Proton dynamics in the medium-strong intramolecular hydrogen bond of acetylacetone (Pentane-2,4-dione) was studied using the mixed quantum-classical method density matrix evolution (DME). The proton involved in the hydrogen bond was treated as a quantum particle, involving several vibrational levels, while the rest of the system was treated using classical mechanics. Molecular simulations were performed in the gas phase and in a chloroform solution. The effects of deuteration were also considered. The vibrational spectrum was calculated by Fourier transform of the time-dependent expectation value for the OH bond length. In the present case, we demonstrate by calculations and experimentally that coupling of the proton to the OO and both CO bonds, which are attached to the hydrogen bond (indirect relaxation mechanism) is more important than coupling to the solvent degrees of freedom in determining spectral shape.