Substituent polarity, dipole moments

High electric polarity (dipole moment) of substituent groups in the organics... 

When the ring has no polar substituents the dipole moments of imidazole and its derivatives are of the order of 3.8-4.0 D.163-165 A nitro substituent in a condensed ring increases this value by 2.0-2.5 D,166 whereas A-arylation lowers the dipole moment due to conjugation of the imidazole and aryl rings.167 Further measurements168 (from dielectric data in benzene at 25°C) have shown that in N-arylimidazoles, the phenyl ring is out of the plane of the imidazole... 

No effect of solvent polarity on the position of the photostationary state was found for 4,4 -CMS [85,153], in contrast with the case of 4,4 -NMS, despite the fact that the nitro and cyano groups are equally polar substituents. The dipole moments of 4,4 -CDS and 4,4 -NDS are similar in the ground state (7 Debye) and only slightly different in the lowest excited singlet state (20-29 and 25-32 Debye, respectively, according to the various methods employed [155-158]). 

Substituents exert a decisive influence on the polarity (dipole moments) and mobility (steric inhibition) of macromolecular chains. 

The unequal distribution of electron density in covalent bonds produces a bond dipole, the magnitude of which is expressed by the dipole moment, having the units of charge times distance. Bonds with significant bond dipoles are described as being polar. The bond and group dipole moments of some typical substituents are shown in Table 1.7. 

Based on the fundamental dipole moment concepts of mesomeric moment and interaction moment, models to explain the enhanced optical nonlinearities of polarized conjugated molecules have been devised. The equivalent internal field (EIF) model of Oudar and Chemla relates the j8 of a molecule to an equivalent electric field ER due to substituent R which biases the hyperpolarizabilities (28). In the case of donor-acceptor systems anomalously large nonlinearities result as a consequence of contributions from intramolecular charge-transfer interaction (related to /xjnt) and expressions to quantify this contribution have been obtained (29). Related treatments dealing with this problem have appeared one due to Levine and Bethea bearing directly on the EIF model (30), another due to Levine using spectroscopically derived substituent perturbations rather than dipole moment based data (31.) and yet another more empirical treatment by Dulcic and Sauteret involving reinforcement of substituent effects (32). 

IR spectroscopy is not only useful for determining the chemical constitution of polymers. It additionally provides profound information on chain orientation and on the orientation of attached lateral substituents of polymers. In this case, polarized IR radiation is applied which is only absorbed by an IR-active bond if the plane in which the electrical field vector E of the IR beam oscillates is parallel to the transition dipole moment p of the vibration to be excited. If, on the other hand, the transition dipole moment p is perpendicular to the electrical field vector E of the IR beam no absorption is observed. Using this effect, the degree of orientation of a polymer sample (film, fiber) can be estimated by comparing the intensity at maximum /(11) and at minimum I ) absorption, i.e., the dichroic ratio. 

F 1608 cm- Aromatic C=C stretch This band is strong since the aromatic ring has polar substituents which increase the dipole moment of the C=C bonds in the ring... 

Complexes of platinum(II) with /J-monothiodiketonates, derived by deprotonation of the parent acid (207), can be prepared from PtCl -. The dark red complex Pt(C3H(Ph)2SO 2 (R = R = Ph) shows IR bands at 1535 cm-1 [v(C=C)], 1410 cm-1 [v(O O)] and 1270 cm-1 [v(O S)].1824 Electronic spectra and dipole moment data for these complexes have been compared with the O.O -diketonate complexes.182s 1826 The structure of the phenyl derivative has been confirmed by X-ray crystallography.1827 Detailed dipole moment measurements using static polarization have been made with fluorinated jS-monothiodiketone complexes. Variations with substituent depend on the magnitude and vector directions of the Ph—X bond moments (aryl substituents), the inductive effect of the meta and para substituent on the phenyl ring, and the mesomeric effect of the substituent X.1828 A useful separation method for bis(monothiotrifluoroacetylacetonates) of platinum(II) is gas chromatography.1829... 

Quantitative structure-activity relationships (QSARs) are important for predicting the oxidation potential of chemicals in Fenton s reaction system. To describe reactivity and physicochemical properties of the chemicals, five different molecular descriptors were applied. The dipole moment represents the polarity of a molecule and its effect on the reaction rates HOMo and LUMO approximate the ionization potential and electron affinities, respectively and the log P coefficient correlates the hydrophobicity, which can be an important factor relative to reactivity of substrates in aqueous media. Finally, the effect of the substituents on the reaction rates could be correlated with Hammett constants by Hammett s equation. 

M.O. calculations have predicted that all of the heterobenzenes have the same direction of polarization with negative end of the dipole towards the heteroatom67-70). This has been confirmed by substituent effects from pyridine, phos-phabenzene and arsabenzene 27). In each case the 4-methyl derivatives have dipole moments which exceed the parent compound since the electron donating methyl group reinforces the ring dipole. 

This study is the first where semiquantitative use of relaxation data was made for conformational questions. A similar computer program was written and applied to the Tl data of several small peptides and cyclic amino acids (Somorjai and Deslauriers, 1976). The results, however, are questionable since in all these calculations it is generally assumed that the principal axis of the rotation diffusion tensor coincides with the principal axis of the moment of inertia tensor. Only very restricted types of molecules can be expected to obey this assumption. There should be no large dipole moments nor large or polar substituents present. Furthermore, the molecule should have a rather rigid backbone, and only relaxation times of backbone carbon atoms can be used in this type of calculation. 

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