We have obtained the room temperature Raman spectra of the three salts n-PrNH3X (X = Cl, Br and I). The concentration of gauche isomers increases when passing from the chloride to the iodide. We have evaluated experimentally the lattice contribution to the energy difference between gauche and anti conformers. A force field calculation yielded an energy difference of 0.85 kcal mol−1 between the two conformations of the propyl-ammonium moiety.