Enthalpy-controlled

Two extreme situations should be noted. If p =0, then 8AG = — 78A5, and the reaction series is entirely entropy controlled it is said to be isoenthalpic. If I/p = 0, then 8AG = 8A//, and the series is enthalpy controlled, or isoentropic. All of these relationships apply also to equilibria, but we will be concerned with kinetic quantities. 

In series with a constant enthalpy, controlled by entropy changes, steric effects (15), or more particularly, kinetic steric effects (13, 14) and solvent effects (14) may be decisive. 

In summary, although the metal-ion selectivity of the cryptands is normally largely enthalpy-controlled, entropic terms may also be quite important. Once again, the factors underlying these respective terms may be quite variable and, as a consequence, a criterion for preferred complexation based solely on a match of the cavity for the cation radius may not always be appropriate. 

Three-membered ring-forming processes involving X-CH2-CH2-F or CH2-C(Y)-CH2F (X = CH2, O, or S and Y = O or S) in the gas phase have been treated by the ab initio MO method with a 6-31+G basis set." When electron correlation effects were considered, the activation (AG ) and reaction (AG°) free energies were lowered by about lOkcal mol indicating the importance of electron correlation in these reactions. The contribution of entropy of activation -TAS ) at 298 K to AG is very small the reactions are enthalpy controlled. 

In this case, enthalpic interactions within the HPLC system exceed the exclusion effects (Eigure 16.3b). The retention volumes of polymer species as a rule exponentially increase with their molar masses. The limitations of the resulting procedures were elucidated in Section 16.3 the retention of (high)polymers is usually so large that these do not elute from the column (Section 16.6). Therefore, the majority of enthalpy controlled HPLC procedures is applicable only to oligomers—up to... 

This result explains the apparent contradiction between growth selection experiments and recrystallization experiments. The problem resulted only from the wrong tacit assumption that the preexponential factor is essentially independent of misorientation so that only the activation enthalpy controls mobility. Growth selection experiments have to... 

It has been shown recently (1) that the transfer of urea from water to water-rich water-tetrahydrofuran (THF) mixtures is an entropy-controlled phenomenon (T AS°t > AH°t ), but from water to water-THF mixtures of mole fraction Z2 > 0.20 it is an enthalpy-controlled phenomenon ( AH°t > T AS°t ) moreover, the transfer is energetically favorable in the former case and unfavorable in the latter. A minimum in the standard free energy transfer... 

The chiral separation of cis enantiomers was improved with a decrease in temperature, whereas that of trans enantiomers was improved with an increase in temperature. The temperature dependence of enantioselectivities was studied to determine the thermodynamic parameters H°, S°, and Tiso. The thermodynamic parameters revealed that the separation of trans enantiomers was controlled by entropy in the range of temperatures examined, whereas enthalpy-controlled separation was observed for cis enantiomers. The separations of both cis and trans enantiomers, however, were controlled by enthalpy in normal phase HPLC [150],... 

Data in Table 2 reveal that with a few exceptions, the stability of the complexes is enthalpy controlled and entropy destabilised. As previously stated, the variations observed in the thermodynamics of complexation of these systems as a result of the medium effect are the result of the solvation changes that the reactants (anion and receptor) and the product (complex anion) undergo in moving from one medium to another. This issue will be carefully addressed in the next section. 

Intramolecular nucleophilic substitution to form thiiranes was studied by means of ab initio MO computations based on the 6-31G basis set <1997JCC1773>. Systems studied included the anions SCH2CH2F and CH2C(=S)CH2F which would afford thiirane and 2-methylenethiirane, respectively (Equations Z and 3). It was important to include electron correlation which was done with the frozen-core approximation at the second-order Moller-Plesset perturbation level. Optimized structures were confirmed by means of vibrational frequency calculations. The main conclusions were that electron correlation is important in lowering AG and AG°, that the displacements are enthalpy controlled, and that reaction energies are strongly dependent on reactant stabilities. 

In general, cycloadditions catalyzed by Lewis acids proceed at significantly lower temperatures and with higher selectivities than their uncatalyzed counterparts. Factors that contribute to the increased selectivity of the catalyzed reactions include lower temperatures and more organized transition states. For enthalpy-controlled reactions, lowering temperatures increases selectivity (recall Section 1.4, equation 1.5). Coordination of a Lewis acid to the enone carbonyl not only activates the enone by electron withdrawal, it also restricts conformational motion and thereby reduces the number of competing transition states. Figure 6.12 illustrates several chiral auxiliaries for dienophile modification that have been used in the Diels-Alder reaction. 

The column feed was preheated by the bottoms then a steam preheater. Preheater steam was controlled by the feed temperature downstream. The feed entha fluctuated with fluctuations in column bottom flow. This interfered with the column product anal3rzer control Problem was cured by a feed enthalpy controller which regulated preheater steam flow. 

Anal5 zer control may fall short of achieving its objectives if column is unstable. A feed enthalpy control may be needed if heat input to the feed fluctuates. 

Interestingly, experimentally determined entropy and enthalpy of activation for the catalyzed reaction indicate that 1E9 does not function as a classical entropy trap, as the activation entropy is even more negative than that of the uncatalyzed reaction in solution (—22.1 e.u. and — 21.5 e.u for catalyzed and uncatalyzed reactions, respectively). Instead the enthalpy-controlled process by which the substrates are arranged to a reactive conformation in the combining site dominates the reaction. Thus, the rate enhancement of 1E9 is derived entirely from a reduction of the activation enthalpy of the reaction (11.3 kcal mol and 15.5 kcal mol for the catalyzed and uncatalyzed reactions, respectively). In fact. 

In this case, the enthalpic interactions within the HPLC system exceed the exclusion effects (see Figure 3(e)). The retention volumes of polymer species as a rale exponentially increase with their molar masses. The important limitation of the resulting procedures was presented in section 11.5.2.3. The retention of (high) polymers is usually so intense that the latter do not elute from the column any more. Therefore, the majority of enthalpy controlled HPLC procedures is applicable only to ohgomers - up to molar mass of few thousands g.mol. Still, the reduced sample recovery may affect results of separation even in case of oligomers. The selectivity of enthalpy driven HPLC separation is much higher than in the case of SEC but, naturally, the sequence of molar masses eluted from the column is reversed. If the effect of enthalpy is reduced, problems with sample recovery are mitigated - but at the same time the separation selectivity is reduced. 

The second- and third-order reactions showed interesting differences. For the second-order reaction, AH values were 9.0 kcal/mol and 4.7 kcal/mol for addition of bromine to p-nitrostyrene and to styrene, respectively, while the corresponding AS values were -39.5 eu and -37.6 eu for these two compounds. For the third-order reaction, however, the values of AH were 0.9 and 0.01 kcal/mol, and the AS values were -50.5 and -37.6 eu, respectively, for addition to m-nitrostyrene and to styrene. Thus, the second-order reaction was said to be "enthalpy controlled," while the third-order reaction was said to be "entropy controlled" (reference 87). 

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