An Fe11 complex showing single-molecule magnet behavior: Theoretical study using density functional methods and Monte Carlo simulations

Theoretical methods based on density functional theory have been employed to study the exchange interactions in a Fe₁₁ complex. This molecule shows 10 different exchange coupling constants. The theoretical J values obtained agree well with the information available for similar complexes, being the single μ₃-O bridging ligands those that present the strongest antiferromagnetic coupling. Monte Carlo simulations performed with the calculated J values reproduce fairly well the experimental data and the S value of 11/2 corresponding to the ground state.     

Theoretical study of the exchange coupling interactions in a polyoxometalate Fe9W12 complex

Theoretical methods based on density functional theory have been employed to analyze the exchange interactions in a Fe₉W₁₂ polyoxometalate complex. The calculated values of the seven exchange coupling constants required by the molecular structure agree well with those reported previously for other Fe^III polynuclear complexes and give an S = 15/2 single determinant ground state, with a first excited state that has S = 5/2.     

Is It Possible To Get High TC Magnets with Prussian Blue Analogues? A Theoretical Prospect

Theoretical methods based on density functional theory have been employed to search for Prussian blue analogues with Curie temperatures higher than the ones reached today. Our study suggests several possible cyano-bridged compounds as candidates to present stronger exchange coupling and higher ordering temperatures than the well known Cr(III)V(II) derivatives.     

Investigation of ligand exchange reactions in aqueous uranyl carbonate complexes using computational approaches

Carbonate anion exchange reactions with water in the uranyl-carbonate and calcium-uranyl-carbonate aqueous systems have been investigated using computational methods. Classical molecular dynamics (MD) simulations with the umbrella sampling technique were employed to determine potentials of mean force for the exchange reactions of water and carbonate. The presence of calcium counter-ions is predicted to increase the stability of the uranyl-carbonate species in accordance with previous experimental observations. However, the free energy barrier to carbonate exchange with water is found to be comparable both in the presence and absence of calcium cations. Possible implications of these results for uranyl adsorption on mineral surfaces are discussed. Density functional theory (DFT) calculations were also used to confirm the trends observed in classical molecular dynamics simulations and to corroborate the validity of the potential parameters employed in the MD scheme.     

Theoretical study of the exchange coupling in a Ni12 single-molecule magnet

The exchange interactions in a Ni12 complex have been studied by using theoretical methods based on density functional theory. The calculated J values reproduce correctly the S = 12 ground state of this system found experimentally and indicate the presence of three different exchange interaction pathways, in agreement with previous inelastic neutron scattering experiments. The three interactions are ferromagnetic, one of them corresponding to a second-neighbor interaction through a syn-anti acetato ligand. A magnetostructural correlation was found for such coupling, confirming the ferromagnetic nature of such an interaction. Our results are in excellent agreement with two new fittings of the experimental magnetic susceptibility data. The spin density distribution of the Ni12 complex is also reported and discussed.     

Conclusive evidence on the insensitivity of additive rules to the combinational details of exchange and correlation functional in hybrid DFT methods

This account is a quest to evaluate the sensitivity of additive rules among total electronic energies of some triple molecular sets to the employed computational level. To achieve the goal, seven randomly generated hybrid functionals have been constructed and employed for calculation on a test set of 29 molecules containing second and third row elements. The computational results based on these nonoptimized DFT methods in conjunction with those of standard B3LYP method clearly demonstrates that in contrast to atomization or isomerization energies, additive rules among total electronic energies are insensitive to reliability or precision of computational method. This insensitivity to details of exchange‐correlation functional indicates that additive rules can be generally well considered using simple computational methods. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

A theoretical study on the electronic structures of TiO2: Effect of Hartree-Fock exchange

The effects of the Fock exchange on the geometries and electronic structures of TiO2 have been investigated by introducing a portion of Hartree-Fock (HF) exchange into the traditional density functional. Our results indicate that the functional with 13% HF exchange can correctly predict the band gap and the electronic structures of rutile TiO2, and such an approach is also suitable to describe the structural and electronic properties of anatase and brookite phases. For the TiO2 (110) surfaces, although the surface relaxations are insensitive to the variation of HF exchange, there are larger effects on the positions of the occupied surface-induced states. When 13% HF exchange is employed, the predicted band gap of the perfect surface and position of defect state of the reduced surface are consistent with the experimental values. Moreover, the electronic structures of TiO2 (110) surface are carefully reexamined by using this hybrid density functional method.     

Exchange coupling in mu-aqua:mu-oxo vs. di-mu-hydroxo dinuclear Cu(II) compounds: a density functional study

Theoretical methods based on density functional theory have been applied to study the differences in exchange coupling between the di-mu-hydroxo and mu-aqua:mu-oxo tautomers of a dinuclear Cu(II) complex. The calculations indicate that the two compounds are totally different from a magnetic point of view. The transfer of one proton from one of the two hydroxo bridges to the other leaves an asymmetric structure with only one effective exchange pathway through the oxo bridge. The coordinated water molecule and an associated perchlorate counterion, although not directly involved in the exchange coupling, play an important role in the magnetic properties of this compound since their presence is crucial in the determination of the geometrical details of the Cu-O(ox)-Cu linkage and hence in the exchange coupling between the copper atoms.     

Theoretical determination of multiple exchange couplings and magnetic susceptibility data in inorganic solids: the prototypical case of Cu2(OH)3NO3

Density functional theory based on hybrid functionals and localized atomic type basis sets is employed to calculate the exchange couplings in the layered three-dimensional compound Cu2(OH)3NO3. We assign accurate values to the six different in-plane exchange couplings. Interlayer exchange interactions through hydrogen bonds are also quantified. The calculated exchange coupling constants are then employed to perform quantum Monte Carlo simulations to yield magnetic susceptibility data, which compare successfully with experiments. Our approach sets the foundations of a viable methodology to extract reliable magnetic susceptibilities from density functional data.     

Quantum Chemical Spin Densities for Radical Cations of Photosynthetic Pigment Models

The spin densities of radical cations of magnesium porphyrin, magnesium chlorine and a truncated chlorophyll a model are calculated with density‐functional theory and multiconfigurational quantum chemical methods. The latter serve as a reference for approximate density‐functional theory which yields spin densities that may suffer from the self‐interaction error. We carried out complete active space self‐consistent field calculations with increasing active orbital spaces to systematically converge qualitatively correct spin densities. In particular, for the magnesium chlorine and chlorophyll a model radical cations, this is not easy to achieve because of the lower symmetry compared to magnesium porphyrin. Strategies had to be employed which allowed us to consider very large active orbital spaces. We explored restricted active space self‐consistent field and density‐matrix renormalization group calculations. Based on these reference data, we assessed the accuracy of different density‐functional approximations. We show that in particular, exchange–correlation model potentials with correct asymptotic behavior yield good spin densities, and we find, in agreement with previous studies on different classes of compounds, that hybrid functionals systematically increase spin‐polarization effects with increasing amounts of exact exchange. Our results provide a starting point for investigations of spin densities of more complex systems such as the hinge model for the primary electron donor in photosystem II.     

Controllable preparation of a hydrophilic/ion‐exchange mixed‐mode stationary phase by surface‐initiated atom transfer radical polymerization using a mixture of two functional monomers

Mixed‐mode chromatographic stationary phases require functionalization with at least two functional groups to yield multiple interactions with analytes. Departing from reported methods, a mixture of two different monomers, glycidyl methacrylate and 2‐dimethylaminoethylmethacrylate, was grafted onto the surface of silica by a one‐step surface‐initiated atom transfer radical polymerization to prepare a novel hydrophilic interaction/anion‐exchange mixed‐mode chromatographic stationary phase. The grafted amounts of functional groups were controlled via varying the ratio of monomers in the polymerization system. The influences of water content, salt concentration and pH in the mobile phase were investigated to illustrate the mixed interaction between the stationary phase and analytes. The retention of various solutes on three columns, especially acidic and basic solutes, showed an obvious dependence on the ratio of the two monomers in the polymerization system. The results indicated that the strategy proposed in this work was beneficial to develop various types of mixed‐mode chromatographic stationary phases with adjustable selectivity to meet the needs of complex samples. Finally, the column was successfully employed in the isolation of melamine in liquid milk.     

DFT study of [2.2]-, [3.3]-, and [4.4]paracyclophanes: strain energy, conformations, and rotational barriers

The three smallest symmetrical paracyclophanes, having tethers with two, three, or four methylene groups, have been examined with four density functional methods (B3LYP, M06-2x, B97-D, ωB97X-D). The geometries predicted by functionals accounting for medium-range correlation or long-range exchange and/or dispersion are in close agreement with experiment. In addition, these methods provide similar estimates of the strain energy of the paracylcophanes, which decrease with increasing tether length. [4.4]Paracyclophane is nearly strain-free, reflecting the small out-of-plane distortion of its phenyl rings. Lastly, the barrier for interconversion of the conformers of [3.3]paracylcophane is computed in close agreement with experiment, and an estimate for phenyl rotation in [4.4]paracyclophane of about 19 kcal mol(-1) is predicted by the DFT methods employed.     

Solvated CH5+ in liquid superacid

The transition states for methane activation in liquid superacid have been studied by experimentally determined secondary kinetic deuterium isotope effects (SKIEs) and computational chemistry. For the first time, the SKIEs on hydrogen/deuterium exchange of methane have been measured by using the methane isotopologues in homogeneous liquid superacid (2HF/SbF5). To achieve high accuracy of the SKIEs, the rate constants for pairs of methane isotopologues were simultaneously measured in the same superacid solution by using NMR spectroscopy. Density functional theory (DFT) and high-level ab initio methods have been employed to model possible intermediates and transition states, assuming that the superacids involved in the exchange reactions are H2F+ ions solvated by HF. Only the unsolvated superacid H2F+ is found to be strong enough to protonate methane, yielding the methonium ion solvated by HF as a potential energy minimum. In contrast, the (HF)x-solvated H2F+ superacids (x = 1-4) do not appear to be strong enough to yield stable solvated methonium ions. However, such ions show up as parts of the transition states of the exchange in which the methonium ions are solvated by (HF)x. The calculated DFT activation barrier is in good agreement with that experimentally observed.     

Electronic structures of 3d-metal mononitrides

Bond distances, vibrational frequencies, electron affinities, ionization potentials, and dissociation energies of the title molecules in neutral, positively, and negatively charged ions were studied by use of density functional methods B3LYP, BLYP, BHLYP, BPW91, and B3PW91. The calculated results are compared with experiments and previous theoretical studies. It was found that the calculated properties are highly dependent on the functionals employed, in particular for the dissociation energy and vibrational frequency. For neutral species, pure density functional methods BLYP and BPW91 have relatively good performance in reproducing the experimental bond distance and vibrational frequency. For cations, hybrid exchange functional methods B3LYP and B3PW91 are good in predicting the dissociation energy. For both neutral and charged species, BHLYP tends to give smaller dissociation energy.     

Functional, star polymeric molecular carriers, built from biodegradable microgel/nanogel cores

Acid and disulfide biodegradable cross-linkers have been employed to generate microgel star polymers, using RAFT-polymer arms. RAFT end-groups were then exploited to attach functional compounds via both thiol-ene and thiol-pyridyl disulfide exchange reactions.     

Multiple Catalytic C−H Bond Functionalization for Natural Product Synthesis

In the past decade, multiple catalytic C?H bond functionalization has been successfully applied in natural product synthesis as a strategy to reduce the number of steps, increase overall yield and employ more easily available starting materials. This minireview presents selected examples making use of multiple C?H bond functionalization in conceptually different ways. First, linear syntheses are discussed, wherein multiple C?H functionalization is employed either from simple (hetero)cyclic cores, at a late stage, or to build polycyclic systems. Second, the use of multiple C?H functionalization as a strategic tool in convergent synthesis to access and couple complex fragments is discussed. Information on the scalability of the employed methods is provided when available. The presented cases indicate that multiple C?H functionalization strategies should play a great role to shape the future synthesis of functional complex molecules with improved sustainability.     

Theoretical study on the magnetic exchange behavior of a hetero-double-bridged binuclear copper(II) complex: orbital complementary effect

The mechanism of magnetic exchange interaction in a μ-chloro and μ-methoxy double-bridged copper(II) complex is investigated by calculations based on density functional theory combined with the broken-symmetry approach (DFT-BS). The calculated results reveal that the complex has a strong antiferromagnetic interaction. By the analyses of magnetic orbital interaction and spin distribution, it is found that there exists orbital complementary effect between the two bridging ligands. In addition, to examine the contribution to the magnetic exchange interaction of each bridging ligand, two single-bridged systems have been studied.     

Heterocumulene metathesis by iridium guanidinate and ureylene complexes: catalysis involving reversible insertion to form six-membered metallacycles

The four-membered metallacycles Cp*Ir((NAr)2C=Z) have been prepared for Z = NAr and Z = O. These complexes undergo facile exchange with free heterocumulenes. This exchange process can be employed to effect the rapid, catalytic metathesis of aryl carbodiimides at room temperature and the metathesis of aryl carbodiimides with aryl isocyanates at slightly elevated temperature. The exchange process appears to proceed via a novel associative mechanism involving ring expansion to form a six-membered metallacycle rather than cycloreversion to give an imido complex. Observation of key intermediates and the results of crossover experiments support this hypothesis.