Exchange polarization effects in the interaction of closed-shell systems

Explicit formulae for the calculation of the exchange polarization energy in the interaction of closed-shell atoms or molecules have been derived by assuming neglect of the electron correlation within the noninteracting systems. The dispersion part of the exchange polarization energy has been represented as a sum of contributions arising from the interaction of two, three or four orbitals at a time. Each of these contributions is given by an integral involving the orbitals engaged in the interaction and the pair functions describing the dispersion interaction between these orbitals. The numerical calculations for the interaction of two ground-state beryllium atoms show that the exchange dispersion energy is positive and quenches about 5 to 10 per cent of the dispersion term. This results in a decrease of the interaction energy, computed as a sum of the SCF and dispersion components, by 6 to 30 per cent for interatomic distances ranging from 10 to 7 bohrs.Simplified formulae for estimating the exchange dispersion energy in the interaction of larger systems are also proposed and their accuracy is discussed.     

Examination of Ortho effects in the collisionally activated dissociation of closed-shell aromatic ions

The collisionally activated dissociation of a variety of isomeric disubstituted aromatic ions formed by ion–molecule reactions were examined in order to characterize ortho effects in closed-shell systems. Closed-shell ions of methoxyacetophenone, hydroxyacetophenone, methoxyphenol, anisaldehyde and hydroxybenzaldehyde were formed by proton transfer, methyl addition or methyne addition by using dimethyl ether or ethylene oxide as chemical ionization reagents, and then the structures of these adducts were studied by deuterium-labelling methods and by collisionally activated dissociation techniques in a triple quadrupole mass spectrometer or a quadrupole ion trap. Typically, the meta and para isomers have qualitatively similar dissociation spectra which reflect the types of dissociation reactions observed for the corresponding monosubstituted aromatic ions. The predominant dissociation pathways of the [M + H]⁺ and [M + 15]⁺ ions are directed by the electron-withdrawing substituents, whereas the major dissociation pathways of the [M + 13]⁺ ions are related to the electron-releasing substituent. In contrast, the dissociation routes of the corresponding ortho isomers are dramatically different. This is attributed to the opportunity for functional group interactions of the ortho isomers which facilitate alternative pathways.     

On the role of higher-order correlation effects on the induction interactions between closed-shell molecules

High-order correlation contributions to the second-order induction energy were studied for various representative van der Waals complexes. It was found that the induction energy obtained by the truncation of the relaxed M?ller-Plesset expansion in the second or third order is in most cases quite close to the induction energy computed with the coupled-cluster method (restricted to single and double excitations). Also, the effect of triples excitations on this perturbation term is usually small. However, given an oscillatory behaviour of the M?ller-Plesset induction corrections, the coupled-cluster method seems to be better suited to a reliable calculation of the induction energy. The sources of the remaining differences between the interaction energies computed by symmetry-adapted perturbation theory and those computed by the supermolecule coupled-cluster method (restricted to single, double, and noniterative triple excitations) are examined. It has been found that they can be attributed to the higher-order correlation terms in the second-order dispersion and exchange-induction corrections.     

Information theory interpretation of the Pauli principle and Hund rule

Making use of different information measures, the Pauli exclusion principle and the first Hund rule were found to be related to a trend toward acquiring a maximum information content of the atoms and molecules, and more generally, for any fermion system. The bosons conversely always have a minimum (zero) information content with respect to the permutation symmetry of their wave functions. Some new interpretations of the Pauli principle are presented.     

New possibilities of Möller-Plesset perturbation theory for calculating correlation energies of closed-shell systems

A new scheme for generating and selecting configurational wave functions (CWF) including the correlation effects is suggested. Standard Möller-Plesset perturbation theory (SMP) is modified in such a way that the newly constructed CWF and the Hartree-Fock reference determinant are the eigenfunctions of a zero-order Hamiltonian. Possessing all advantages of SMP, the calculation scheme allows for more than 80% of the empirical correlation energy in the second order. The results for diatomic hydrides are comparable in accuracy to those obtained by the configuration interaction method.     

Ab initio Hartree-Fock instabilities in closed-shell molecular systems

The Hartree-Fock instability of twelve polyatomic systems is studied at theab initio level. It is found that all the systems with at least one double bond, exhibit a non-singlet instability. On the other hand instabilities of singlet type as well as instabilities of non-real type appear only in a small number of cases. The existence of these instabilities is discussed with respect to the location of low-lying excited states and to the weight of ionic structure.     

Modulation effects in the electron spin echo of biradical systems

The results of a theoretical and experimental investigation of the modulation effects in the electron spin echo of biradical systems are presented. SO ₄ ^– radicals stabilized as pairs in UV-irradiated potassium persulfate and frozen solutions of the stable N,N-[bis-(, -diphenylpicrylhydrazyl)--picryl-3]-piperazine biradical (A) were chosen as models. The modulation effects in the ESE of SO ₄ ^– radicals in potassium persulfate are shown to be caused by a dipole-dipole interaction between the radicals. The results of the study of the ESE of the A-biradicals leads to the assumption that the observed modulation effects are caused by an exchange interaction between unpaired electrons.The authors are grateful to V. E. Khmelinskii, M. D. Shchirov, A. M. Raitsimring, and R. I. Zusman for their help in carrying out the experiments. The authors are especially grateful to R. O. Matevosyan for kindly supplying biradical A.     

Molecular design of ambipolar redox-active molecules II: closed-shell systems

This mini-review highlights key structural features that should be taken into account when creating ambipolar redox-active closed-shell metal-free molecules. This type of compound is strongly required for the fabrication of all-organic ‘poleless’ batteries and semiconductors. The suggested strategies aimed at stabilization of both oxidized (cationic) and reduced (anionic) redox-states are based on the comprehensive analysis of the most successful structures taken from the recent publications.     

A note on the orbital energy-splitting of closed-shell systems in the DODS scheme

Investigating the lowering of the total energy and the orbital energy-splittings of electrons with spins α and β, respectively, in the DODS scheme, ab initio calculations have been performed for the formaldehyde molecule. The results obtained by changing the CO distance and the orbital exponents of the C and O atoms indicate that the decrease of the electron density in the CO bond region causes larger orbital energy-splittings and lowering of the energy.     

Many-orbital cluster expansion for the exchange-repulsion energy in the interaction of closed-shell systems

The energy of exchange repulsion between two closed-shell systems described by determinantal wave functions has been represented as a sum of contributions arising from the interaction of two, three and four orbitals at a time. These contributions have been calculated for the interaction of two neon atoms. It has been found that in the van der Waals minimum region the two-orbital components are of secondary importance and that about 90% of the total exchange energy originates from the three-orbital interactions ofL-shell electrons. The four-orbital as well as the double-exchange terms have been found negligible. The approximate algorithms for evaluation of the exchange repulsion energy have been tested and discussed.This work was partly supported by the Polish Academy of Sciences within the project MR-I.9.     

Tunable photoluminescence of closed-shell heterobimetallic Au-Ag dicyanide layered systems

The excited-state properties of the layered La[Ag(CN)(2)](3) and La[Au(CN)(2)](3) systems have been examined and compared with mixed-metal systems of varying metal ratios such as La[Ag(0.78)Au(0.22)(CN)(2)](3), La[Ag(0.55)Au(0.45)(CN)(2)](3), La[Ag(0.33)Au(0.67)(CN)(2)](3), and La[Ag(0.19)Au(0.81)(CN)(2)](3). We have found that these mixed-metal systems luminesce quite strongly at room temperature at an energy that is tunable and depends on the Au:Ag stoichiometric ratio. The emission energy of the mixed-metal samples lies between those of the pure Au and Ag systems. This provides evidence that the excited states responsible for this emission are delocalized over the Ag and Au centers. The strong luminescence of the mixed-metal systems at ambient temperatures is in stark contrast to the weak luminescence behavior of pure La[Au(CN)(2)](3) and La[Ag(CN)(2)](3) samples, which makes the mixed-metal systems more viable than the pure systems for practical applications.     

Investigation of the mobility of amphiphilic polymer--AOT reverse microemulsion systems using electron spin resonance

Amphiphilic polymers can be used as tools to manipulate the behavior of reverse microemulsions. EPR spectroscopy employing the spin probe 5-doxyl stearic acid was used to study the adsorption of a comb type polymer (polymaleic anhydride octyl vinyl ether) and a diblock polymer (polybutadiene-ethylene oxide) onto reverse microemulsion droplets formed from Aerosol-OT/heptane/water. The findings indicate that the comb type polymer was adsorbed by the reverse microemulsion drops at low polymer concentrations causing a structural change of the micelle.     

On a new partitioning of the Hamiltonian in many-body calculation of pair-correlation energies in closed-shell systems

We introduce here a new partitioning of the Hamiltonian in calculating pair-correlation energies using many-body perturbation theory, by which we are able to eliminate the off-diagonal particle–hole (p–h) ladders exactly to all orders in the perturbation expansion. In this formulation, the particle states turn out to be different for each distinct pair of hole states in the correlation energy calculation. We have also included the contributions of the diagonal particle–particle (p–p) and hole–hole ladders exactly to all orders. The effect of the off-diagonal p–p ladders has been estimated for each pair by computing the third-, foruth- and fifth-order energies. For highly symmetric systems the present partitioning yields in general symmetry-broken orbitals. Here one may use an average kind of partitioning for all the partners of the degenerate sets, which restores the symmetry and at the same time ensures cancellation of the p–h ladders exactly at the lowest order and approximately at the higher orders. Results are presented for a selection of 6π-electron conjugated systems. The correlation energy for each pair is in excellent agreement with that obtained from a partial CI calculation involving all double excitations from this pair. The advantages of implementing the present scheme in larger systems has been discussed.     

Light-induced excited-state spin trapping in tetrazole-based spin crossover systems

Ab initio calculations have been performed on Fe (II) (tz) 6 (tz = 1- H-tetrazole) to establish the variation of the energy of the electronic states relevant to (reverse) light-induced excited-state spin trapping (LIESST) as function of the Fe-ligand distance. Equilibrium distances and absorption energies are correctly reproduced. The deactivation of the excited singlet is proposed to occur in the Franck-Condon region through overlap of vibrational states with an intermediate triplet state or an intersystem crossing along an asymmetric vibrational mode. This is followed by an intersystem crossing with the quintet state. Reverse LIESST involves a quintet-triplet and a triplet-singlet intersystem crossing around the equilibrium distance of the high-spin state. The influence of the transition metal is studied by changing Fe (II) for Co (II), Co (III), and Fe (III). The calculated curves for Fe (III) show remarkable similarity with Fe (II), indicating that the LIESST mechanism is based on the same electronic conversions in both systems.     

Accurate all-electron correlation energies for the closed-shell atoms from Ar to Rn and their relationship to the corresponding MP2 correlation energies

All-electron correlation energies E(c) are not very well-known for atoms with more than 18 electrons. Hence, coupled-cluster calculations in carefully designed basis sets are combined with fully converged second-order M?ller-Plesset perturbation theory (MP2) computations to obtain fairly accurate, nonrelativistic E(c) values for the 12 closed-shell atoms from Ar to Rn. These energies will be useful for the evaluation and parameterization of density functionals. The results show that MP2 overestimates ∣E(c)∣ for heavy atoms. Spin-component scaling of the MP2 correlation energy is used to provide a simple explanation for this overestimation.     

Topological symmetry control in spin density distribution: spin chemistry of phenalenyl-based neutral monoradical systems

[structure: see text] Topological symmetry-based extensions of a pi-conjugation network in an odd alternant phenalenyl radical have enabled us to control the spin density distribution systematically. ESR/ENDOR and NICS studies on the topological isomers of oxophenalenoxyl have revealed that the unpaired electron tends to localize in the antiaromatic ring systems.     

Cooperativity and electron correlation effects on hydrogen bonding in infinite systems

Structural and electronic properties of hydrogen-bonded infinite chains of hydrogen cyanide and formamide molecules have been investigated by the ab initio crystal orbital method using several, partly highly polarized, atomic basis sets of increasing size at the Hartree–Fock (HF ) level and by including electron correlation effects in the second order of Møller–Plesset perturbation theory. The results obtained show that hydrogen bonding in molecular crystals of the type investigated is a highly cooperative phenomenon, both from the structural and energetic points of view. Comparison with clusters of up to four monomers demonstrate how various structural parameters converge toward their limiting values in the infinite system. The results obtained for infinite HCN chains show an excellent agreement with those observed for solid HCN, whereas the infinite formamide chain proves to be a reasonable model for the corresponding liquid phase. © 1994 John Wiley & Sons, Inc.