Understanding structure-property relationships is foundational to numerous modern chemistries, such as proton-coupled electron transfer (PCET). However, an experimentally measured property is the result of the behavior from an ensemble of molecules. Neglecting ensemble effects, especially under complex chemical environments, may obfuscate these relationships and lead to discrepancies between theory and experiment. In this work, we demonstrate the impact of configurational entropy and local chemical environments on hydroxide bond dissociation free energies [BDFE(O-H)] for a set of polyoxovanadate nanoclusters, at ambient conditions. The O-H bond strengths are investigated via density functional theory (DFT) coupled with statistical thermodynamic analysis and bilinear modeling, and compared with previous experimental results on the same systems, namely electrochemical solutions of: [V6O13-x(OH)x(TRIOLR)2]-2 (x = 2, 4, 6; R = NO2, Me) and [V6O11-x(OMe)2(OH)x(TRIOLNO2)2]-2 (x = 2, 4). Interestingly, we find that ensemble effects, even at room temperature, can account for a significant portion of the BDFE(O-H) trend with the degree of reduction via H atom binding, which cannot be fully captured by single-structure, static DFT calculations. Moreover, we find that the ensemble effects may be replicated statistically, requiring only enumeration of energetically accessible H-binding sites. With the ensemble effects resolved, we present a simple bilinear model to reconcile remaining biases between experiment and ensemble-informed theory, which corelate with cluster-specific electronic environment differences. The bilinear model achieves outstanding accuracy vs experiments with a root-mean squared error of 0.4 kcal/mol. Finally, based on the physicochemical characteristics of hydrogen interaction with polyoxometalates, we present a simple methodology that captures the BDFE(O-H) trend while dramatically reducing required DFT calculations by 98% and achieving accuracy within 1 kcal/mol. Overall, this work elucidates the roles and structural origins of configurational entropy and chemical effects on polyoxometalate hydroxide bond energies, with potential applicability to various atomically precise metal oxide systems. Importantly, it introduces models for rapid and highly accurate property calculations in connection with experiments.
Protein cavity and surface hydration play critical roles in determining a protein's structure, flexibility, dynamics, and overall function. Yet, uncovering the precise relationships among these factors has remained challenging. To address this, atomistic molecular dynamics (MD) simulations were performed on aqueous systems containing rat liver fatty acid-binding protein (rLFABP)─a β-barrel protein─both in its apo or ligand-free and holo or oleate ligand bound forms at room temperature. The dynamic properties of water at the exterior surface of the protein as against those that are confined within the protein internal core were investigated. This study focused on elucidating how structural fluctuations of the protein (in both apo and holo states) and the bound oleate ligands impact the diffusivity and hydrogen-bonding characteristics of these separate water ensembles. A primary finding was the emergence of pronounced spatial heterogeneity and retarded dynamics of water molecules within the protein's internal cavity, contrasting sharply with the comparatively uniform solvent dynamics at the exterior surface. The structural reorganization of the β-barrel cavity in the holo form was notably correlated with a dynamical transition in the trapped water population. It has been demonstrated that the restricted mobility of core water arises directly from alterations in the kinetics of hydrogen bond formation and dissociation, reflecting a restructured hydrogen-bond network within the confined core volume. Importantly, our findings highlight heterogeneous dynamical behavior of interfacial water across different surface regions of the protein, thus emphasizing intricate coupling between protein structural transitions and local hydration dynamics.
Aerosol particles that catalyze ice nucleation alter the optical properties and precipitation cycles of clouds. Although mineral dust aerosol particles containing metal oxides are susceptible to the formation of oxygen vacancies (VO) on their surfaces, the impact of these defects on ice nucleation activity has not been addressed. To investigate the impact of VO sites, we conducted a droplet immersion freezing assay on zinc aluminate (ZnAl2O4) and magnesium aluminate (MgAl2O4) spinels annealed under air, nitrogen, and oxygen atmospheres. We observe that samples annealed under nitrogen promote ice nucleation at warmer temperatures compared to those treated in oxidizing atmospheres, with the effect being most pronounced for ZnAl2O4. To further understand these results, we investigated the immersion freezing of zinc oxide (ZnO) and magnesium oxide (MgO). Here, we observe that ZnO nucleates ice at substantially warmer temperatures than MgO after annealing under nitrogen. We hypothesize that the trends in ice nucleation activity are due to the varying concentrations of VO that form during the annealing process on the oxide surfaces, which tend to be higher in the absence of O2. Density functional theory (DFT) calculations support our hypothesis, indicating that VO is more stable on the surfaces of the Zn-containing oxides. The study suggests that oxygen vacancies, which are common defects on metal oxide surfaces that affect their adsorption and catalytic properties, can influence the efficiency with which mineral dust aerosol particles activate ice formation and affect cloud radiative forcing.
Reactive uptake of methylglyoxal (Mgly) on aerosol particles is an important source of secondary organic aerosol (SOA), yet its significance remains highly uncertain due to the poorly constrained uptake coefficients (γMgly). Here, we quantified γMgly on deliquesced pH-buffered ammonium nitrate (AN) and sulfate/nitrate/ammonium (SNA) aerosols via flow tube experiments by directly measuring SOA formation under variable relative humidity (RH, 75-92%) and pH (3.1-4.4). For AN aerosols, γMgly ranged from 1.92 × 10-4 to 7.29 × 10-4, increasing with enhanced RH due to salting-out effects. Moreover, γMgly decreased by a factor of 2 to 10 as pH rose from 3.15 to 4.4 with NH3 addition, suggesting that acid-catalyzed reactions dominate the Mgly uptake. The pH dependence was captured by a first-order reaction rate constant (kI = 102.44-0.85·pH, R2 = 0.93). This kinetic parameter, together with effective Henry's law constants, can be implemented to update the γMgly parametrization. Addition of sulfate aerosols was found to strongly suppress γMgly, reducing kI to 12-38% of the estimation on AN-alone aerosols at a similar pH. Our findings underscore the critical role of aerosol pH and composition in Mgly uptake and provide kinetic parameters to atmospheric models to improve predictions of Mgly SOA.
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