Anisotropic distribution, of electron

XB is a particularly directional interaction, more directional than HB. The angle between the covalent and non-covalent bonds around the halogen in D- X-Y is approximately 180° [48]. As discussed above, the origin of this directionality is in the anisotropic distribution of electron density around the halogen atom. Figure 5 shows the Cambridge Structure Database (CSD, ver-... 

For instance, both theoretical and experimental evidence indicates that Si bound to electron-withdrawing groups displays an anisotropic distribution of electron density, and four regions of positive electrostatic potential are present on the... 

Fig. 3 Due to the anisotropic distribution of the electron density, halogen atoms show a negative electrostatic potential and a larger radius (rmax) in the equatorial region and a positive electrostatic potential and a smaller radius (rmjn) in the polar region. As a consequence of this, halogens behave as nucleophiles at the equator and as electrophiles at the pole...

The electrons modify the magnetic field experienced by the nucleus. Chemical shift is caused by simultaneous interactions of a nucleus with surrounding electrons and of the electrons with the static magnetic field B0. The latter induces, via electronic polarization and circulation, a secondary local magnetic field which opposes B0 and therefore shields the nucleus under observation. Considering the nature of distribution of electrons in molecules, particularly in double bonds, it is apparent that this shielding will be spatially anisotropic. This effect is known as chemical shift anisotropy. The chemical shift interaction is described by the Hamiltonian... 

Let us consider an anisotropic triangular lattice with alternating values of the one-site energy a. The corresponding general formulas for the interaction between w-site segments are more cumbersome because of a more complicated dependence on the distribution of electrons through the lattice in zero PT order. 

The first attempt to clarify the physical basis of the Jahn-Teller theorem was due to Ruch, [3] in an introductory presentation to the 1957 annual meeting of the Bunsen-Gesellschaft in Kiel, which was organised by H. Hartmann. Ruch discussed the general connection between symmetry and chemical bonding, and also touched upon the Jahn-Teller effect in transition-metal complexes. He explained that degeneracy can always be related to the existence of a higher than twofold rotational axis and a wave function which is not totally symmetric under a rotation around this axis. Provided that the wave function is real the electron densities for such a wave function are bound to be anisotropic. The combination of an anisotropic distribution of the electron cloud and a symmetric nuclear frame leads to electrostatic distortion forces where the nuclear frame adapts itself to the anisotropic attraction force. 

The transient absorption method utilized in the experiments reported here is the transient holographic grating technique(7,10). In the transient grating experiment, a pair of polarized excitation pulses is used to create the anisotropic distribution of excited state transition dipoles. The motions of the polymer backbone are monitored by a probe pulse which enters the sample at some chosen time interval after the excitation pulses and probes the orientational distribution of the transition dipoles at that time. By changing the time delay between the excitation and probe pulses, the orientation autocorrelation function of a transition dipole rigidly associated with a backbone bond can be determined. In the present context, the major advantage of the transient grating measurement in relation to typical fluorescence measurements is the fast time resolution (- 50 psec in these experiments). In transient absorption techniques the time resolution is limited by laser pulse widths and not by the speed of electronic detectors. Fast time resolution is necessary for the experiments reported here because of the sub-nanosecond time scales for local motions in very flexible polymers such as polyisoprene. 

In contrast to the situation in plasmas in steady state or in time-dependent plasmas, the electron density n z) in space-dependent plasmas always depends on the z coordinate, and this happens already if only conservative inelastic collisions are considered. As an immediate consequence, it no longer makes sense to separate the density from the isotropic and anisotropic distribution of the electrons. 

The analysis of the cluster atomic and electronic structure, distribution of electron and spin density, as well as the calculation of isotropic and anisotropic hyperfine coupling constants (IHC and AHC) was carried out. 

CSA reflects the anisotropy inherent in the distribution of electronic currents about nuclei which screen (tr) them fi om the applied magnetic field Bo- The local magnetic field experienced by a nucleus is anisotropic and therefore three dimensional, so the nuclear screening constant tr is in fact a tensor and may be described [1,32] by... 

The external applied magnetic field Bq produces electronic currents in a molecule, and these currents in turn produce local magnetic fields at the various constituent nuclei. Because the electronic environment about a nucleus is not uniform, i.e. the distribution of electrons is directionally sensitive or anisotropic, the local magnetic field experienced by a nucleus is three-dimensional, and its magnitude and molecular orientation may be described [5] by the chemical shift tensor a, given by... 

The anisotropic distribution of the electron density in halogen atoms of monovalent halogen derivatives accounts for their well-established amphoteric behavior and the different geometry of interactions formed with different entering groups. 

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