Modifying the Fullerene Surface Using Endohedral Noble Gas Atoms: Density Functional Theory Based Molecular Dynamics Study of C₇₀O₃

We have performed a series of ab initio molecular orbital and molecular dynamics calculations to ascertain the influence of an endohedral noble gas atom on the reactivity of the surface of the model system C₇₀O₃. Our simulations show that the minimum energy pathways for the ozone ring-opening reaction are influenced by the presence of the endohedral atom. The effect is isomer dependent, with the enthalpy of the reaction increasing for <i>a</i>,<i>b</i>-C₇₀O₃ and decreasing for <i>e</i>,<i>e</i>-C₇₀O₃ when doped with the heavy noble gas atoms Xe and Rn

Hybrid QM/QM Simulations of Excited-State Intramolecular Proton Transfer in the Molecular Crystal 7‑(2-Pyridyl)-indole

A subtractive implementation of the QM/QM hybrid method for the description of photochemical reactions occurring in molecular crystals is presented and tested by applying it in a simulation study of the ultrafast intramolecular excited-state proton transfer reaction in the crystal form of 7-(2-pyridyl)-indole, an organic compound featuring an intramolecular hydrogen bond within a six-membered ring. By propagating molecular dynamics on the excited-state potential energy surface, a mean proton transfer time was calculated as 80 fs. The reaction mechanism is discussed in terms of three-dimensional reaction coordinate diagrams. Proton transfer was found to be barrierless and to be strongly coupled to vibrational modes of the photoexcited molecule that modulate the proton donor–acceptor distance. Some 300 fs after the initial photoexcitation, the excited state molecule reached an S₁/S₀ conical intersection through the mutual twist of the pyridyl and indolyl moieties

Liquid Methanol from DFT and DFT/MM Molecular Dynamics Simulations

We present a comparative study of computational protocols for the description of liquid methanol from <i>ab initio</i> molecular dynamics simulations, in view of further applications directed at the modeling of chemical reactivity of organic and organometallic molecules in (explicit) methanol solution. We tested density functional theory molecular dynamics (DFT-MD) in its Car–Parrinello Molecular Dynamics (CPMD) and Quickstep/Born–Oppenheimer MD (CP2K) implementations, employing six popular density functionals with and without corrections for dispersion interactions (namely BLYP, BLYP-D2, BLYP-D3, BP86, BP86-D2, and B97-D2). Selected functionals were also tested within the two QM/MM frameworks implemented in CPMD and CP2K, considering one DFT molecule in a MM environment (described by the OPLS model of methanol). The accuracy of each of these methods at describing the bulk liquid phase under ambient conditions was evaluated by analyzing their ability to reproduce (<i>i</i>) the average structure of the liquid, (<i>ii</i>) the mean squared displacement of methanol molecules, (<i>iii</i>) the average molecular dipole moments, and (<i>iv</i>) the gas-to-liquid red-shift observed in their infrared spectra. We show that it is difficult to find a DFT functional that describes these four properties equally well within full DFT-MD simulations, despite a good overall performance of B97-D2. On the other hand, DFT/MM-MD provides a satisfactory description of the solvent–solute polarization effects with all functionals and thus represents a good alternative for the modeling of methanol solutions in the context of chemical reactivity in an explicit environment

The Influence of the Hofmeister Bias and the Stability and Speciation of Chloridolanthanates on Their Extraction from Chloride Media

<p>The possibility of recovering rare earth elements from solutions containing their chloridometalate anions [LnCl<i>_x</i>]^{(<i>x</i>−3)−} via the process: LnCl<i>_x</i>^{(<i>x</i>−3)−} + (<i>x</i> − 3)<i>L</i>_org + (<i>x</i>–3)H⁺ ⇌ [(LH)<i>_x</i>₋₃LnCl<i>_x</i>]_org has been tested using 2-(1,3-bis(hexylamino)-1,3-dioxopropan-2-yl)-4,6-di-<i>tert</i>-butylpyridine (PMA), tri-<i>n-</i>butylphosphate (TBP), and tri-<i>n</i>-octylamine (TOA), which are known to be strong extractants for transition metal chloridometalates. While DFT calculations indicate that the formation of the neutral assembly [(PMAH)₃LaCl₆] in the gas phase is favorable, no uptake of La(III) from 6 M HCl by toluene solutions of PMA (or of TBP or TOA) was observed in solvent extraction experiments. Successful uptake of the [PtCl₆]²⁻ dianion by PMA and the failure to extract the [IrCl₆]³⁻ trianion under the same conditions indicate that the higher hydration energy of the latter makes transfer to the toluene solution less favorable and that this militates against extraction of La(III) chlorido complexes carrying charges of −3 or larger in which all the inner-sphere water molecules have been replaced. Computational results confirm literature observations that, in contrast to transition metal trications, formation of REE metalate anions such as [LnCl<i>_x</i>]^{(<i>x</i>−3)−} is not very favorable, particularly so for chloride, compared with nitrato or sulfato systems. Also, they indicate that the formation of <i>outer-sphere</i> assemblies such as {[La(H₂O)₉]·<i>x</i>Cl} in which water ligands are retained in the inner sphere, H-bonded to anions, is more stable than <i>inner-sphere</i> complexes containing an equivalent number of anions. The high level of hydration of such species disfavors their transfer into nonpolar water-immiscible solvents. It is unlikely that recovery of [LnCl<i>_x</i>]^{(<i>x</i>−3)−} from acidic solutions can be achieved efficiently using currently available anion exchange extractants operating in a “pH-swing” process. Receptors giving very high binding energies to chloridolanthanates will be needed to offset the high dehydration energies required.</p

Elucidating the Breathing of the Metal–Organic Framework MIL-53(Sc) with ab Initio Molecular Dynamics Simulations and in Situ X‑ray Powder Diffraction Experiments

Ab initio molecular dynamics (AIMD) simulations have been used to predict structural transitions of the breathing metal–organic framework (MOF) MIL-53­(Sc) in response to changes in temperature over the range 100–623 K and adsorption of CO₂ at 0–0.9 bar at 196 K. The method has for the first time been shown to predict successfully both temperature-dependent structural changes and the structural response to variable sorbate uptake of a flexible MOF. AIMD employing dispersion-corrected density functional theory accurately simulated the experimentally observed closure of MIL-53­(Sc) upon solvent removal and the transition of the empty MOF from the <i>closed-pore</i> phase to the <i>very-narrow-pore</i> phase (symmetry change from <i>P</i>2₁/<i>c</i> to <i>C</i>2/<i>c</i>) with increasing temperature, indicating that it can directly take into account entropic as well as enthalpic effects. We also used AIMD simulations to mimic the CO₂ adsorption of MIL-53­(Sc) in silico by allowing the MIL-53­(Sc) framework to evolve freely in response to CO₂ loadings corresponding to the two steps in the experimental adsorption isotherm. The resulting structures enabled the structure determination of the two CO₂-containing <i>intermediate</i> and <i>large-pore</i> phases observed by experimental synchrotron X-ray diffraction studies with increasing CO₂ pressure; this would not have been possible for the <i>intermediate</i> structure via conventional methods because of diffraction peak broadening. Furthermore, the strong and anisotropic peak broadening observed for the <i>intermediate</i> structure could be explained in terms of fluctuations of the framework predicted by the AIMD simulations. Fundamental insights from the molecular-level interactions further revealed the origin of the breathing of MIL-53­(Sc) upon temperature variation and CO₂ adsorption. These simulations illustrate the power of the AIMD method for the prediction and understanding of the behavior of flexible microporous solids

Elucidating the Breathing of the Metal–Organic Framework MIL-53(Sc) with ab Initio Molecular Dynamics Simulations and in Situ X‑ray Powder Diffraction Experiments

Ab initio molecular dynamics (AIMD) simulations have been used to predict structural transitions of the breathing metal–organic framework (MOF) MIL-53­(Sc) in response to changes in temperature over the range 100–623 K and adsorption of CO₂ at 0–0.9 bar at 196 K. The method has for the first time been shown to predict successfully both temperature-dependent structural changes and the structural response to variable sorbate uptake of a flexible MOF. AIMD employing dispersion-corrected density functional theory accurately simulated the experimentally observed closure of MIL-53­(Sc) upon solvent removal and the transition of the empty MOF from the <i>closed-pore</i> phase to the <i>very-narrow-pore</i> phase (symmetry change from <i>P</i>2₁/<i>c</i> to <i>C</i>2/<i>c</i>) with increasing temperature, indicating that it can directly take into account entropic as well as enthalpic effects. We also used AIMD simulations to mimic the CO₂ adsorption of MIL-53­(Sc) in silico by allowing the MIL-53­(Sc) framework to evolve freely in response to CO₂ loadings corresponding to the two steps in the experimental adsorption isotherm. The resulting structures enabled the structure determination of the two CO₂-containing <i>intermediate</i> and <i>large-pore</i> phases observed by experimental synchrotron X-ray diffraction studies with increasing CO₂ pressure; this would not have been possible for the <i>intermediate</i> structure via conventional methods because of diffraction peak broadening. Furthermore, the strong and anisotropic peak broadening observed for the <i>intermediate</i> structure could be explained in terms of fluctuations of the framework predicted by the AIMD simulations. Fundamental insights from the molecular-level interactions further revealed the origin of the breathing of MIL-53­(Sc) upon temperature variation and CO₂ adsorption. These simulations illustrate the power of the AIMD method for the prediction and understanding of the behavior of flexible microporous solids

Inter- versus Intramolecular Structural Manipulation of a Dichromium(II) Pacman Complex through Pressure Variation

The effect of pressure on the intranuclear M···M separation and intermolecular secondary interactions in the dinuclear chromium Pacman complex [Cr₂(L)]­(C₆H₆) was evaluated because this compound contains both a short Cr···Cr separation and an exogenously bound molecule of benzene in the solid state. The electronic structure of [Cr₂(L)] was determined by electron paramagnetic resonance spectroscopy, SQUID magnetometry, and density functional theory calculations and shows a diamagnetic ground state through antiferromagnetic exchange, with no evidence for a Cr–Cr bond. Analysis of the solid-state structures of [Cr₂(L)]­(C₆H₆) at pressures varying from ambient to 3.0 GPa shows little deformation in the Cr···Cr separation, i.e., no Cr–Cr bond formation, but instead a significantly increased interaction between the exogenous arene and the chromium iminopyrrolide environment. It is therefore apparent from this analysis that [Cr₂(L)] would be best exploited as a rigid chemical synthon, with pressure regulation being used to mediate the approach and secondary interactions of possible substrates

On the Extraction of HCl and H₂PtCl₆ by Tributyl Phosphate: A Mode of Action Study

<p>Combining computational modeling with experimental measurements has revealed the self-assembly of nano-aggregate structures in the transfer of HCl and PtCl₆^2– from an aqueous phase into toluene by the common industrial extractant tributyl phosphate (TBP). Molecular dynamics simulations have been coupled to analytical measurements to provide an atomistic interpretation of the mode of action of TBP under 6 M and 10 M HCl conditions. The structures conform to reverse micelles, where the Cl^– or PtCl₆^2– core is encapsulated by a hydration shell that acts as a mediating bridge to the electronegative oxygen atom in the TBP phosphate groups. For the 6 M HCl extraction model, the data support stable aggregates forming from 2–3 TBP molecules around one chloride anion if the number of water molecules encapsulating the chloride anion is no more than five; increasing the water content to 10 molecules allows a fourth TBP molecule to coordinate. For the 10 M HCl extraction model, stable structures are obtained that conform to the empirical formula (TBP.HCl.H₂O)_3–5. At 6 M HCl, extraction of PtCl₆^2– is achieved by encapsulation by four TBP molecules; the data for extraction at 10 M HCl indicate larger aggregates containing multiple PtCl₆^2– anions are likely to be forming. In all cases, the hydrated core regions of the reverse micelles are considerably exposed. The diameters of the self-assembled structures around chloride ions agree well with available literature data from small-angle neutron-scattering experiments.</p

Experimental and DFT‑D Studies of the Molecular Organic Energetic Material RDX

We have performed simulations utilizing the dispersion-corrected density functional theory method (DFT-D) as parametrized by Grimme on selected polymorphs of RDX (cyclotrimethylenetrinitramine). Additionally, we present the first experimental determination of the enthalpy of fusion (Δ<i>H</i>_fus) of the highly metastable β-form of RDX. The characteristics of fusion for β-RDX were determined to be 186.7 ± 0.8 °C, 188.5 ± 0.4 °C, and 12.63 ± 0.28 kJ mol^–1 for the onset temperature, peak temperature (or melting point), and Δ<i>H</i>_fus, respectively. The difference in experimental Δ<i>H</i>_fus for the α- and β-forms of RDX is 20.46 ± 0.92 kJ mol^–1. Ambient-pressure lattice energies (<i>E</i>_L) of the α- and β-forms of RDX have been calculated and are in excellent agreement with experiment. In addition the computationally predicted difference in <i>E</i>_L (20.35 kJ mol^–1) between the α- and β-forms is in excellent agreement with the experimental difference in Δ<i>H</i>_fus. The response of the lattice parameters and unit-cell volumes to pressure for the α- and γ-forms have been investigated, in addition to the first high-pressure computational study of the ε-form of RDXthese results are in very good agreement with experimental data. Phonon calculations provide good agreement for vibrational frequencies obtained from Raman spectroscopy, and a predicted inelastic neutron scattering (INS) spectrum of α-RDX shows excellent agreement with experimental INS data determined in this study. The transition energies and intensities are reproduced, confirming that both the eigenvalues and the eigenvectors of the vibrations are correctly described by the DFT-D method. The results of the high-pressure phonon calculations have been used to show that the heat capacities of the α-, γ-, and ε-forms of RDX are only weakly affected by pressure

Inter- versus Intramolecular Structural Manipulation of a Dichromium(II) Pacman Complex through Pressure Variation

The effect of pressure on the intranuclear M···M separation and intermolecular secondary interactions in the dinuclear chromium Pacman complex [Cr₂(L)]­(C₆H₆) was evaluated because this compound contains both a short Cr···Cr separation and an exogenously bound molecule of benzene in the solid state. The electronic structure of [Cr₂(L)] was determined by electron paramagnetic resonance spectroscopy, SQUID magnetometry, and density functional theory calculations and shows a diamagnetic ground state through antiferromagnetic exchange, with no evidence for a Cr–Cr bond. Analysis of the solid-state structures of [Cr₂(L)]­(C₆H₆) at pressures varying from ambient to 3.0 GPa shows little deformation in the Cr···Cr separation, i.e., no Cr–Cr bond formation, but instead a significantly increased interaction between the exogenous arene and the chromium iminopyrrolide environment. It is therefore apparent from this analysis that [Cr₂(L)] would be best exploited as a rigid chemical synthon, with pressure regulation being used to mediate the approach and secondary interactions of possible substrates