Molecular dynamics analysis of coupling between librational motions and isomeric jumps in chain molecules

ABSTRACT: The coupling between librational motions and rotational isomeric jumps in polymers is investigated. Librations originate from bond stretching, bond angle distortion, and small-amplitude fluctuations in the bond dihedral angles. Molecular dynamics trajectories for a polymer chain of 100 bonds in vacuo are used for assessing the contribution of librations to the dynamic behavior of chains. Each trajectory is resolved into two sets of components, low and high frequency, representing bond rotameric transitions and librations, respectively. Librational motions are observed to be strongly correlated with the rotational jumps of bonds between isomeric states. In fact, the passage from an isomeric state to another is accompanied by an enhancement in librational motions. Examination of the contribution of librations to the local relaxation behavior of polymers indicates that librations affect the mechanism of motion via two opposing effects. On the one hand, they provide an additional degree of freedom which causes a faster relaxation of the conformational correlation functions associated with the bond dihedral angles. On the other hand, the orientational correlation functions associated with the spatial reorientation of backbone bonds exhibit a slower time decay due to the compensating effect of librational motions. In fact, librational motions are not random but highly directed such that they collectively preserve local chain direction and help in accommodating the local reorientations induced by isomeric jumps, which would otherwise result in larger displacements of chain segments. Finally, it is shown that replacing the librations of a trajectory with random, uncorrelated fluctuations leads to an overestimation of the relaxation rate of configurational correlations