Dipole aromatic molecules

Figure 1. Nonlinearity of some monosubstituted aromatic molecules in terms of their dipole moments (ir electron contributions).

Polymer films were produced by surface catalysis on clean Ni(100) and Ni(lll) single crystals in a standard UHV vacuum system H2.131. The surfaces were atomically clean as determined from low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Monomer was adsorbed on the nickel surfaces circa 150 K and reaction was induced by raising the temperature. Surface species were characterized by temperature programmed reaction (TPR), reflection infrared spectroscopy, and AES. Molecular orientations were inferred from the surface dipole selection rule of reflection infrared spectroscopy. The selection rule indicates that only molecular vibrations with a dynamic dipole normal to the surface will be infrared active [14.], thus for aromatic molecules the absence of a C=C stretch or a ring vibration mode indicates the ring must be parallel the surface. 

The results obtained show that the dipole-relaxational motions in protein molecules are really very retarded as compared to such motions in the environment of aromatic molecules dissolved in liquid solvents (where they occur on a time scale of tens of picoseconds).(82) Dipole-relaxational motions on the nanosecond time scale have been observed for a variety of proteins. For example, such motions were recorded for apohemoglobin and bovine serum albumin0 04 105) labeled with the fluorescent probe 2,6-TNS. 

Azulene. The absorption spectrum of azulene, a nonbenzenoid aromatic hydrocarbon with odd-membered rings, can be considered as two distinct spectra, the visible absorption due to the 1Lb band (0-0 band near 700 nm) and the ultraviolet absorption of the 1L0 band.29 This latter band is very similar to the long wavelength bands of benzene and naphthalene CLb) and shows the same 130 cm-1 blue shift when adsorbed on silica gel from cyclohexane.7 As in the case of benzene and naphthalene, this blue shift is due to the fact that the red shift, relative to the vapor spectra, is smaller (305 cm"1) for the adsorbed molecule than in cyclohexane solution (435 cm"1). Thus it would appear that the red shifts of the 1La band are solely due to dispersive forces interacting with the aromatic molecule, in agreement with Weigang s prediction,29 and dipole-dipole interaction is negligible. 

In 1969, Suppan reviewed experimental data about dipole moment changes in excited states of substituted aromatic molecules and suggested a theoretical approach according to which charge transfer occurs if the lowest vacant orbitals are very close in energy.41 In 1978, Birks introduced the term horizontal radiationless transition, which was applied to intramolecular rotation in stilbene and polyene derivatives.37 In this... 

Induced dipolar interactions. The electron clouds in many (especially large) organic molecules are readily polarised resulting in the formation of induced dipoles that can interact, resulting in complex stabilisation. Both cations and anions can induce dipoles in aromatic molecules, for example. 

Here o represents the electron density per carbon-carbon bond, k is the strength of the induced magnetic dipoles in a given polyaromatic hydrocarbon relative to that in benzene, and / is a factor that corrects for the assumption that the electrons move in rectilinear segments, chosen by Pauling to be 1.23. With this approach, the magnetizability of almost any aromatic molecule may be calculated on the back of an envelope asf = oeW r nj ... 

Let us remark that in crystals consisting of aromatic molecules, to which the theory of Sternlicht and McConnell (26) was applied, the excited triplet states are not three-fold degenerate even when an external magnetic field is absent. Due to the dipole spin-spin interaction between electrons the degeneracy is totally or partially removed, depending on the symmetry of the excited state wavefunction. By a phenomenological description of this splitting the so-called Spin-Hamiltonian is usually applied... 

Fig. 1.2. The field induced magnetic moment is depicted schematically in this drawing. This effect is most pronounced in aromatic molecules such as fluorobenzene, where comparatively strong electron ring currents may be induced, leading to a field induced, molecular magnetic dipole moment which opposes the exterior field. Trying to align the induced moment, the exterior field will exert a torque ind X H on the molecule and will thus perturb the overall rotation. This perturbation is seen as a splitting in the rotational spectra. Since there will be a torque only in the case that md and H are not aligned, i.e., if Jg is anisotropic, only the anisotropies of the molecular susceptibility tensor can be obtained from the splittings of the rotational lines...

Compounds of the type M[Al2(CH3)eX] react with small aromatic molecules to form nonstoichiometric liquid complexes for those cases in which the anion structure has both a separation of organic and inorganic areas and an appreciable dipole moment. For example, the structure of the anion of a substance which exhibits this type of behavior, K[Al2(CH3)eN3] (10), was found to be ... 

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