Comparison of the Structures of Ammonium Myristate, Palmitate, and Stearate by X-ray Diffraction, Infrared Spectroscopy, and Infrared Hole Burning

X-ray diffraction studies of ammonium myristate and palmitate show crystals that belong to space group P21/n with the methylene groups packing in a triclinic subcell. The infrared spectra of the palmitate and stearate are similar as expected, but the bands of the myristate salt show additional splitting in the methylene rocking and twisting bands. A number of other factors of the myristate spectrum and of the infrared hole burning are unique as well. The differences show that the myristate salt has two molecules per spectroscopic unit cell, which are postulated to differ by small rotations of the polar carboxyl groups relative to the plane of the chain. A second form of ammonium myristate is found by crystallizing under different conditions.