It is shown that absorption line shapes in the condensed phase can be well described with an adiabatic quantum model. This model reproduces the results of other line-shape models and includes spectral-diffusion and motional-narrowing effects. The motional narrowing is described in a purely quantum-mechanical manner and is found to result from the delocalized character of the wave functions of the low-frequency modes to which the optically active mode is coupled. In many line-shape models it is assumed that an increase in the rate of spectral diffusion automatically leads to motional narrowing of the absorption line. However, this assumption is no longer correct if the low-frequency motion is affected by a change in the quantum state of the high-frequency mode. It is demonstrated that this quantum effect can easily be accounted for in the adiabatic quantum model and that this effect leads to different dependencies of motional narrowing and spectral diffusion on the low-frequency motion.