Raman and neutron experiments using specific isotope labeling were combined in order to study the dynamics and structure of l-alanine. Inelastic neutron and Raman scattering data of C ₂H ₄(ND ₂)CO ₂D are discussed in relation to the doubling of the lattice parameter a observed by means of neutron powder diffraction in C ₂D ₄ (NH ₂)CO ₂H. The major changes accompanying the phase transition are found in the vibrational frequencies involving the torsional vibration τ(CO ₂ ^-), which is clearly affected by the hydrogen bonds between the protons of the ammonium group and the oxygen atoms of the carboxylate group. At lower temperatures the rearrangement of identifiable hydrogen bonds induces changes in the bending vibration δ(ND ₃), confirming some orientational disorder.