The intramolecular nuclear wave packet dynamics of carboxy-deuterated phthalic acid monomethylester following excitation of the OD stretching vibration in the hydrogen bond is investigated. When one starts from a Cartesian reaction surface Hamiltonian, a nine-dimensional model is identified, which comprises the most strongly coupled degrees of freedom. The anharmonic coupling is demonstrated to lead, in particular, to low-frequency modulations of the OD dynamics. Partial vibrational energy randomization takes place on a time scale of 2 ps, which was not seen in earlier single configuration calculations.