The ultrafast dynamics of N-methylacetamide (NMA), a peptide-linkage model compound, are reported for the first time as a function of temperature and dilution. The rotational-diffusive timescale of NMA was observed to behave in a manner inconsistent with established hydrodynamic theory for simple liquids, exhibiting sharp decreases at elevated temperatures and low concentrations. These were associated with non-stoichiometric changes in far-infrared and terahertz Raman transition strengths. The data is consistent with a model in which neat NMA forms hydrogen bonded aggregates, featuring cooperative effects upon the polarizability and dipole moment that are disrupted by increased temperature or decreased NMA mole fraction. (c) 2006 Elsevier B.V. All rights reserved.