Dipoles zwitterions

Recognition that a further correlation contribution exists, due to two-dimensional permanent dipole (zwitterionic) head-group fluctuations confined to a surface accounts for the discrepancy in equilibrium lamellar spacings [31,32]. 

Examine the charge on the methylidene group, as well as the magnitude and direction of the molecule s dipole moment. Are they consistent with representation of the ylide as a hypervalent molecule or as a zwitterion ... 

Many of the properties oj -hydroxypyridines are typical of phenols. It was long assumed that they existed exclusively in the hydroxy form, and early physical measurements seemed to confirm this. For example, the ultraviolet spectrum of a methanolic solution of 3-hydroxypyridine is very similar to that of the 3-methoxy analog, and the value of the dipole moment of 3-hydroxypyridine obtained in dioxane indicates little, if any, zwitterion formation. However, it has now become clear that the hydroxy form is greatly predominant only in solvents of low dielectric constant. Comparison of the pK values of 3-hydroxypyridine with those of the alternative methylated forms indicated that the two tautomeric forms are of comparable stability in aqueous solution (Table II), and this was confirmed using ultraviolet spectroscopy. The ratios calculated from the ultraviolet spectral data are in good agreement with those de-... 

The ultraviolet spectra of the pyridinecarboxylic acids (334) were initially interpreted assuming that the proportion of the zwitterion structure 335 was not appreciable,and the early pK work was inconclusive. However, Jaffe s calculations based on the Hammett equation indicated that about 95% of nicotinic and isonicotinic acids existed in the zwitterion form, and ultraviolet spectral data showed that the actual percentages of picolinic, nicotinic, and isonieotinie acids existing in the zwitterion form in aqueous solution are 94, 91, and 96%, respectively.This was later confirmed by Stephenson and Sponer, who further demonstrated that the proportion of the zwitterion form decreases in solvents of low dielectric constant, becoming very low in ethanol. Dipole moment data indicate that isonicotinic acid exists as such in dioxane, and 6-hydroxypyridine-3-carboxyiic acid has been shown to exist in form 336 u.sing pK data. ... 

Amino acid zwitterions are internal salts and therefore have many of the physical properties associated with salts. They have large dipole moments, are soluble in water but insoluble in hydrocarbons, and are crystalline substances with relatively high melting points. In addition, amino acids are amphiprotic they can react either as acids or as bases, depending on the circumstances. In aqueous acid solution, an amino acid zwitterion is a base that accepts a proton to yield a cation in aqueous base solution, the zwitterion is an add that loses a proton to form an anion. Note that it is the carboxylate, -C02-, that acts as the basic site and accepts a proton in acid solution, and it is the ammonium cation, -NH3+, that acts as the acidic site and donates a proton in base solution. 

The problem of the structure of 1,2- and 1,4-quinone diazides was investigated by Le Fevre s group (1949, 1954) by measuring dipole moments. The observed moments in benzene are in the range 2.9 to 5.0 D, compared with calculated values of 1.6 to 4.0 D for the quinone diazide structure and 15.7 and 27.4 D respectively for the 1,2-and 1,4-zwitterionic forms. No attempts were made by Lowe-Ma et al. (1988) to calculate dipole moments for the mesomeric structure 4.4 that they proposed. 

Olefins (enamines) unsymmetrically substituted with strong electron-donating (amino) group and CS generate zwitterions (1,4-dipoles) [32, 33]. Polar additions are proposed here to be reactions in the pseudoexcitation band. 

The pathways are discussed which lead to stereoisomeric SiN-ring compounds via a multiple step mechanism including zwitterionic intermediates (1.4-dipoles). We prefer path 2 because experimental evidence is in favour of this route. 

Typical examples are the conversion of the neutral form of an amino acid into its zwitterionic form, the helix-coil transitions in polypeptides and polynucleotides, and other conformational changes in biopolymers. Reactions of higher molecularity in which reactants and products have different dipole moments are subject to the same effect (association of the carboxylic acids to form hydrogen-bonded dimers). Equilibrium involving ions are often more sensitive to the application of an electric field ... 

Karelson et al. [268] used the AMI D02 method with a spherical cavity of 2.5 A radius to study tautomeric equilibria in the 4-hydroxyisoxazole system (they did not specify which hydroxyl rotamer they examined). Tautomer 17 predominates in aqueous solution. Although AMI predicts 16 to be about 10 kcal/mol more stable in the gas-phase than 17, its dipole moment is only predicted to be 0.68 D. Tautomer 17 has a predicted dipole moment of 2.83 D in the gas-phase. With the small cavity, the two dipole moments increase to 0.90 and 4.56 D, respectively, and this is sufficient to make 17 0.3 kcal/mol more stable than 16 in solution. Zwitterion 18 is much better solvated than either of the other two tautomers, but AMI predicts its gas-phase relative energy to be so high that it plays no equilibrium role in either the gas phase of solution. 

These compounds can be considered as zwitterions in the ground state. Upon excitation, electron transfer occurs from the oxygen atom to the center of the aromatic system. The dipole moment is thus about 15 D in the ground state whereas it is nearly zero in the excited state. 

Figure 1.12 Reichardt s betaine dye in its zwitterionic ground state (a) and first excited state (b). The ground state has a larger dipole moment (15 D) than the excited state (6 D). Measurement of the energy of the transition between these two states (n - n ) is the basis for the Ej scale of solvent polarity...

Photo-aggregation of the nitro-BIPS compounds continues to fascinate researchers and articles are being produced still regarding this phenomenon [24,38-45,60-62]. It is, of course, noteworthy before we continue with this discussion that spiropyran merocyanines are zwitterions possessing a significant dipole moment and that their stability in a nonpolar solvent will therefore be minimal. Spontaneous precipitation upon formation may be expected as the final outcome of the photo-formation of the merocyanine. 

The reaction of the sterically crowded thiocarbonyl ylide 69 with highly electron-deficient alkenes such as 2,3-dicyano fumarate and maleate, tetracya-noethene, a-cyano cinnamates, and l,2-bis(trifluoromethyl)ethene-l,2-dicarboni-trile occurred in a nonstereospecific manner (27,89,96,97,136-138). The formation of a mixture of cis/trans tetrahydrothiophenes of type 82 is the result of a stepwise reaction involving zwitterionic intermediates of type 81 (Scheme 5.29). Ylide 69 fulfills the fundamental requirements for a two-step reaction with electron-deficient alkenes. This species corresponds to an electron-rich 1,3-dipole that also contains a bulky substituent at one terminus (89). 

Summary of the Average Dipole Moments y. isoNEUTRAL Gas Phase isoion zwitterion in the Gas Phase solv Solvated Alanine (Average from the... 

One of the common charging mechanisms is the preferential adsorption of an ion from a solution on an initially uncharged surface. An example of this mechanism is the binding of a Ca2+ ion on a zwitterionic (which implies that the surface group consists of surface dipoles but no net charge) head group of a lipid layer. 

ZWITTERION. An ion carrying charges of opposite sign, which thus constitutes an electrically neutral molecule with a dipole moment looking like a posilive ion at one end and a negative ion at the other. Most aliphatic amino acids form such dipolar ions, hence react with both strong acids and strong bases. 

The rates of Diels-Alder reactions are little affected by the polarity of the solvent. If a zwitterionic intermediate were involved, the intermediate would be more polar than either of the starting materials, and polar solvents would solvate it more thoroughly. Typically, a large change of solvent dipole moment, from 2.3 to 39, causes an increase in rate by a factor of only 10. In contrast, stepwise ionic cycloadditions take place with increases in rate of several orders of magnitude in polar solvents. This single piece of evidence rules out stepwise ionic pathways for most Diels-Alder reactions, and the only stepwise mechanism left is that involving a diradical. 

According to Mechanism 1, the ion-dipole interaction proposed by Shah and Schulman (II), Ca++ binds to the oxygen of a polarized P-O bond in the structure of the phosphorylcholine zwitterion. In spite of the speculative claims of Shah and Schulman (II) and the theoretical affirmations of Gillespie (20), this mechanism must be rejected on the basis of the theoretical arguments presented here and elsewhere (2,5,6). In brief, if such a bond existed (and the evidence (IR or NMR) is not available), it should not bear any direct relation to AV and surface dipole moments, which are generated only by ionized species and not by silent ion pairs nor by partial charges of polarized covalent bonds (2,5). 

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