Real-time vibrational dynamics of the cyclic dimer of deuterated acetic acid in solution, a model system for hydrogen-bonded units of biomolecules, are elucidated by ultrafast infrared spectroscopy. Upon excitation of the O-D stretching mode, we demonstrate coherent nuclear motions along several intermolecular low-frequency modes of the dimer. The anharmonic coupling of the fast O-D stretching and slow low-frequency motions is isolated in the nonlinear vibrational response whereas other couplings are suppressed. The large-amplitude motions persist for several pico-seconds, potentially allowing for optical control of (bio)chemical dynamics. (C) 2003 Elsevier Science B.V. All rights reserved.