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“Molecular squares” formed from Re(CO)3Cl corners and porphyrin sides have potential applications as hosts for catalytic sites and as building blocks for membranes. In these materials, knowledge of the conformations of the squares is important. Molecular-mechanics (MM) and density-functional (DF) calculations have been used iteratively in this work to find the minimum-energy configurations of several porphyrin molecular squares. MM predicts that the steric and torsional interactions at connecting junctures of the square framework determine the overall geometry. Torsional degrees of freedom around these junctures were therefore analyzed using DF methods, giving further insight and helping choose among MM force-field options. Single-point DF calculations on the entire squares showed that the energy and conformation of the entire square could be reliably obtained by performing DF calculations on the critical elements of the square and then piecing them together. This “piecewise” strategy allows for both the major torsional motions and the most important local relaxations of large supramolecular species such as molecular squares. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Spin-polarized calculations of tetrahedral Fe4, Ni4, and Co4 clusters have been performed by the self-consistent field discrete variational Xα method. Partially carbonylated C3v clusters Co4(μ-CO)3 and (OC)3CoCo3(μ-CO)3 have also been examined to explore how magnetic moments vary with cluster coordination environment. The bare tetrahedral clusters are magnetic containing 12, 6, and 2 unpaired electrons for Fe4, Co4, and Ni4, respectively. Exchange splittings also decrease (2.68, 1.60, and 0.56 eV) along this series. Analysis of density of states plots for these clusters supports the notion that 4s and 4p mixing into the metal d band yields hybrid orbitals better suited for metal–metal bonding. Electronic transitions are predicted to occur in the near IR absorption spectra of the M4 clusters, a region yet to be studied experimentally. Calculations for C3v Co4(μ-Co)3 yield six unpaired electrons with a large spin density on the unligated apical cobalt. In (OC)3CoCo3(μ-CO)3, where the apical cobalt is coordinately saturated, all the spin density (four unpaired electrons) is localized in the basal plane. Bridging CO groups in the basal plan are antiferromagnetically coupled to cobalt. [ABSTRACT FROM AUTHOR]

A formalism is given for predicting reactivity of complex systems by combining electronic structure calculations with forcefield calculations within a transition state theory framework. The theory is employed in combination with the Fukui function to produce a simulation method capable of the ensemble sampling needed to examine sterically complex systems. An important linkage between reactivity information and energetic quantities is provided by introduction of the Fukui overlap integral. This spatial overlap integral measures the coincidence of electron donating regions on a nucleophile with electron accepting regions on the corresponding electrophilic reactant. We show that configurations with high values of this overlap integral tend to have lower density-functional theory energies. Thus, Fukui functions calculated once on single isolated reactants can be used to quickly estimate the reactivity of configurations generated using conventional forcefield-based simulations. The correlation between energies and high overlap integrals can also be used to identify initial guess configurations for transition state searches. However, in the present implementation, real transition states are not accessible because intramolecular geometry relaxation is not allowed. The proposed method is tested on electrophilic aromatic alkylation reactions. Simulation results successfully reproduce experimental substituent effects in a series of variously substituted aromatics. Especially encouraging is the ability of the simulations to predict steric effects in the reaction of toluene with a series of electrophiles of varying bulkiness. Further applications, previously inaccessible to simulation, are expected in systems where steric effects play a dominant role in determining reaction selectivity. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]