Redox thermochemistry of SrFe1-xCoxO3-d

The enthalpy of oxidation of SrFe_1-xCo_xO_3-d with x=0.33 and 0.67 has been determined by adiabatic calorimetry; average values for x=0.33 and 0.67 are -67±11 and -49.5±9 kJ (mol O₂)^-1. These data and the previously reported value for SrFeO_3-d suggest that the enthalpy of oxidation for pure (perovskite-type) SrCoO_3-d is close to zero. Earlier reported composition - partial pressure data for SrFe_0.67Co_0.33O_3-d are reproduced when preferential oxidation of iron is assumed for low partial pressures of oxygen. This revised version was published online in August 2006 with corrections to the Cover Date.     

Mechanistic proposal for the zeolite catalyzed methylation of aromatic compounds

Alkylation and methylation reactions are important reactions in petrochemical production and form part of the reaction mechanism of many hydrocarbon transformation processes. Here, a new reaction mechanism is explored for the zeolite catalyzed methylation of arenes using quantum chemical calculations. It is proposed that the most substituted methylbenzenes, which will reside predominantly on the protonated form when adsorbed in a zeolite, can react directly with a neutral methanol molecule in the vicinity, thereby initiating the methylation reaction without having to return a proton to the zeolite surface. The calculated barriers are quite low, indicating that the suggested mechanism is plausible. This route might explain how the most substituted methylbenzenes can function as efficient reaction intermediates in the methanol to hydrocarbons reaction without themselves acting as catalyst poisons as a consequence of their high proton affinities.     

MCSCF calculation of response properties of Argon

Summary Analytic equations of the multiconfigurational SCF (MCSCF) response theory are combined with the finite-field (FF) approach to compute static and frequency dependent electric and magnetic properties of the Argon atom. A complete active space (CAS SCF) function including the 3s, 3p, 3d, 4s and 4p orbitals in the active space and a large (17s 13p 7d 5f 3g) basis set are employed. This permits an accurate determination of various linear and non-linear response properties such as e.g. electric dipole polarisability and second hyperpolarisability, Verdet constant, magnetisability and second hyperpolarisability. The results, both for the static values and for the frequency dependence of these properties, compare well with other most recent experimental and theoretical data.Dedicated to Jan Linderberg on the occasion of his 60th birthday     

Light-induced copper(II) coordination by a bicyclic tetraaza chelator through a ligand-to-metal charge-transfer reaction

To enable utilization of the broad potential of copper isotopes in nuclear medicine, rapid and robust chelation of the copper is required. Bowl adamanzanes (bicyclic tetraaza ligands) can form kinetically stable copper complexes, but they are usually formed at low rates unless high pH values and high temperatures are applied. We have investigated the effects of the variation in the pH, different anions, and UV irradiation on the chelation rate. UV spectra of mixtures of Cu(2+) and [2(4).3(1)]adz in water show the existence of a long-lived two-coordinated copper(II) intermediate (only counting coordinated amine groups) at pH above 6. These findings are supported by pH titrations of mixtures of Cu(2+) and [2(4).3(1)]adz in water. Irradiation of this complex in the ligand-to-metal charge-transfer (LMCT) band by a diode-array spectrophotometer leads to photodeprotonation and subsequently to formation of the four-coordinated copper(II) complex at a rate up to 7800-fold higher at 25 °C than in the dark. Anions in the solution were found to have three major effects: competitive inhibition due to Cu(II) binding anions, inhibition of the photoinduced transchelation from UV-absorbing anions, and photoredox inhibition from acido ligands capable of acting as electron donors in LMCT reactions. Dissolved O(2) was also found to result in photoredox inhibition.     

Variational response-function formulation of vibrational circular dichroism

The atomic axial tensor (AAT) of vibrational circular dichroism is expressed as the frequency derivative at zero frequency of a linear response function for operators referencing a nuclear displacement and a magnetic field. This is used in the density matrix-based quasienergy derivative Lagrangian approach of Thorvaldsen et al. [J. Chem. Phys., 2008, 129, 214108] to express the AAT in a form where the need to solve response equations for the nuclear displacements is removed, significantly reducing the computation cost compared to existing formulations. The density matrix-based quasienergy derivative Lagrangian approach also allows us straightforwardly to use London atomic orbitals to remove the gauge-origin dependence and to account for the atomic orbitals' dependence on the nuclear coordinates. The formalism is entirely based on atomic-orbital density and integral matrices and therefore amenable to linear scaling for sufficiently sparse matrices and given a linearly scaling response solver.