The characteristic dependence of J(31P,31P) spin-spin coupling constants of alkali metal tetraphosphane-1,4-diides on structure and composition has been analyzed by density functional methods. The computations confirm that the structure of the contact ion pairs is conserved in solution. Calculations on model systems M2P4H4, on naked P4H4(2-) anions, and on models including point charges, show that the role of the cations is mainly structural and to a smaller extent electrostatic. Three of the four J(P,P) coupling constants depend characteristically on the conformation of the anion, which in turn is determined by the substituents R and by cation-anion interactions. Several couplings exhibit a large through-space component and are thus strongly dependent on the relative orientation of nonbonding electron pairs on the phosphorus atoms involved. This is shown by visualization of coupling pathways using the recently introduced coupling energy density (CED), in combination with the electron localization function (ELF).
found that hydrogen atoms trapped in silasesquioxanes unexpectedly exhibit g-values significantly larger than the free-electron value, with an appreciable dependence on the external substituents of the cage. Here it is shown by density functional calculations and detailed analyses of the g-values that (a) the positive g-shifts are predominantly due to spin-orbit coupling from the cage oxygen atoms; (b) in second-order perturbation theory the g-shifts arise mainly from couplings of t 1g oxygen-based cage orbitals to the a 1g singly occupied molecular orbital, and (c) the reduction of the g-shifts by external substituents is mainly caused by lower spin density on the cage oxygen atoms, due in part to reduced delocalization of the singly occupied MO onto the cage. Calculations with the hydrogen atom displaced from the center of the cage provide explanations for the negligible temperature dependence of the g-values compared to an experimentally notable dependence of the hyperfine couplings. Large direct halogen spin-orbit contributions and negative g-shifts are predicted for systems with heavy halogen atoms as external substituents. Comparison of different exchange-correlation functionals, and of spin-restricted and -unrestricted calculations reveals unexpected trends for spin density distribution and g-values: Spin polarization increases the computed g-shifts, whereas the admixture of Hartree-Fock exchange in hybrid functionals decreases them.
It is investigated whether cluster models can be used to describe the (001) CdTe (2×1) reconstructed surface and adsorption of Cd atoms on three different sites upon it. Surface structures and the cadmium binding energies are evaluated for clusters of different size and by slab calculations. While the structure of the pure surface depends only weakly on the surface model, a proper structure of the most stable adsorption site requires inclusion of at least the fourth next neighbor to the adsorbed Cd atom. Prediction of the structure of less bound adsorption sites is shown to be also possible with cluster models but in this case particular care has to be spent to avoid asymmetric distortions or non-realistic geometries due to border effects of the cluster. The binding energy of a Cd atom on the (001) CdTe (2×1) surface is predicted to be 1.9-2.1 eV. The motion of a Cd atom in (100)-direction (along Te-Te dimer rows) is unlikely as it requires about all of the binding energy. Adsorption of a Cd atom between two Te-dimer rows probably leads to a reconstruction of the dimer structure.
The experimental observations of novel behavior of the kinetic properties of a two-dimensional electron gas in the presence of microwave radiation are reviewed, and the various theoretical models that have been proposed for explaining them are described.
Kinetics of spatially uniform distribution of 2D electrons in crossed electric and magnetic fields in the presence of microwave radiation has been studied. In the present model the contribution from the microwave radiation and the effects of Landau quantization are considered exactly, while scattering is treated perturbatively.Here Landau-Floquet states interact with the impurity potential that causes transitions between them. The linear response of a non-equilibrium system to a weak measurement field has been considered for the case when the non-equilibrium state of the carriers can be described by the average values of the total energy and the number of particles.
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