthe-fly electronic structure computations.…”

“…In order to get satisfactory sampling, each component of each normal mode was coupled to a chain of three Nosé–Hoover thermostats, , with mass parameters set at Q k = k normalB T / ω k 2 , where ω k represents the frequency of the k th normal mode. , Adequate convergence of the different average values typically required harvesting statistics along ∼0.5–1 ns long trajectories. The number of beads was set to P = 128; this number is similar to the ones adopted in several previous studies. ,,,, …”

“…Similarly to what we implemented in previous analyses, 34,47,48 our strategy to collect statistics along the complete interval of relevant values of the order parameter joining reactive and product states in eq 3 resorted to standard umbrella sampling techniques. 62 For the sake of conciseness, full details of our implementation can be found in the Supporting Information.…”

“…The latter element represents a clear fingerprint of deep-tunneling regimes. 34,47,48,70 Incidentally, we remark that appropriate sampling at the vicinities of the sharp transitions between nontunneling and tunneling controlled transfers required a careful procedure in which the umbrella windows were moved in opposite directions, i.e., from reactant to transition states and vice versa; additional details can be found in the Supporting Information.…”

Electrostatics in Computational Biophysics and Its Implications for Disease Effects

“…Thus, recent work focused on the implications of nuclear quantum fluctuations on equilibrium and dynamical properties relating to bifurcation routes in hydrogen-bond dimers of water and ammonia. It was shown that the classical over-the-hill approach is substituted with a tunneling-controlled mechanism that, from the perspective of the path integral, can be modeled as coordinated inter-basin migrations of polymer beads [ 73 ]. The application of quantum mechanical approaches was further extended to probe the development of neighboring hydrogen bonds with water molecules.…”