Dipol character

We see that the electric dipole allowed transitions are, in general, much more intense than the magnetic dipole allowed transitions. In fact, the magnetic dipole contribution to an optical transition of a center dominated by an electric dipole character is usually completely masked by the much more intense J electric dipole transitions. 

For electric multipolar interactions, the energy transfer mechanism can be classified into several types, according to the character of the involved transitions of the donor (D) and acceptor (A) centers. Electric dipole-dipole (d-d) interactions occur when the transitions in D and A are both of electric dipole character. These processes correspond, in general, to the longest range order and the transfer probability varies with l/R, where R is the separation between D and A. Other electric multipolar interactions are only relevant at shorter distances dipole-quadrupole (d-q) interaction varies as l/R, while quadrupole-quadrupole interaction varies as l/R °. 

The Tg is related to chain stiffness and the geometry of the polymer chain. Flexible polymers with methylene and oxygen atoms in the chain, such as polyethylene, polyoxymethylene, and polysiloxane (silicone), have relatively low Tg values. The Tg of polyoxymethylene is somewhat higher than would be anticipated because of the dipole character of the C—O—C group, which increases the intermolecular forces and restricts segmental motion. 

In carbon monoxide the bond between the atoms depends, as in the N2O molecule, on an asymmetrical electron shift, electrons of the 0 atom moving toward the C atom, and the CO molecule having a dipole character. In this case, too, metal electrons are displaced toward the adsorbed molecule and taken from the electron gas, as shown by the change of the electrical resistance of thin nickel films on carbon monoxide adsorption (18). 

These correlated fluctuations themselves ride on a further set of coherent fluctuations taking place at a much lower frequency scale and normally attributed to the phonons, the traditional exchange Bosons associated with superconductivity. Real systems are never devoid of ionic or nuclear motion, and at the very least it is now Hamiltonian (3) (and eventually its extension to alloys) that applies for a full discussion of superconductivity density fluctuations in the nuclear coordinates are omnipresent and of course their effects on electronic ordering have been evident for quite some time. An elementary estimate of the relative importance of (monopole) polarization arising from phonons and the (multipole) equivalents arising from internal fluctuations, primarily of a dipole character, can now be easily given. 

The spectroscopy of the SD0 level is also a valuable tool to investigate the point symmetry of Eu3+ ions in materials. It is well-known that the 5D0 - 7F2 transition is electric-dipole in nature, while 5D0- -7Fi shows a magnetic-dipole character. 5D0-> 7F2 is totally forbidden in presence of an inversion center and it is allowed in the opposite case. Usually, in vitreous phases, where the symmetry is low, electric dipolar transitions exhibit the strongest intensity. The fluorescence spectra displayed in Fig. 8 show that both transitions have about the same intensity indicating that Eu3+ ions are in high-symmetry sites in fluorozirconate glasses, relatively to oxide glasses. 

The narrow absorption and emission bands of rare-earth 0-diketonates in the visible, near ultra-violet and near infra-red is attributed to 4f-4f transitions. These transitions are electric dipole forbidden to first order, but are allowed by the electric quadrupole, vibronic, magnetic dipole and forced electric dipole mechanisms. The magnetic dipole character of the Dq F transition of the Eu + ion was demonstrated in 1939 by... 

A standard question in spectroscopy is does a given molecule have a permanent dipole moment This can usually be decided immediately by inspection, but the formal group-theoretical answer is that a molecule can exhibit a component of permanent dipole for each occurrence of the totally symmetric To in the dipole character T (/u.) = xyz = Ft- Similar reasoning can be extended to other multipole moments, polarisabilities and corresponding magnetic properties. 

Complex Formation — The hydroxyl groups offer two types of interactions with molecules having either a clearly dipole character or charge, i.e., ions. The hydrogen atoms of these groups are capable of interactions with electron-excessive sites of dipoles and anions, whereas lone electron pairs of the oxygen atom are electron donors for cations and the positive side of dipolar molecules. 

In principle, a chemical shift calculation represents a perturbation theory, because of the presence of an external field Bz and magnetic moments due to the dipole character of nuclei. Therefore, perturbations to the Hamiltonian and the wave function have to be considered. The next important point is that the origin of the vector potential Az is not fixed due to the relation Bz = rot Az- Any change of the gauge origin Rq should not change any measurable observable. Therefore, a gauge transformation of the wave function 1%) and Hamilton operator h is essential... 

Dipole character n is a measure of the ability of the solvent to interact with a solute by dipolar and polarization forces. 

PVDF membranes have been used occasionally (Hicks and Vecoli, 1987 Williams, 1990). They have binding characteristics similar to charged nylon although the binding is hydrophobic, with possible electrostatic interactions since PVDF polymers have a strong dipole character. This may be the reason why PVDF functions very well in alkaline conditions. 

In general, transitions have partial magnetic dipole and partial induced electric dipole character. The total experimental dipole strength then contains contributions of both electric dipole and magnetic dipole transitions ... 

Due to its dipole character, water is a good solvent of ionic compounds and is particularly suitable for the formation of addition compounds with ionic substances or with substances having a dipole character (Fig. 3.9). The... 

Fig. 3.9. Tbe effect of polarity (of dipole character) of water molecules on the solution of ionic compounds, a — crystalline lattice of NaCl, b — water molecules surround the chloride anions and sodium cations, c — water molecules remove the Na and Cl ions from crystalline lattice, surround them and prevent their return to the lattice...

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