Intramolecular entropy

The inter- and Intramolecular contributions to the entropy and energy of fusion are calculated for several linear aliphatic polyesters and polyamides assuming the fusion process consists of two Independent contributions the volume expansion (intermolecular contribution) and the increase in the conformational freedom of each polymer chain on melting (intramolecular contribution). The intramolecular entropy and energy contributions are obtained from the configurational partition function and Its temperature coefficient calculated for an isolated, unperturbed polymer chain using the RIS approximation. 

V (the potential) is identified with the enthalpy, i.e. the number n of base pairings (contacts), and T corresponds to the entropy. At each stage in the folding process, as many as possible new favourable intramolecular interactions are fonned, while minimizing the loss of confonnational freedom (the principle of sequential minimization of entropy loss, SMEL). The entropy loss associated with loop closure is (and the rate of loop closure exp... 

The reaction is controlled, not primarily by the alteration of FMO energies, but by the chelation of the substrates to the catalyst leading to a favorable entropy of the pseudo-intramolecular intermediates. 

On the basis of the evidence presented above as well as some other pertinent data (e.g. negative entropies of activation), Darwish and Braverman have suggested that the rearrangement of allylic 2,6-dimethylbenzenesulfinates (6a-f) to corresponding sulfones (7a-f) proceeds by a cyclic intramolecular mechanism involving a five-membered transition state which may be represented by a resonance hybrid (8) of the following resonance structures. 

The metalloalkyne complex Ru ( )-CH=CH(CH2)4C CH Cl(CO)(P,Pr3)2 exhibits behavior similar to that of cyclohexylacetylene (Scheme 10).40 Thus, it reacts with OsHCl(CO)(P Pr3)2 to give the hydride-vinylidene derivative (P Pr3)2 (CO)ClRu ( )-CH=CH(CH2)4CH=C OsHCl(CO)(P,Pr3)2, which evolves in toluene into the heterodinuclear-pi-bisalkenyl complex (P Pr3)2(CO)ClRu (is)-CH=CH(CH2)4CH=CH-( ) OsCl(CO)(P,Pr3)2. Kinetic measurements between 303 and 343 K yield first-order rate constants, which afford activation parameters ofAH = 22.1 1.5, kcal-mol-1 andAS = -6.1 2.3 cal-K 1-mol 1. The slightly negative value of the activation entropy suggests that the insertion of the vinylidene ligand into the Os—H bond is an intramolecular process, which occurs by a concerted mechanism with a geometrically highly oriented transition state. 

---