Crystal structure of butyric acid

Abstract

According to single-crystal threc-dimensional X-ray data, n-butyric acid crystals at-43 °C. are monoclinic with a=8.01_+0.08, b=6"82+0"02, c=10.14_+0.03A; fl-lll °27'_+15'. 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-55 °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 CO = 1.22 and 1.35 /~, CC = 1.54, 1"53 and 1-53 /~ (in order from carboxyl to methyl), each _+ 0.02 A. The hydrogen bond length is 2.62/~.