An investigation of the longitudinal acoustic mode (LAM) in a series of perfluoro-n-alkanes under pressure has been carried out using Raman spectroscopy in conjunction with a diamond anvil cell. Perfluoro-n-alkanes are shown to undergo a phase transition from a helical to a planar zigzag structure at 5 kbar. This conformational change with pressure results in an abrupt increase in the LAM frequency. Since the chain axis elastic modulus of a planar zigzag molecule is larger than that of a helical one, such a conformational change leads to stiffening of the backbone giving rise to the observed change in the LAM frequency. © 1991 American Institute of Physics.