Dipole change

Just as the dipole changes in an external field, so do all the other moments, and we can develop a set of equations for the quadrupole polarizability (and hyperpolarizabilities), the octupole polarizability, and so on. These esoteric quantities are rarely met in chemistry. 

For probes with known structure and electric dipoles change A/< jie - the spectral shift measured experimentally AvaJ enables to calculate the value of... 

These temporary dipoles change constantly, but the net result of their existence is to produce attractive forces between nonpolar molecules. 

Maximum enhancement will be obtained by molecules adsorbed in such a way as to maximise vibrational dipole changes perpendicular to the particle surface. 

Thermal effects (dielectric heating) can result from dipolar polarization as a consequence of dipole-dipole interactions of polar molecules with the electromagnetic field. They originate in dissipation of energy as heat, as an outcome of agitation and intermolecular friction of molecules when dipoles change their mutual orientation at each alternation of the electric field at a very high frequency (v = 2450 MHz) [10, 11] (Scheme 3.1). 

Pure rotational spectra only appear for molecules with permanent dipole moments and vibrational spectra require a change of dipole during the motion. However, electronic spectra are observed for all molecules, and changes in the electron distribution in a molecule are always accompanied by dipole changes. As a result even homonuclear molecules (H2 or N2) which have no rotation or vibration spectra, do give electronic spectra with vibrational and rotational structure from which rotational constants and bond vibration frequencies may be derived. 

The dipole moment also plays a significant role in the transition of a molecule because a molecule can absorb radiation only when its dipole changes. In general the more symmetrical a molecule, the light will be absorbed very slowly Greater the transition dipole, greater is the absorption intensity. The ease of excitation is of the following order ... 

The spectra of methane, adsorbed at 90° K., showed a weak band at 2,899 cm.", in addition to a strong band (vt) at 3,006 cm. h This weak band was assigned to the I l symmetrical breathing frequency of methane, which is normally observed only in the bulk state in the Raman spectrum at 2,916 cm. h No over-all dipole change is associated with the vi vibration consequently, it is forbidden in the infrared spectra of liquid and gaseous methane. The appearance of this band is a direct measure of the... 

Band intensity in Raman spectra depends on bond polarizability rather than molecular dipole changes. 

The soft-mode absorption of cyclohexane on the stepped Ni[5(lll) X (110)] surface is clearly present at ca. 2620 cm, but with much reduced intensity compared with that on Ni(lll) (7), implying, as expected, less convenient CH contacts with the metal surface. The band positions in the fingerprint region are similar, but notable changes in relative intensities, e.g., of the prominent ca. 520-cm 1 absorption, suggest a nonparallel orientation with respect to the surface. Indeed the on-specular spectrum of the species on Ni[5(l 11) X (110)] bears a remarkable resemblance to the off-specular spectrum of the species on Pt(lll) (222) whereby the modes with dipole changes parallel to the surface, i.e., parallel to the median Q, plane,... 

From the geometrical similarity between piperidine and cyclohexane it may be assumed that the N-U vibration causing the dipole change in 94 is parallel to the major axis of rotation (largest moment of inertia /c) and accordingly the Q branch should be strong and the P and R bands, weak (parallel-type band). On the other hand the N-H equatorial bond in 93 has 28% parallel character (N-H bond 1110 to / ), and P, Q, and R bands of... 

Microwave Absorption of radiation due to dipole change during rotation (A = 0.1—30 cm 300-1 GHz in frequency) Mean value off—2 terms potential function IO- >s 10-2 pa (10- torr) Mean value of/-2 does not occur at rB even For harmonic motion. Dipole moment necesanry. Only one component may be detected. Analysis difficult for large molecules of low symmetry... 

Vibrational infrared Absorption of radiation due to dipole change during vibration (A - 10—10— cm) Qualitative for large molecules 10- 3 S 100 Pa (1 torrl Useful for characterization. Some structural information from number of bands, position, and possibly isotope effects. All States of matter... 

Spin-spin relaxation of nuclei is accelerated when the nuclei participate in a dipolar bond (O — H, N — H, 13C—1H). Spin-spin relaxation involving dipole-dipole interaction is very effective in solids and viscous liquids with slow molecular motion, since the magnetic fields caused by slowly tumbling dipoles change very slowly. 

We assumed in Fig. 4.2 that no surface charge or surface dipole is present in the semiconductor. In general, however, both surface charges and surface dipoles are present in the semiconductor owing to adsorption equilibria for various ions between the electrolyte and the semiconductor surface as well as formation of polar bonds at the semiconductor surface. Such surface charges and surface dipoles change the potential difference in the (outer) Helmholtz layer and thus cause shifts in the surface band positions, as shown schematically in Fig. 4.3. The shifts can be expressed as changes in 0(0) or in the above equations, with the... 

IR transmission 4000-co. 1100" >1 High Finely divided metals Yes Dipole change perpendicular to surface Very great... 

DRIFTS 4000-ca. 1100° >1 High Powdered metal/oxides Yes Dipole change perpendicular to surface Limited as yet... 

RAIRS 4000-800 (-200) ca. 1 Low Flat, preferably single-crystal, metal surfaces Yes Dipole change perpendicular to surface Moderate... 

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