Recent theoretical studies of water oxidation in photosystem II

In the present mini-review, computational work over the past decade on water oxidation in photosystem II (PSII) is summarized. The size of the chemical model used for the oxygen evolving complex (OEC) has during this time increased from the initial 20 atoms to the present day 220 atoms. The electronic structure methods used have during the same period only undergone minor improvements. It is concluded that the results have now reached a high level of convergence and the predictions for both the structure of the OEC and the O-O bond formation mechanism are most probably of higher accuracy than presently available from experiments.     

Core-electron spectroscopy of alkaline-earth and transition-metal ions in solution

Core-electron spectra of alkaline-earth and transition-metal ions in solution have been recorded. Solvation energies for the core-ionized species are derived using thermodynamic cycles which involve the free-ion to dissolved-ion binding energy shifts. The distinction between “soft” and “hard” ion behaviour is discussed in relation to the solvation energies and multi-line features in the transition-metal core spectra.     

A critical study of basis set effects and the use of approximate natural orbitals in SCF-CI calculations of molecular geometries and heats of reaction

SCF-CI calculations have been performed on a number of chemical reactions between closed shell molecules in order to determine the heats of reaction. Contracted Gaussian type atomic basis sets of three different qualities were used and the CI calculations were performed in a truncated approximate natural orbital space. The conclusions to be drawn from these calculations are rather pessimistic. For heats of reaction, errors up to 6 kcal/mole are obtained on the SCF-level with a double zeta plus polarization atomic basis. A further improvement is only possible if extended basis sets are used. Correlation effects on heats of reaction are of the same size and CI calculations are therefore only meaningful with large atomic basis sets.For the CI calculations a one-electron space of approximate natural orbitals, obtained from second order RS perturbation theory, was used. Different truncations, using the occupation number as criterion, were tested. The general conclusion is that errors in energy differences obtained with a truncated basis set are of the same magnitude as the error in the total correlation energy. In practice this means that not more than 20–30% of the approximate natural orbitals can be deleted if the error is to be kept less than a few kcal/mole.Finally the truncation error in calculations of bond distances was tested for a few cases. Errors of around 10% of the total change due to correlation were found when 30% of the lowest occupied natural orbitals were deleted.     

Chemical shifts of auger electron lines and electron binding energies in free molecules. Sulfur compounds

The first paper describing a series of systematic investigations of the chemical shifts in Auger electron spectra from various free molecules is presented. Excitation is performed by means of a fine focus electron beam. The Auger electrons are retarded in a four component lens system and recorded at high resolution in a new multidetector system placed in the focal plane of the ESCA instrument. A calibration procedure against the KL₂L₃ Auger electron line of Ne is described. The first study concerns Auger electron line shifts for sulfur in some small molecules and the results are compared to the corresponding chemical shifts in the core photoelectron spectra. A formalism based on a transition potential model is briefly presented which takes account of the relaxation energies involved in Auger transitions as well as in single photoionization.     

Chemical shifts in ESCA and NMR

A detailed examination of the relationship between chemical shifts in ESCA and NMR is presented. It is demonstrated that even in series of closely related compounds a linear correlation between ESCA and NMR shifts cannot be expected. Included is a discussion of reorganization effects on electron binding energies. The close relation between chemical shifts of ESCA and NMR and the spin—rotation constants of molecules is pointed out.     

ESCA applied to liquids

The conditions for applying ESCA on liquid samples are discussed. A “liquid beam” technique is developed which meets the various requirements. The first spectrum of a liquid is presented, namely from formamide. It is shown that one can adjust the liquid beam so that a complete separation between the ESCA signals from the liquid and the vapour is achieved. The binding energies of the core levels of the elements in the molecule differ from each other in the liquid and the vapour phase by about 1.6 eV. One can also obtain electron spectra from dissolved chemical species. This is illustrated by potassium iodide dissolved in formamide.