Enthalpy dipolar effect

A decrease in the activation energy AG is certainly a major effect. Because of the contribution of enthalpy and entropy to the value of AG (= AH -TAS ), it might be predicted that the magnitude of the -TAS term would increase in a microwave-induced reaction, because organization is greater than with classical heating, as a consequence of dipolar polarization. 

As a consequence of the model employed, values of Hb(H20) and N ought to be independent of the choice of the cosolvent as long as specific structural effects are absent. Therefore, we applied Equation 3 to the enthalpies of solution of n-Bu4NBr in DMSO-water mixtures (iO), since DMSO is a dipolar aprotic solvent like DMF. The best fit of the AHE values in this mixture yields Hb(H20) = —49.2 kj mol-1 and N/4 = 6.4, in excellent agreement with our values at 25°C given in Table III. 

TheD term accounts for part of the effects of solution enthalpy. Enthalpy of mixing results when the solute-solvent interaction force is different from the solute-solute and the solvent-solvent interactions. Intermolecularforces can be further characterized as dispersion, dipolar, and hydrogen-bond forces. In the mobile order solubility approach, dispersion and dipolar forces were not separated. The effects of these two forces on solubility were expressed in terms of modiLed solubility parameters, S andSj. The relationship between solubility and solubility parameters can be derived in the... 

Solid acid zeolites, beside inducing reactions known from superacids are indeed moderate acids " The moderate acidity allows zeolite catalyzed reactions to not suffer from side chains reactions." Enthalpies of reaction compare actually more closely with gas phase reactions than with reactions catalyzed by homogeneous high dipolar acids. However, zeolite micropores have been shown to have a stabilizing effect on the carbocationic transition states with respect to gas phase results, which can reach 10 to 30 % of the activation energies This stabilization has been demonstrated to be mainly of short range electrostatic nature ... 

The proposed mechanism is in accordance with the observed solvent dependence of the reaction. Whereas the dipolar sulfoxide is expected to be more strongly solvated with an increase in solvent polarity, the less dipolar sulfenate should be relatively insensitive to such a solvent change. Stabilization of the sulfoxide, relative to the less dipolar activated complex (which should be similar to the sulfenate intermediate), increases the enthalpy of activation, AH. This is refiected in the necessity of breaking increasingly strong solute-solvent interactions. On the other hand, because desolvation on activation is expected to increase the degrees of freedom in the system, a more positive AS is expected to work in the opposite direction and effect a compensating increase in k with... 

The effect of transfer from dipolar aprotic to protic solvent on the enthalpy and entropy of activation of Su2 reactions is a topic which is still in its infancy. Up till now, the discussion in this article has been in terms of free energies, but much more can be learnt about solvent effects on rate, if changes in the entropy and enthalpy of solvation of reactants and transition states are known and understood. Arnett and McKelvey (1966) have reported some preliminary measurements of enthalpies of transfer from water to DMSO for some ions and molecules, but as yet neither these, nor measurements of entropies and enthalpies of activation, are sufficiently extensive for any calculation of entropies and enthalpies of transfer for 8 2 transition states. 

In all the evaluations of Table 4, the solvent effects on the activation free enthalpies are positive, increase with increasing solvent dielectric constants and tend to be larger for the endo than for the exo adducts. This behaviour, in accord with the experimental trend, is due to the electrostatic contribution the CDS and CDR contributions, in fact, are rather independent of the isomeric reaction considered and, moreover, appear to obtain comparable values in every 1,3-dipolar cycloaddition. For the Tomasi parametrisation in water, for example, the CDR" contribution for the cycloadditions of diazomethane and nitrile oxides to substituted alkenes amounts to -1.85 0.14 kcal mok This finding can be traced back to the view that the CDR term is approximately proportional to the solvent accessible surface area (the cavity area) of solutes and to the feature of TSs of having very alike structures of the new forming pentatomic ring so that the changes of the cavity areas from reactants to TSs are similar. 

The effects of the electric field, E, have been determined, by ah initio calculations, by evaluating the saving in the standard enthalpy of formation (AHf) of different ion-pairs M+X on application of electric fields (regarded as dipolar, uniform, and continuous) of different types [46]. Calculations were based on the effect of the electric field E (with optimum effects when considering E to be collin-ear with the dipole moment) according to ion-pair dissociation by considering the stabilization (decreases in AHf) as a function of the interatomic distance, d, between Li+ and F and the nature of ions at their equilibrium distance for several... 

C, however, Ca + does not have a dehydrating effect on Li+ since these ions possess similar enthalpies of hydration in saturated aqueous solutions. The nuclear magnetic relaxation rates and shifts of Li+ and Cs+ in aqueous solutions containing Fe + and various counter-anions are interpreted in terms of a dipolar attraction between Li+ and the unpaired electrons on the Fe + ion, and the formation of an ion pair between Cs+ and ferric halide complex. An increase of pressure in the range 0—1000 bar results in an enhancement in the hydration of the ions Na+ and K+ in their aqueous chloride solutions. The enhancement is more pronounced at 20 than at 45 °C. These conclusions... 

Relaxation Phenomena Hilczer et al. [2002] studied the relaxation behavior of nanoceramic-polymer composites. The smdy focused on a PVDF/PZT composite with 30-nm particles. The dielectric relaxation time of PVDF as well as that of the low-temperature component followed VFTH [Eq. (13.2)]. By contrast, the relaxation time of the high-temperature component obeyed the Arrhenius equation. It is interesting to note that the activation enthalpy increased strongly in composites. The effect was ascribed to the wide-angle oscillation of dipolar groups of PVDF. 

It should be mentioned that even in the absence of dipolar, polarizable, or ionic reaction partners, high electric fields may cause shifts in chemical distributions. Such a field effect requires, however, that the solvent phase has a finite temperature coefficient of the dielectric permittivity or a finite coefficient of electrostriction an additional condition is that the chemical reactions proceed with a finite reaction enthalpy (AH) or a finite partial volume change (A V). Electric field induced temperature and pressure effects of this type are usually very small they may, however, gain importance for isochoric reactions in the membrane phase. 

Abstract - This work is concerned with the origin of effects on rate equilibrium processes when reactions are performed in dipolar aprotic media such as dimethyl sulfoxide and dimethyl-formamide compared to typical hydroxylic media. An example of processes studied in our laboratory is the competition between proton abstraction and a-complex (Meisenheimer adduct) formation in the interaction of nitroaromatic compounds with basic systems (e.g. 1,3-dinitrobenzene in DMF-D20-Na0D). Other reactions studied are the base catalyzed isotopic exchange of D2 in aqueous DMSO mixtures, and also isotopic exchange of fluoroform. It has been found possible in certain cases to dissect the initial state and transition state contributions to the reaction rates from the kinetically measured enthalpies of activation and the thermodynamically evaluated enthalpies of transfer of the reactants. This procedure affords insight into transition state properties and behaviour. 

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