Dipole effect

The pA of 1,3-dithiane is 36.5 (Cs" ion pair in THF). The value for 2-phenyl-1,3-dithiane is 30.5. There are several factors which can contribute to the anion-stabilizing effect of sulfur substituents. Bond dipole effects contribute but carmot be the dominant factor because oxygen substituents do not have a comparable stabilizing effect. Polarizability of sulfur can also stabilize the carbanion. Delocalization can be described as involving 3d orbitals on sulfur or hyperconjugation with the a orbital of the C—S bond. MO calculations favor the latter interpretation. An experimental study of the rates of deprotonation of phenylthionitromethane indicates that sulfur polarizability is a major factor. Whatever the structural basis is, there is no question that thio substituents enhance... 

The steric and dipole-dipole effects of the CF3 group on valence isomerization in the Dewar pyridine-azaprismane-pyridine system have been studied. These reveal themselves in the high stability, compared to the pyridine, of the valence isomer arising out of the large activation energy for rearomatization. The transformation of a 1-Dewar to 2-Dewar pyridine was observed (89T3115). 

The metastable species involved in these reactions may be either excited charge-transfer complexes or biradicals, although a charge-transfer complex would be expected to result in a predominance of head-to-head dimers. Dipole effects, on the other hand, would favor formation of head-to-tail dimers, especially in relatively nonpolar solvents ... 

Reactions of n-n excited molecules have been termed half-allowed reactions on this basis. 131> The zn-n state should be stabilized by the polar solvent I82,i33)( and one would then expect less of the head-to-head dimer. This argument ignores the dipole-dipole effect and could be invalid on those grounds. 

A related methodology that makes use of the calculated surface charges at the cavity surface to estimate the interaction with the solvent has been described in Ref. [54] in addition, the reaction field model can be extended to include the effects of higher order multipoles [55], In the present implementation, only dipole effects are considered. 

The higher nucleophilicity of the yS-oxide ion may be attributed to a steric factor in combination with a kinetically effective stereoelectronic effect that results from repulsions of lone electron pairs, dipole effects, or both (Scheme 5) (45,46). This effect should be more pronounced in anomericy3-oxide ions... 

He stressed the significance of measurements of molecular dipole moment, the principles of which had been established in 1912 by Debye.52 Ingold predicted that mesomeric systems would display measurable dipole effects. 53 He later said that he realized at this time that the theory of dipolar charge and intermediate form was... 

Additions to the anti,anti and anti,syn aldehydes exhibited a more typical matched/mismatched profile (equations 28 and 29). It was concluded that dipole effects of the /S-OTBS group must play an important role in the mismatching observed with these latter two aldehydes. 

About 200 mV of these potential differences arises from dipole effects at the interface and should be subtracted from each value to give the double-layer contribution to the measured potentials. Compare these corrected values with the values of p0 calculated by the equation given. 

There is evidence to suggest that competing dipole effects will alter the preferred conformation. Thus, for example, halogens will prefer a conformation in which the dipoles are anti to one another. This is often described as the Cornforth model In this model the highly polarized group will take the place of the L-group of the Cram model. 

The positions of substitution, orientation, and configuration of the stable form are determined by a balance between opposing steric and dipole effects.16,17,18,19 There is less agreement regarding the factors influencing kinetically controlled reaction (see below). Essentially neutral conditions, such as provided by an acetate or pyridine buffer, are required to avoid isomerization. Frequently, however, bromination will not proceed under these conditions, and a compromise has been used in which a small amount of acid is added to start and maintain reaction, while the accumulation of hydrogen bromide is prevented by adding exactly one equivalent of acetate... 

Molecules in polar liquids such as water, liquid ammonia, sulfuric acid, and chloroform are held together by dipole-dipole and hydrogen bonding interactions. For molecules of comparable size, these are stronger than London forces resulting in the familiar trends in boiling points of nonmetal hydrides. For the heavier molecules, such as H2S, H,Se, PH3, and HI, dipole effects are not particularly important (the elec-... 

Davidson (1971) determined that the most important rotation inhibition interactions between guest molecules (in adjacent cages) were the dipole-dipole interactions, but even they were of minor importance. Within a single cage, both nonpolar and polar guest molecules such as EO, THF, and acetone have only small barriers to rotational freedom, which approximates that in the vapor phase. The rotational freedom is probably due to the fact that the sum of the cage water dipoles effectively cancel near the center of each cage, and even the quadrupolar fields are relatively small. 

The discontinuity of the interface leads to two contributions to the second order nonlinear polarizability, the electric dipole effect due to the structural discontinuity and the quadrupole type contribution arising from the large electric field gradient at the surface. Under the electric dipole approximation, the nonlinear susceptibility of the centrosymmetric bulk medium 2 is zero. If the higher order magnetic dipole... 

If the metal ion to which a ligand is co-ordinated is in a non-zero oxidation state, it will exert an electrostatic effect upon the bonding electrons of the ligand. This will result in the induction of a net permanent dipole in the ligand, with any associated chemical and physical effects. Even zero-oxidation state metal centres may induce a polarisation in the ligand through electronegativity or induced dipole-dipole effects. 

Solvent effects on nuclear magnetic resonance (NMR) spectra have been studied extensively, and they are described mainly in terms of the observed chemical shifts, 8, corrected for the solvent bulk magnetic susceptibility (Table 3.5). The shifts depend on the nucleus studied and the compound of which it is a constituent, and some nuclei/compounds show particularly large shifts. These can then be employed as probes for certain properties of the solvents. Examples are the chemical shifts of 31P in triethylphosphine oxide, the 13C shifts in the 2-or 3-positions, relative to the 4-position in pyridine N-oxide, and the 13C shifts in N-dimethyl or N-diethyl-benzamide, for the carbonyl carbon relative to those in positions 2 (or 6), 3 (or 5) and 4 in the aromatic ring (Chapter 4) (Marcus 1993). These shifts are particularly sensitive to the hydrogen bond donation abilities a (Lewis acidity) of the solvents. In all cases there is, again, a trade off between non-specific dipole-dipole and dipole-induced dipole effects and those ascribable to specific electron pair donation of the solvent to the solute or vice versa to form solvates. 

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