Closed-shell Species Research Articles

Electron Correlation in Weakly Coupled Transition Metal Compounds: Poly‐Decker Systems

AbstractThe Thouless instability conditions in the poly‐decker sandwich compounds 1–8 have been investigated by means of semiempirical INDO calculations. Hartree‐Fock (HF) fluctuations of the singlet, non‐singlet (triplet) and non‐real type have been detected. The reasons for the various instability conditions are analysed. The charge density waves (CDW) of the singlet route are of helical type, the spin density wave (SDW) of the non‐singlet route corresponds to a longitudinal modulation. Singlet fluctuations in complexes with identical 3d centers are the result of intraatomic angular and interatomic left‐right correlation. The non‐singlet variation corresponds to a spin decoupling between the two 3d centers. The HF instabilities in the closed shell species 1 – 8 are compared with doublet fluctuations in open shell complexes and are extrapolated to infinite polymer chains.

Radiation less transitions in gas phase molecular ions

Open shell molecular ions are particularly interesting species for studying radiationless transitions. With respect to closed shell species, there are both structural and instrumental advantages. The important experimental parameters to determine are fluorescence decay patterns and quantum yields. These are measured using a technique in which the ions are produced by photoionization and are detected in delayed coincidence with the photons emitted from their excited electronic states. A brief survey is given of the theory of radiationless transitions between radiant |s> and quasi-nonradiant |l> levels, paying particular attention to the following three cases: 1) resonance limit, where the density ρ l of {|l>} states is very small; 2) statistical limit, where the {|l>} states form a quasi continuum; 3) intermediate case. Experimental results on [Math] are interpreted in terms of the resonance limit, while those on a series of polyfluorobenzene ions are considered in terms of the statistical limit. Emission quantum yields and multiexponential decay behaviour of mono- and di-chloroacetylene ions are interpreted in terms of the strong coupling theory of the intermediate case. Coupling energies, densities of states and level decay rates are determined in each case. Vibrationally hot ground electronic state ions can result from the electronic relaxation process; their fate may involve intramolecular nonradiative vibrational redistribution and fast infrared emission. Problems for further study are identified in the conclusion.

Plasmon effects in electron spectroscopies and chemisorption

We propose a soluble quantum mechanical model for the photoemission spectrum of a surface molecule interacting with the surface plasmon field. Plasmon polarization and hole recoil are included in our model. These terms should be particularly important in the XPS spectra of chemisorbed electronegative species like F and Cl. Further our exact solution implies the existence of a new, plasmon mediated, chemisorption mechanism. This new kind of chemisorption bond is involved in the absorption of closed shell species, like Xe, on transition metals, and explains the microscopic physical mechanism underlying well known phenomenological arguments like the “charge transfer no bond” model of chemisorption.

Hydrogen bonding: The structure of HF–HCl

The structure of the hydrogen bonded complex formed between HF and HCl has been determined by molecular beam electric resonance spectroscopy. The molecule HF–HC1 is a slightly asymmetric prolate top. The spectroscopic constants determined from K=0 spectra of several isotopic species are 〈eqQDa〉 for HF D35Cl is 146(15) kHz. The equilibrium atomic arrangement is HF–HCl with the internal proton colinear with the two heavy atoms spaced 2.12 Å from the fluorine atom. The exterior proton is off axis by 50°. The bonding of this complex is discussed with respect to other gas phase complexes and hydrogen halide crystal structures. It appears that a model which involves electron donation from the highest occupied molecular orbital (HOMO), of one submolecule to the lowest unoccupied molecular orbital (LUMO) of the other gives a useful qualitative description of complex formation between closed shell species.

Computational study of vertical ionization potentials using the ΔINDO + FOCI method

The computational procedure devised by Segal has been employed to estimate ionization potentials by energy differences. In order to obtain similar accuracy with both open- and closed-shell species, a first-order configuration interaction (FOCI) calculation is performed after the SCF procedure for the open-shell system. In this work, the SCF scheme is based on the INDO approximation. In general, these ΔINDO + FOCI computations provide adequate interpretations of photo-electron spectra. For closed-shell parent molecules, comparison of the ionization potentials calculated by Koopmans' Theorem and those by the ΔSCF + FOCI method leads to an estimate of the electronic reorganization taking place on ionization. The results of the present calculations indicate that electronic rcargauization is insufficient to affect the ordering of the first few cationic states predicted by Koopmans' Theorem. The major exception to this occurs for the molecules H2CO and F2CO.

The influence of polarization functions on molecular orbital hydrogenation energies

Polarization functions are added in two steps to a split-valence extended gaussian basis set: d-type gaussians on the first row atoms C. N, O and F and p-type gaussians on hydrogen. The same d-exponent of 0.8 is found to be satisfactory for these four atoms and the hydrogen p-exponent of 1.1 is adequate in their hydrides. The energy lowering due to d functions is found to depend on the local symmetry around the heavy atom. For the particular basis used, the energy lowerings due to d functions for various environments around the heavy atom are tabulated. These bases are then applied to a set of molecules containing up to two heavy atoms to obtain their LCAO-MO-SCF energies. The mean absolute deviation between theory and experiment (where available) for heats of hydrogenation of closed shell species with two non-hydrogen atoms is 4 kcal/mole for the basis set with full polarization. Estimates of hydrogenation energy errors at the Hartree-Fock limit, based on available calculations, are given.

Bonding in boron hydrides

Unique preferred localized valence structures for B2H6, B4H10, B5H,, and B6H10 have been found from accurate self-consistent field wavefunctions by maximizing Coulomb repulsions of electron pairs within orbitals. Objective evidence has thus been obtained for two-centre BH, three-centre bridge BHB, two-centre BB and central three-centre BBB bonds, but not, as yet, for open three-centre BBB bonds. Locali7ation in B5H9 is ambiguous, as it is in organic molecules such as benzene. More thermodynamical stable isomers or plausible reaction pathways appear to have more resonance structures, and less reactive (or more plausible) structures or intermediates appear to have more nearly uniform charge distributions. INTRODUCTION Given the same number of valence orbitals as carbon but one less electron, boron is said to form electron-deficient compounds. However, in another sense these compounds are not deficient because the extended topologies of carbon chemistry are not found in boron compounds. Instead, a contracted topology occurs, mainly exemplified geometrically by bridge hydrogens and by boron triangles. The simplest valence-theoretical description of this contracted topology is the three-centre two-electron bond, which has gradually developed into a general theory' of bonding in boron hydrides, their derivatives and their plausible reaction intermediates. Only among the simplest structures are unique, or preferred, valence descriptions possible. More complex structures require resonance descriptions when three-centre and two-centre bonds are used as a basis6. Nevertheless, in the sense that the most stable orbitals are filled, and that a substantial energy gap exists towards the unfilled (excited) state, these electron-deficient' molecules are indeed closed-shell species. Molecular orbital theory in the form of the extended Huckel theory was originated and first applied to these larger boron hydrides6'7 in order to provide an alternative to these resonance descriptions. Subsequently, both the logical basis and the source of parameters for these molecular orbital studies were greatly improved, and relationships were thereby established8'9 to the rigorous molecular self-consistent field (SCF) method' .By the averaging of Coulomb repulsions, but the inclusion of exchange interactions and use of

Correlation Studies on H3+. II. Electron Densities and Expectation Values

Effects of correlation on one-electron distribution and other properties are calculated for the H3+ molecule ion. Effects due to singly substituted and doubly substituted configurations are essentially additive for this closed shell species. The effect of doubly substituted configurations is to shift charge from the molecular center to nuclear regions, mainly outside the molecule. This is rationalized in terms of the many-electron theory of Sinanoğlu. The singly substituted configurations cause a slight electronic contraction similar to that caused by initial atomic—molecular formation. Neglect of singly substituted configurations is not justified when one is seeking improvement of properties beyond the HF-SCF level.

The solvation of monatomic ions

Abstract The significance of plots of hydration enthalpies vs ionization potentials is questioned. Aqueous ionization enthalpies for transition-metal ions (crystal-field-corrected), and ionization potentials, are linearly related, but a hook sequence in similar plots for closed-shell species is remarked. Several other solution properties are shown to follow such a sequence, and the basis for this is discussed. The persistence of the hook relationship through many apparently dissimilar solvents is explained by the comparability of the solvent moieties. The solvation free energies of the alkali ions relative to the H + values are shown to be largely determined by the H + -moiety interaction involving the protonic charge and average moiety polarizability.

The independent yields of 140La, 90Y, and 91Y in the thermal neutron fission of 235U and 233U

The independent yields of 140La have been radiochemically determined for the thermal neutron fission of 235U and 233U to be 4·5 × 10 −3 per cent and 2·4 × 10 −2 per cent respectively. A study of the hypotheses advanced to date concerning charge distribution in fission has indicated that the equal charge displacement postulate still gives the best agreement with experimental values, although additional data may prove this to be fortuitous. The 140La yields are considerably lower than the values estimated from this postulate, as is to be expected for an 83 neutron configuration if closed shell species are favoured in the fission process. An investigation of the apparent independent yield of 91Y has produced evidence that some mechanism other than direct formation has contributed to the high yields observed. Attempts have been made to measure the independent yield of 90Y, and an upper limit of 4 × 10 −4 per cent has been estimated for 235U and 233U fission. If, however, selectivity does exist in the region of closed shells, 90Y yields much below these limits can be anticipated due to the effects of the 50 neutron shell.

Studies of the Interaction between Stable Molecules and Atoms. I. A Molecular Orbital Theory of the Activation Energy between Molecules and Atoms

The molecular orbital method of calculation is applied to the interaction between closed-shell species; the results discussed in this paper and presented in the following papers of this series indicate that this method will yield quantitative results for energies of a system as function of internuclear parameters if one is interested in the differences in energy for two different geometric ground state configurations of the system. The molecular orbitals are used as a basis of constructing correlation diagrams for predicting mechanisms and calculating activation energies for chemical reactions. The orbitals of the reactants and products are correlated with the orbitals of a state defined as the ambivalent complex. The low-lying excited states of the interacting molecules seem to be of importance in determining the activation energy of a chemical reaction. Considerable qualitative insight into the activation process is gained by this method; the relative values of the activation energies for the interaction between two closed-shell molecules, a molecule and a radical, and two radicals are predicted in agreement with experiment.