V. Van Speybroeck

For the development of ab-initio derived force fields, atomic charges must be computed from electronic structure computations, such that (i) they accurately describe the molecular electrostatic potential (ESP) and (ii) they are transferable to the force-field application of interest. The Iterative Hirshfeld (Hirshfeld-I or HI) scheme meets both requirements for organic molecules. For inorganic oxide clusters, however, Hirshfeld-I becomes ambiguous because electron densities of nonexistent isolated anions are needed as input. Herein, we propose a simple Extended Hirshfeld (Hirshfeld-E or HE) scheme to overcome this limitation. The performance of the new HE scheme is compared to four popular atoms-in-molecules schemes, using two tests involving a set of 248 silica clusters. These tests show that the new HE scheme provides an improved trade-off between the ESP accuracy and the transferability of the charges. The new scheme is a generalization of the Hirshfeld-I scheme and it is expected that its improvements are to a large extent applicable to molecular systems containing elements from the entire periodic table.

The methanol to olefins process is an alternative for oil-based production of ethene and propene. However, detailed information on the reaction mechanisms of olefin formation in different zeolite is lacking. Herein a first principle kinetic study allows elucidating the importance of a side-chain mechanism during methanol conversion in H-SAPO-34. Starting from the experimentally observed hexamethylbenzene, a full low-barrier catalytic cycle for ethene and propene formation is found. The olefin elimination steps exhibit low free energy barriers due to a subtle interplay between an sp3 carbon center of the organic intermediate, stabilizing non-bonding interactions and assisting water molecules in the zeolite material.

When adsorbing guest molecules, the porous metal-organic framework MIL-53(Cr) may vary its cell parameters drastically while retaining its crystallinity. A first approach to the thermodynamic analysis of this 'framework breathing' consists of comparing the osmotic potential in two distinct shapes only (large-pore and narrow-pore). In this paper, we propose a generic parametrized free energy model including three contributions: host free energy, guest-guest interactions, and host-guest interaction. Free energy landscapes may now be constructed scanning all shapes and any adsorbed amount of guest molecules. This allows to determine which shapes are the most stable states for arbitrary combinations of experimental control parameters, such as the adsorbing gas chemical potential, the external pressure, and the temperature. The new model correctly reproduces the structural transitions along the CO₂ and CH₄ isotherms. Moreover, our model successfully explains the adsorption versus desorption hysteresis as a consequence of the creation, stabilization, destabilization, and disappearance of a second free energy minimum under the assumptions of a first order phase transition and collective behavior. Our general thermodynamic description allows to decouple the gas chemical potential μ and mechanical pressure P as two independent thermodynamic variables and predict the complete (μ,P) phase diagram for CO₂ adsorption in MIL-53(Cr). The free energy model proposed here is an important step towards a general thermodynamics description of flexible metal-organic frameworks.

Diphosphonylated diazaheterocyclic compounds were synthesized in a one-step reaction by using dimethyl trimethylsilyl phosphite (DMPTMS) under acidic conditions. The reaction of DMPTMS with 1,5-naphthyridine yielded the corresponding diphosphonylated product through a tandem 1,4–1,2 addition under microwave conditions. This tandem 1,4–1,2 addition was also evaluated for other substrates, namely, 1,10-phenanthroline, 1,7-phenanthroline and 4,7-phenanthroline. Reactions under reflux and microwave conditions were compared. 1,5-Naphthyridine and the phenanthroline derived substrates are less reactive than previously investigated quinolines. The experimental trends in reactivity were rationalized by means of theoretical calculations. The intrinsic properties, such as aromaticity and proton affinities, showed distinct differences for the various substrates. Furthermore, the calculated free energies of activation for the rate-determining step of the tandem addition reaction enabled us to rationalize the differences in product yields. Both the theoretical and the experimental results show the substantial influence of the position of the nitrogen atoms in the (poly)aromatic compounds on the reaction outcome.

A new polarizable force field (PFF), namely atom-condensed Kohn-Sham density functional theory approximated to second order (ACKS2), is proposed for the efficient computation of atomic charges and linear response properties of extended molecular systems. It is derived from Kohn-Sham density functional theory (KS-DFT), making use of two novel ingredients in the context of PFFs: (i) constrained atomic populations and (ii) the Legendre transform of the Kohn-Sham kinetic energy. ACKS2 is essentially an extension of the Electronegativity Equalization Method (EEM) [W. J. Mortier, S. K. Ghosh, and S. Shankar, J. Am. Chem. Soc. 108, 4315 (1986)]10.1021/ja00275a013 in which two major EEM shortcomings are fixed: ACKS2 predicts a linear size-dependence of the dipole polarizability in the macroscopic limit and correctly describes the charge distribution when a molecule dissociates. All ACKS2 parameters are defined as atoms-in-molecules expectation values. The implementation of ACKS2 is very similar to that of EEM, with only a small increase in computational cost.