Enthalpy factors

Phenol and hexamethylphoshoramide (HMPA) were selected, respectively, as the standard H-bond donor and H-bond acceptor with their values fixed on free energy and enthalpy H-bond scales for phenol, -2.50 for the H-bond donor enthalpy factor (TJ and also for the H-bond donor free energy factor (Ca) for HMPA, 2.50 for the H-bond acceptor enthalpy factor ( J and 4.00 for the H-bond acceptor free energy factor (CJ. 

Edipiobe) = enthalpy factor of the probe H-bond donor 1/3... 

Use HYBOT s acceptor and donor enthalpy factors to estimate the optimum H-bonding energy via Eq. (3),... 

A similar contribution from the counterion in the transition state for propagation would require formation of an energetically unfavoured four membered ring. Parallel data have been reported for the molecular weights of poly-(isobutenes) produced by y-ray irradiation (free ions) and chemical catalysis (ion pairs) (161), though in this case it was shown that entropy rather than enthalpy factors were more important. 

Entropy dominates equilibrium constants in the difference between inter- and intramolecular reactions. In Chapter 6 we explained that hcmiacetal formation is unfavourable because the C=0 double bond is more stable than two C-0 single bonds. This is clearly an enthalpy factor depending simply on bond strength. That entropy also plays a part can be clearly seen in favourable intramolecular hemiacetal formation of hydroxyaldehydes. The total number of carbon atoms in the two systems is the same, the bond strengths are the same and yet the equilibria favour the reagents (MeCHO + EtOH) in the inter- and the product (the cyclic hemiacetal) in the intramolecular case. 

Phase separation occurs when AG rises above 0. This may be triggered by a rise in enthalpy (i.e., AH) or a decline in entropy (i.e., AS). To allow for the formation of a uniform network polymer, phase separation must be delayed until crosslinking is well enough advanced to prevent individual molecules from demixing. This delay is achieved by either reducing AH or by raising AS (in concert with T). The enthalpy factor (AH) is controlled by the difference in Hildebrand s solubility parameter (5) between the various reacting components, since... 

The free energy of micelle formation has been found to be more dependent on entropy than on enthalpy factors (Kavanau, 1965 Elworthy, 1968). Micelle formation has been treated theoretically either... 

When using more active emulsifiers of the C-10 type (Yeliseyeva, 1966) the effective rate and the equilibrium value of adsorption increase in the polymerization of polar monomers, which reduces the intensity of the processes of initial flocculation. Moreover, in this case the charge density is not the only stabilizing factor it is su(f>lemented by the enthalpy factor which arises from hydration of oxyethylenic chains of emulsifier molecules. The presence of these emulsifiers creates conditions that enable tbe obtain-ment of polar polymer latexes of high stability. 

The enthalpy contributions were also estimated and H-bond donor Ehd and H-bond acceptor Eha enthalpy factors were also calculated. 

Nucleophilic reactivity is in the same order DMF < DMSO < HMPT. In both cases, the activation parameters are characterized by highly negative activation entropy factors (AS = -40 to -65 e.u.). Only the enthalpy factors are markedly different for Si and P (AH — 0 and AH 7 kcal/mol). As shown for silicon compounds, identical rate laws (eqs. [101] and [102]) and energy data are con-... 

---