The ultrafast dynamics of diphenylmethane and its solutions have been measured. A mode at 80 cm(-1) has been identified as being of intramolecular origin. The frequency of this mode is observed to be a strong function of solvent, shifting to lower frequency in more weakly interacting solvents. Such a shift is very similar to that observed for molecular librations in solution. It is concluded that the potential energy curve associated with a low frequency mode depends not only on intramolecular forces, but also on its interaction with the environment. (C) 2003 Elsevier B.V. All rights reserved.