Surprising Effects on the Conformational Entropy due to Nonrandom Distributions of Local Conformations Along Unperturbed Chains

Abstract

Summary: Nonrandomness in the distribution of rotational isomeric states along a flexible unperturbed chain reduces its conformational entropy. Pairwise interdependence of the bonds is a necessary, but not sufficient, condition for a significant reduction. The reduction in conformational entropy from this source can be as severe as a factor of three. It is generally more severe for isotactic chains than for the syndiotactic chains constructed from the same monomer. Surprising effects are sometimes seen, such as the nearly identical reductions in conformational entropy for polydimethylsiloxane, a very flexible chain, and for poly(methyl methacrylate), a much stiffer chain.Fractional difference in conformational entropy due to nonrandomness versus probability of helix in helix‐coil transition.magnified imageFractional difference in conformational entropy due to nonrandomness versus probability of helix in helix‐coil transition.

Statistical thermodynamics of the isomerization reaction between n-heptane and isoheptane

“…For alkane or polyethylene chains of length five carbons or longer, it is well known that structures with g + g À or g À g + configurations for successive torsions are either excluded or made unfavorable by steric repulsion; this is called the pentane effect. [45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63] Naively one might simply assume that these structures are missing and that the number of structures would be less than the ideal number. In fact, Tables 2 and 3 show that none of the n-heptane or isoheptane structures contains g + g À or g À g + .…”