Dipolar equilibria

Thus, basically two types of electric-chemical coupling may be differentiated, (a) permanent or induced dipolar equilibria, and (b) ionic (dissociation and association) processes involving (macro-)ions and low molecular weight ions (of preferably opposite charge sign). Whereas dipolar equilibria in electric fields are accessible to thermodynamic analysis, ionic processes involving free ions require a kinetic approach. - )... 

It is recalled that when z, = T, AZi — AH/T, where AH is the reaction enthalpy representing the enthalpy difference of one stoichiometric transition. When Zi = P, AZf = —AV, where AV is the molar partial volume change for one stoichiometric transformation. Finally, when z = E, the measured electric field, then AZ = AM, where AM is the molar reaction dipole moment. It will be shown below that AM of dipolar equilibria refers to the components parallel to E, of the dipole moments of the interacting dipolar molecules or macromolecular substructures. 

If Y is independent of E, by AM = AM the reaction moment is independent of (or of the concentrations of the reaction partners). In any case, the relationship (4.42) provides the basis for the analysis of electric field-induced concentration shift in dipolar equilibria. Whereas this part of the account dealt with thermodynamic foundations of the analysis of chemical electric field effects, the second part (Chapters) covers some kinetic and mechanistic aspects of macromolecular bioelectric processes. 

The general formalism developed here is particularly useful for the description of electric field effects on the rate constants of dipolar equilibria. We may choose z( ) = =0 as a suitable reference and specify Eqs. (2.11) as... 

Note the strong dependence on the charge number. As shown elsewhere, the equilibrium constant of a 1 1 weak electrolyte like acetic acid is increased by an electric field of 100 kV cm to about 14%, that for a 2 2 electrolyte like MgS04 to about 110%. ° Compared to simple dipolar equilibria of small molecules where electric-field-induced changes in K are very small, we see that the dissociation step of simple ion pairs is associated... 

We recall that the (practical) equilibrium constant (concentration ratio) is given by K=kjk . Thus we may formally express K of ionic (-dipolar) equilibria of the type (2.13) by... 

It should be remarked that chemical contributions of interacting anisotropic molecules are usually negligibly small if simple dipolar equilibria are concerned. Appreciable field effects are encountered only in macromolecular dipolar systems at high field intensities. On the other hand, the second Wien effect and structural changes coupled to ionic dissociation-association processes may occur at already low field intensities. 

Experimental experience demonstrates that in any case large reaction dipole moments AM) are required to produce major displacements of dipolar equilibria high ionic valencies are necessary for larger dissociation field effects in ionic association-dissociation reactions/ ... 

It must be pointed out that high fields shift any equilibria which involve a change in mobility and/or a change in dipole moment. It can be shown using Maxwell s equations that even for shifting dipolar equilibria without ion formation or combination there is a conductance change associated with the process that can be used for detection. 

The reaction dipole moment zfM of a dipolar equilibrium may be obtained from the measurement of continuum properties such as the dielectric permittivity as well as from direct monitoring of concentration shifts produced by an externally applied electric field. In both approaches to reaction properties it is primarily the chemical part of the total polarization that is aimed at. However, the chemical processes are intimately connected with the physical processes of polarization and dipole rotation. In the case of small molecules the orientational relaxations are usually rapid compared to the diffusion limited chemical reactions. When, however, macromolecular structures are involved, the rotational processes of the macromolecular dipoles may control a major part of the chemical relaxations. Two types of processes may be involved if a vectorial perturbation like an external electric field is applied a chemical concentration change and a change in the orientation of the reaction partners. 

In the absence of exchange (and ignoring dipolar relaxation), each z magnetization will relax back to equilibrium at a rate governed by its own T, as in (B2.4.44). 

Recently the solvent effect on the [4+2] cycloaddition of singlet oxygen to cyclic dienes has been subjected to a multiparameter analysis. A pre-equilibrium with charge-transfer character is involved, which is affected by the solvent through dipolarity-polarisability (n ) and solvophobic interactions ( Sjf and Another multiparameter analysis has been published by Gajewski, demonstrating the... 

When R = H, in all the known examples, the 3-substituted tautomer (129a) predominates, with the possible exception of 3(5)-methylpyrazole (R = Me, R = H) in which the 5-methyl tautomer slightly predominates in HMPT solution at -17 °C (54%) (77JOC659) (Section 4.04.1.3.4). For the general case when R = or a dependence of the form logjRTT = <2 Za.s cTi + b Xa.s (Tr, with a>0,b <0 and a> b, has been proposed for solutions in dipolar aprotic solvents (790MR( 12)587). The equation predicts that the 5-trimethylsilyl tautomer is more stable than the 3-trimethylsilylpyrazole, since experimental work has to be done to understand the influence of the substituents on the equilibrium constant which is solvent dependent (78T2259). There is no problem with indazole since the IH tautomer is always the more stable (83H(20)1713). 

The interaction of diazomethane with 1-azirines was the first example of a 1,3-dipolar cycloaddition with this ring system (64JOC3049, 68JOC4316). 1,3-Dipolar addition produces the triazoline adduct (87). This material can exist in equilibrium with its valence tautomer (88), and allylic azides (89) and (90) can be produced from these triazolines by ring cleavage. 

Even iV-aryldiaziridines can be obtained. Compound (274) is formed on irradiation of its 1,3-dipolar isomer, which is in thermal equilibrium with its head-to-tail dimer (82TH50800). 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

The differenee in reaction rates of the amino alcohols to isobutyraldehyde and the secondary amine in strong acidic solutions is determined by the reactivity as well as the concentration of the intermediate zwitterions [Fig. 2, Eq. (10)]. Since several of the equilibrium constants of the foregoing reactions are unknown, an estimate of the relative concentrations of these dipolar species is difficult. As far as the reactivity is concerned, the rate of decomposition is expected to be higher, according as the basicity of the secondary amines is lower, since the necessary driving force to expel the amine will increase with increasing basicity of the secondary amine. The kinetics and mechanism of the hydrolysis of enamines demonstrate that not only resonance in the starting material is an important factor [e.g., if... 

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

One of the most rational means for displacing a broad zone is electrolyte desorption under the conditions of decreasing degree of ionization, i.e., when counterions are converted into dipolar ions, uncharged molecules and coions. This conversion corresponds to a sharp decrease in distribution coefficients of the desorbed substance. Hence, the displacement of equilibrium parame ters at a high rate of mass-exchange is one of the methods of selective stepwise chromatography. 

Here, is the magnetization of spin i at thermal equilibrium, p,j is the direct, dipole-dipole relaxation between spins i and j, a-y is the crossrelaxation between spins i and j, and pf is the direct relaxation of spin i due to other relaxation mechanisms, including intermolecular dipolar interactions and paramagnetic relaxation by dissolved oxygen. Under experimental conditions so chosen that dipolar interactions constitute the dominant relaxation-mechanism, and intermolecular interactions have been minimized by sufficient dilution and degassing of the sample, the quantity pf in Eq. 3b becomes much smaller than the direct, intramolecular, dipolar interactions, that is. 

Transient nOe represents the rate of nOe buildup. The nOe effect (so-called equilibrium value) itself depends only on the competing balance between various complex relaxation pathways. But the initial rate at which the nOe grows (so-called transient nOe) depends only on the rate of cross-relaxation t, between the relevant dipolarly coupled nuclei, which in turn depends on their internuclear distance (r). 

Diazaphospholes are known to undergo facile 1,3-dipolar cycloaddditions with a variety of dipoles [2, 4, 7, 98], During recent years, some interesting [2+3] cycloaddition reactions have been reported. 2-Acyl-[l,2,3]diazaphospholes 6 were reported to undergo [2+3] cycloaddition with diazocumulene 92, the minor equilibrium isomer of a-diazo-a-silyl ketones 91, to form a bicyclic cycloadduct 93 (Scheme 29). Thermolysis of the cycloadduct results in the formation of tricyclic phosphorus heterocycle 94, which can be explained due to the possibility of two parallel reactions of cycloadduct. On the one hand, extrusion of molecular nitrogen from 93... 

Despite the lack of success in the attempts at intramolecular cycloaddition with substrates 83 and 91, a moderately promising outcome was observed for the nitroalkene substrate (98, Scheme 1.10c). Heating a dilute solution of oxido-pyridinium betaine 98 in toluene to 120 °C produced a 20 % conversion to a 4 1 mixture of two cycloadducts (110 and 112), in which the major cycloadduct was identified as 110. While initially very encouraging, it became apparent that the dipolar cycloaddition reaction proceeded to no greater than 20 % conversion, an outcome independent of choice of reaction solvent. Further investigation, however, revealed that the reaction had reached thermodynamic equilibrium at 20 % conversion, a fact verified by resubmission of the purified major cycloadduct 110 to the reaction conditions to reestablish the same equilibrium mixture at 20 % conversion. 

Relaxation processes are probably the most important of the interactions between electric fields and matter. Debye [6] extended the Langevin theory of dipole orientation in a constant field to the case of a varying field. He showed that the Boltzmann factor of the Langevin theory becomes a time-dependent weighting factor. When a steady electric field is applied to a dielectric the distortion polarization, PDisior, will be established very quickly - we can say instantaneously compared with time intervals of interest. But the remaining dipolar part of the polarization (orientation polarization, Porient) takes time to reach its equilibrium value. When the polarization becomes complex, the permittivity must also become complex, as shown by Eq. (5) ... 

As shown in Scheme 12.1, reaction of the phosphine with an allenic ester gives all-carbon 1,3-dipole 262. This dipolar intermediate reacts at the a-position to form the cyclic intermediate 263, which is in equilibrium with 264 via hydrogen shift. Finally, the reaction affords the cycloadduct along with the regeneration of PPh3 as the catalytically active species. 

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