Crystal structure of butyric acid

With 4 molecules per unit cell, the calculated density is 1.135 g.cm. -3. This structure exists from the melting point down to about 5 5 °C. where it changes to a second phase of unknown structure. The structure is described in space group C2/m. The molecules occur as dimers with the oxygen and carbon atoms in the mirror plane, but with large amplitudes for out-of-plane vibrations. A structure in space group C2 is not ruled out by the calculations, but is considered less likely than the structure in C2/m. The bond distances, after corrections of up to 0.03 A for thermal motion, are C-O = 1.22 and 1.35 /~, C-C = 1.54, 153 and 1-53 /~ (in order from carboxyl to methyl), each _+ 0.02 A. The hydrogen bond length is 2.62/~.