Force Field Validation for Molecular Dynamics Simulations of IRMOF-1 and Other Isoreticular Zinc Carboxylate Coordination Polymers

Molecular dynamics simulations were conducted to validate a hybrid force field for metal−organic framework-5 (IRMOF-1). In this force field, only nonbonded parameters are used to describe Zn−O interactions. The CVFF force field was used with slight modifications to describe the benzene dicarboxylate linker. The force field correctly predicts a wide range of structural properties of this MOF, including a negative thermal expansion of approximately 1% at 30 and 293 K, in agreement with both theory and experiment. Compressibility results and the associated elastic moduli are in also good agreement with published density functional theory calculations and nanoindentation experiments. The force field predicts a decrease in elastic moduli as temperature increases, which would greatly affect the mechanical properties of MOFs. Calculated vibrational frequencies for Zn−O modes agree with experiment, and a low-frequency mode representing a 180° rotation of the phenyl groups is seen. This rotation becomes more prevalent as the temperature is increased from 300 to 400 K, in agreement with NMR data. Simulations were also carried out with adsorbed guests, including ethanol, cyclohexane, and several chloromethanes. It is shown that the IRMOF-1 lattice parameter depends on the nature of the guest−framework interaction; strongly hydrophilic guests, such as ethanol, cause a decrease (−0.9%) in unit cell volume, while hydrophobic guests cause an increase (0.7−1.5%) in unit cell volume. The calculated free volumes in IRMOF-1 range from 53.5% to 56.0%, in good agreement with experiment. Finally, the activation energy for benzene self-diffusion calculated at low loadings is in good agreement with previous simulations and NMR results, but the magnitude of the diffusion constant is underestimated, most likely because of deficiencies in the CVFF portion of the force field. The results demonstrate, however, that employing a rigid force field results in much poorer agreement with experimental data. Additionally, a flexible force field approach is required when simulating framework stability because of physical changes or the presence of adsorbates. The use of a general-purpose force field for the organic components allows our approach to be extended to other Zn-based frameworks

Force Field Validation for Molecular Dynamics Simulations of IRMOF-1 and Other Isoreticular Zinc Carboxylate Coordination Polymers

Molecular dynamics simulations were conducted to validate a hybrid force field for metal−organic framework-5 (IRMOF-1). In this force field, only nonbonded parameters are used to describe Zn−O interactions. The CVFF force field was used with slight modifications to describe the benzene dicarboxylate linker. The force field correctly predicts a wide range of structural properties of this MOF, including a negative thermal expansion of approximately 1% at 30 and 293 K, in agreement with both theory and experiment. Compressibility results and the associated elastic moduli are in also good agreement with published density functional theory calculations and nanoindentation experiments. The force field predicts a decrease in elastic moduli as temperature increases, which would greatly affect the mechanical properties of MOFs. Calculated vibrational frequencies for Zn−O modes agree with experiment, and a low-frequency mode representing a 180° rotation of the phenyl groups is seen. This rotation becomes more prevalent as the temperature is increased from 300 to 400 K, in agreement with NMR data. Simulations were also carried out with adsorbed guests, including ethanol, cyclohexane, and several chloromethanes. It is shown that the IRMOF-1 lattice parameter depends on the nature of the guest−framework interaction; strongly hydrophilic guests, such as ethanol, cause a decrease (−0.9%) in unit cell volume, while hydrophobic guests cause an increase (0.7−1.5%) in unit cell volume. The calculated free volumes in IRMOF-1 range from 53.5% to 56.0%, in good agreement with experiment. Finally, the activation energy for benzene self-diffusion calculated at low loadings is in good agreement with previous simulations and NMR results, but the magnitude of the diffusion constant is underestimated, most likely because of deficiencies in the CVFF portion of the force field. The results demonstrate, however, that employing a rigid force field results in much poorer agreement with experimental data. Additionally, a flexible force field approach is required when simulating framework stability because of physical changes or the presence of adsorbates. The use of a general-purpose force field for the organic components allows our approach to be extended to other Zn-based frameworks

BAC-MP4 Predictions of Thermochemistry for Gas-Phase Compounds in the Si−H−O−Cl System

A self-consistent set of thermochemical parameters for 39 molecules in the Si−H−O−Cl system have been calculated using the BAC-MP4 method. The BAC-MP4 method combines ab initio electronic structure calculations with empirical corrections to obtain accurate heats of formation. Both stable and radical species are included in the study, as well as several complexes formed by reaction with gas-phase water. Although there are almost no experimental data available for comparison, trends within homologous series and calculated bond dissociation energies are consistent with previous BAC-MP4 predictions for silicon compounds. Polynomial fits of the predicted thermodynamic data over the 300−3000 K temperature range are included in the Supporting Information. The thermodynamic data are used to evaluate the energetics of reactions that may be involved in the oxidation and hydrolysis of silicon tetrachloride, in particular the reactions of SiCl3 and SiCl2 with O2 and H2O

The Interaction of Water with MOF-5 Simulated by Molecular Dynamics [<i>J. Am. Chem. Soc.</i><b> 2006</b>, <i>128</i>, 10678−10679].

The Interaction of Water with MOF-5 Simulated by Molecular Dynamics [J. Am. Chem. Soc. 2006, 128, 10678−10679]

BAC-MP4 Predictions of Thermochemistry for Gas-Phase Tin Compounds in the Sn−H−C−Cl System

In this work, the BAC-MP4 method is extended for the first time to compounds in the fourth row of the periodic table, resulting in a self-consistent set of thermochemical data for 56 tin-containing molecules in the Sn−H−C−Cl system. The BAC-MP4 method combines ab initio electronic structure calculations with empirical corrections to obtain accurate heats of formation. To obtain electronic energies for tin-containing species, the standard 6-31G(d,p) basis set used in BAC-MP4 calculations is augmented with a relativistic effective core potential to describe the electronic structure of the tin atom. Both stable compounds and radical species are included in this study. Trends within homologous series and calculated bond dissociation energies are consistent with previous BAC-MP4 predictions for group 14 compounds and the limited data available from the literature, indicating that the method is performing well for these compounds

Thermochemistry of Molecules in the B−N−Cl−H System: <i>Ab Initio </i>Predictions Using the BAC-MP4 Method

A self-consistent set of thermochemical data for 33 molecules in the B−N−Cl−H system are obtained from a combination of ab initio electronic structure calculations and empirical corrections. Calculations were performed for both stable and radical species. Good agreement is found between the calculations and experimental heats of formation for most molecules containing B, H, and Cl. In addition, the BAC-MP4 and experimental heats of formation for H3B:NH3 are also in reasonable agreement, suggesting that the bond additivity parameters chosen for B−N bonds will provide reasonably accurate heats of formation for compounds containing this type of bond. Transition-state energies for two reactions involving BCl3 and NH3 are also predicted. Polynomial fits of the predicted thermodynamic data over the 300−4000 K temperature range are included in the Supporting Information

Thermochemistry of the Chromium Hydroxides Cr(OH)<i>_n</i>, <i>n</i> = 2−6, and the Oxyhydroxide CrO(OH)₄: Ab Initio Predictions

We here present a high-level ab initio study of the thermochemistry of the chromium hydroxides Cr(OH)n, n = 2−6, and of the oxyhydroxide CrO(OH)4. Optimum geometries and harmonic vibrational frequencies were determined at the B3LYP level of theory using basis sets of triple-ζ quality including polarization and diffuse functions. Heats of formation were obtained from isogyric reaction energies computed at the CCSD(T) level of theory using large basis sets and including corrections for core-valence correlation, scalar relativistic effects, and basis set incompleteness. Additionally, polynomial fits were performed for the heat capacity and the standard enthalpy and entropy over the 100−3000 K temperature range. While our computed heats of formation agree well with previously obtained experimental data for some of these species, our results suggest that revision of the experimental data for others may be appropriate

The Interaction of Water with MOF-5 Simulated by Molecular Dynamics

Force field parameters for use with metal−organic framework-5 (MOF-5 or IRMOF-1) are presented. Flexibility within the framework is included in this model, so that structural changes upon interaction with adsorbate molecules can be observed and quantified. The model was validated by comparing simulated lattice parameters of pure MOF-5 with X-ray diffraction results. For the first time, molecular dynamics simulations have been performed that show how water interacts with MOF-5. The framework is stable at water contents up to 2.3% by mass, but distortion in the lattice structure is already evident. At water contents of 3.9% and higher, the framework collapses because of the replacement of MOF O atoms by water O atoms in the Zn coordination shells. As a result, inorganic MOF O atoms are no longer coordinated by four Zn ions, and benzene dicarboxylate linkers are no longer tethered to Zn centers

Thermochemistry of the Chromium Hydroxides Cr(OH)<i>_n</i>, <i>n</i> = 2−6, and the Oxyhydroxide CrO(OH)₄: Ab Initio Predictions

We here present a high-level ab initio study of the thermochemistry of the chromium hydroxides Cr(OH)n, n = 2−6, and of the oxyhydroxide CrO(OH)4. Optimum geometries and harmonic vibrational frequencies were determined at the B3LYP level of theory using basis sets of triple-ζ quality including polarization and diffuse functions. Heats of formation were obtained from isogyric reaction energies computed at the CCSD(T) level of theory using large basis sets and including corrections for core-valence correlation, scalar relativistic effects, and basis set incompleteness. Additionally, polynomial fits were performed for the heat capacity and the standard enthalpy and entropy over the 100−3000 K temperature range. While our computed heats of formation agree well with previously obtained experimental data for some of these species, our results suggest that revision of the experimental data for others may be appropriate

Thermochemistry of the Chromium Hydroxides Cr(OH)<i>_n</i>, <i>n</i> = 2−6, and the Oxyhydroxide CrO(OH)₄: Ab Initio Predictions

We here present a high-level ab initio study of the thermochemistry of the chromium hydroxides Cr(OH)n, n = 2−6, and of the oxyhydroxide CrO(OH)4. Optimum geometries and harmonic vibrational frequencies were determined at the B3LYP level of theory using basis sets of triple-ζ quality including polarization and diffuse functions. Heats of formation were obtained from isogyric reaction energies computed at the CCSD(T) level of theory using large basis sets and including corrections for core-valence correlation, scalar relativistic effects, and basis set incompleteness. Additionally, polynomial fits were performed for the heat capacity and the standard enthalpy and entropy over the 100−3000 K temperature range. While our computed heats of formation agree well with previously obtained experimental data for some of these species, our results suggest that revision of the experimental data for others may be appropriate