Molecular Mechanisms of Water-Mediated Proton Transport in MIL-53 Metal–Organic Frameworks

Metal–organic frameworks have recently been proposed as promising proton conducting materials for application in fuel cell technologies. Here, molecular dynamics simulations are used to reveal the microscopic mechanisms associated with water-mediated proton transport in the MIL-53 materials as a function of temperature, water loading, and pore size. The structure of the hydrated proton is found to resemble that of a distorted Zundel complex when the framework closes into a narrow-pore configuration. A transition to Eigen-like structures is then observed at higher water loading when the pores open as a result of the breathing effect. Although the free-energy barriers to proton transfer at room temperature are lower than in bulk water, proton transport in MIL-53 is largely suppressed, which is attributed to the low water mobility inside the pores. Faster proton diffusion is found at higher temperature, in agreement with conductivity measurements

Theoretical Prediction of Spin-Crossover Temperatures in Ligand-Driven Light-Induced Spin Change Systems

Spin-crossover compounds exhibit two alternative spin states with distinctive chemical and physical properties, a particular feature that makes them promising materials for nanotechnological applications as memory or display devices. A key parameter that characterizes these compounds is the spin-crossover temperature, <i>T</i>_1/2, defined as the temperature with equal populations of high and low-spin species. In this study, a theoretical/computational approach is described for the calculation of <i>T</i>_1/2 for the <i>trans</i>-[Fe­(styrylpyridine)₄(NCX)₂] (X = S, Se, and BH₃, styrylpyridine in the <i>trans</i> configuration) ligand driven light-induced spin change (LD-LISC) complexes. In all cases, the present calculations provide an accurate description of both structural and electronic properties of the LD-LISC complexes and, importantly, predict spin-crossover temperatures in good agreement with the corresponding experimental data. Fundamental insights into the dependence of <i>T</i>_1/2 on the nature of the axial ligands are obtained from the direct analysis of the underlying electronic structure in terms of the relevant molecular orbitals

Systematic Study of Structural and Thermodynamic Properties of HCl(H₂O)_<i>n</i> Clusters from Semiempirical Replica Exchange Simulations

The structural and thermodynamic properties of HCl­(H₂O)_<i>n</i> clusters with <i>n</i> = 4–10 are studied using Born–Oppenheimer replica exchange molecular dynamics simulations with the PM3-MAIS semiempirical Hamiltonian. Independently of the cluster size, the simulations predict that HCl exists in the dissociated form in all low-energy isomers. Different local structures are identified within the clusters due to the presence of the dissociated proton, including Zundel, Eigen, Eigen-like, H₇O₃⁺, and intermediate Zundel–Eigen configurations. As the cluster size increases, several groups of isomers are identified, whose relative stabilities vary as a function of temperature. A detailed analysis of the heat capacity indicates that the melting behavior of HCl­(H₂O)_<i>n</i> clusters is strongly size-dependent. In particular, melting is observed in clusters with <i>n</i> = 7–10 in the temperature range <i>T</i> = 100–150 K. By contrast, melting is not observed in clusters with <i>n</i> = 4–6. Minimum energy structures for HCl­(H₂O)_<i>n</i> clusters with <i>n</i> = 11–15 and <i>n</i> = 21 are also characterized

Assessing Many-Body Effects of Water Self-Ions. I: OH^–(H₂O)_<i>n</i> Clusters

The importance of many-body effects in the hydration of the hydroxide ion (OH^–) is investigated through a systematic analysis of the many-body expansion of the interaction energy carried out at the CCSD­(T) level of theory, extrapolated to the complete basis set limit, for the low-lying isomers of OH^–(H₂O)_<i>n</i> clusters, with <i>n</i> = 1–5. This is accomplished by partitioning individual fragments extracted from the whole clusters into “groups” that are classified by both the number of OH^– and water molecules and the hydrogen bonding connectivity within each fragment. With the aid of the absolutely localized molecular orbital energy decomposition analysis (ALMO-EDA) method, this structure-based partitioning is found to largely correlate with the character of different many-body interactions, such as cooperative and anticooperative hydrogen bonding, within each fragment. This analysis emphasizes the importance of a many-body representation of inductive electrostatics and charge transfer in modeling OH^– hydration. Furthermore, the rapid convergence of the many-body expansion of the interaction energy also suggests a rigorous path for the development of analytical potential energy functions capable of describing individual OH^––water many-body terms, with chemical accuracy. Finally, a comparison between the reference CCSD­(T) many-body interaction terms with the corresponding values obtained with various exchange-correlation functionals demonstrates that range-separated, dispersion-corrected, hybrid functionals exhibit the highest accuracy, while GGA functionals, with or without dispersion corrections, are inadequate to describe OH^––water interactions

Guest-Dependent Stabilization of the Low-Spin State in Spin-Crossover Metal-Organic Frameworks

Computer simulations are carried out to characterize the variation of spin-crossover (SCO) behavior of the prototypical {Fe­(pz)­[Pt­(CN)₄]} metal-organic framework (MOF) upon adsorption of chemically and structurally different guest molecules. A detailed analysis of both strength and anisotropy of guest molecule–framework interactions reveals direct correlations between the mobility of the guest molecules inside the MOF pores, the rotational mobility of the pyrazine rings of the framework, and the stabilization of the low-spin state of the material. On the basis of these correlations, precise molecular criteria are established for predicting the spin state of {Fe­(pz)­[Pt­(CN)₄]} upon guest adsorption. Finally, predictions of the SCO temperature upon adsorption of various toxic gases demonstrate that in silico modeling can provide fundamental insights and design principles for the development of spin-crossover MOFs for applications in gas detection and chemical sensing

Molecular-Level Characterization of the Breathing Behavior of the Jungle-Gym-type DMOF-1 Metal–Organic Framework

Fundamental insights into the molecular mechanisms that determine the breathing behavior of the jungle-gym-type DMOF-1 metal–organic framework upon adsorption of benzene and isopropyl alcohol are gained from computer simulations. In all cases, good agreement is obtained between the calculated and experimental structural parameters. In the case of benzene adsorption, DMOF-1 is predicted to exist in a narrow pore configuration at high loadings and/or low temperature. A structural transition into a large pore configuration is then observed as the temperature increases and/or the loading decreases, which is directly related to the spatial distribution and molecular interactions of the benzene molecules within the pores. The isopropyl alcohol adsorption simulations indicate that DMOF-1 undergoes two distinct structural transitions (from large pore to narrow pore and then back to large pore) as the number of adsorbed molecules increases, which is explained in terms of the formation of hydrogen bonds between the isopropyl molecules and the framework

Elucidating the Competitive Adsorption of H₂O and CO₂ in CALF-20: New Insights for Enhanced Carbon Capture Metal–Organic Frameworks

In light of the pressing need for efficient carbon capture solutions, our study investigates the simultaneous adsorption of water (H2O) and carbon dioxide (CO2) as a function of relative humidity in CALF-20, a highly scalable and stable metal–organic framework (MOF). Advanced computer simulations reveal that due to their similar interactions with the framework, H2O and CO2 molecules compete for the same binding sites, occupying similar void regions within the CALF-20 pores. This competition results in distinct thermodynamic and dynamical behaviors of H2O and CO2 molecules, depending on whether one or both guest species are present. Notably, the presence of CO2 molecules forces the H2O molecules to form more connected hydrogen-bond networks within smaller regions, slowing water reorientation dynamics and decreasing water entropy. Conversely, the presence of water speeds up the reorientation of CO2 molecules, decreases the CO2 entropy, and increases the propensity for CO2 to be adsorbed within the framework due to stronger water-mediated interactions. Due to the competition for the same void spaces, both H2O and CO2 molecules exhibit slower diffusion when molecules of the other guest species are present. These findings offer valuable strategies and insights into enhancing the differential affinity of H2O and CO2 for MOFs specifically designed for carbon capture applications

Spin Crossover in the {Fe(pz)[Pt(CN)₄]} Metal–Organic Framework upon Pyrazine Adsorption

The spin-crossover behavior of the {Fe­(pz)­[Pt­(CN)₄]} metal–organic framework (MOF) upon pyrazine adsorption is investigated through hybrid Monte Carlo/molecular dynamics (MC/MD) simulations. In contrast to previous theoretical studies, which reported a transition temperature of ∼140 K, the present MC/MD simulations predict that the high-spin state is the most stable state at all temperatures, in agreement with the experimental observations. The MC/MD simulations also indicate that the pyrazine molecules adsorbed in the MOF pores lie nearly parallel but staggered by 60° relative to the pyrazine ligands of the framework. The analysis of the magnetization curve as a function of the temperature demonstrates that the staggered configuration assumed by the guest pyrazine molecules within the framework is responsible for the stabilization of the high-spin state. Both the guest pyrazine molecules and the pyrazine ligands of the framework are effectively locked into the minimum-energy configuration and do not display any rotational mobility

Water Dynamics in Metal–Organic Frameworks: Effects of Heterogeneous Confinement Predicted by Computational Spectroscopy

The behavior of water confined in MIL-53­(Cr), a flexible metal–organic framework (MOF), is investigated through computational infrared spectroscopy. As the number of molecules adsorbed inside of the pores increases, the water OH stretch band of the linear infrared spectrum grows in intensity and approaches that of bulk water. To assess whether the water confined in MIL-53­(Cr) becomes liquid-like, two-dimensional infrared spectra (2DIR) are also calculated. Confinement effects result in distinct chemical environments that appear as specific features in the 2DIR spectra. The evolution of the 2DIR line shape as a function of waiting time is well described in terms of the orientational dynamics of the water molecules, with chemical exchange cross peaks appearing at a time scale similar to the hydrogen bond rearrangement lifetime. The confining environment considerably slows the hydrogen bond dynamics relative to bulk as a result of the competition between water–framework and water–water interactions

Infrared and Raman Spectroscopy of Liquid Water through “First-Principles” Many-Body Molecular Dynamics

Vibrational spectroscopy is a powerful technique to probe the structure and dynamics of water. However, deriving an unambiguous molecular-level interpretation of the experimental spectral features remains a challenge due to the complexity of the underlying hydrogen-bonding network. In this contribution, we present an integrated theoretical and computational framework (named many-body molecular dynamics or MB-MD) that, by systematically removing uncertainties associated with existing approaches, enables a rigorous modeling of vibrational spectra of water from quantum dynamical simulations. Specifically, we extend approaches used to model the many-body expansion of interaction energies to develop many-body representations of the dipole moment and polarizability of water. The combination of these “first-principles” representations with centroid molecular dynamics simulations enables the simulation of infrared and Raman spectra of liquid water under ambient conditions that, without relying on any <i>ad hoc</i> parameters, are in good agreement with the corresponding experimental results. Importantly, since the many-body energy, dipole, and polarizability surfaces employed in the simulations are derived independently from accurate fits to correlated electronic structure data, MB-MD allows for a systematic analysis of the calculated spectra in terms of both electronic and dynamical contributions. The present analysis suggests that, while MB-MD correctly reproduces both the shifts and the shapes of the main spectroscopic features, an improved description of quantum dynamical effects possibly combined with a dissociable water potential may be necessary for a quantitative representation of the OH stretch band