Simulating Hydrogen-Bond Structure and Dynamics in Glassy Solids Composed of Imidazole Oligomers.

We simulated structural and dynamical properties of imidazoles tethered to aliphatic backbones to determine how chain length influences the competition between extended hydrogen-bond networks and imidazole reorientation dynamics. We performed molecular dynamics simulations on hypothetical solids using the GAFF Amber force field over the temperature range 300-800 K, for chain lengths varying from monomers to pentamers. We investigated the effect of heterogeneity by simulating monodisperse and polydisperse solids with the same average chain length. We computed hydrogen-bond cluster sizes and percolation ratios; orientational order parameters associated with imidazole rings, tethering linkers, and backbones; and orientational correlation functions for imidazole rings. We found the surprising result that chain-length heterogeneity negligibly affects system density at standard pressure, the fraction of percolating hydrogen-bonded clusters, and the order parameters for backbone, linker, and imidazole ring. Decreasing oligomer chain length from pentamers to shorter chains decreases the tendency to form percolating hydrogen-bond networks while dramatically decreasing imidazole ring reorientation times from a broad range of 100-700 ps for pentamers down to 20 ps for monomers, hence quantifying the competition between hydrogen-bond cluster size and reorientation rate. The computed orientational order parameters suggest the following hierarchy of structural excitations: imidazole ring reorientation in the range 400-500 K, linker motion around 500-600 K, and backbone relaxation at 600-700 K in this model. The question remains for this class of systems which of these motions is crucial for facile proton transport.