Chain Formation and Addition Drive the Debye Relaxation of Methanol.

The Debye relaxation in alcohols has generally been attributed to "collective motion" driven by hydrogen bonding, but the molecular mechanism of relaxation has yet to be conclusively identified. Taking methanol as a model alcohol, we apply an oscillating electric field in molecular dynamics simulations to directly evaluate the molecular motions and identify the mechanism. Methanol forms short-lived chains of hydrogen-bonded molecules through the hydroxyl group. We find that the hydroxyl group rotates in response to the field with a frequency dependence that tracks the Debye peak, implicating these chains in the Debye relaxation. Focusing on the hydroxyl OH rotation, we consider the events (e.g., diffusion of chains, birth, growth) over the lifetime of chains that could contribute to this frequency dependence. We find that molecular participation in chains is responsible for the OH alignment relative to the field: molecules align incrementally, ″clicking in″ during chain formation and during molecular addition to existing chains.