Hydrogen Bond Studies. LX.* A Single Crystal Neutron Diffraction Study of Trichloroacetic Acid Dimer

The crystal structure of trichloroacetic acid (CCl₃COOH) has been determined by application of direct methods of phase determination to single crystal neutron diffraction data. The structure is monoclinic (P2₁/c) with cell constants at 22°C: a=6.152(2) Å, b=9.705(4) Å, c=12.020(9) Å, β=124.29(5)°, $D_c\text{= 1.830\hspace{0.17em}}\mathrm{g}{\mathrm{cm}}^{\text{−3}}{\mathrm{,D}}_m\text{= 1.81}\mathrm{g}\hspace{0.17em}{\mathrm{cm}}^{\text{−3}}$ . The value of R(F) for 615 independent reflections is 0.057. There are four molecules, crystallizing as two centrosymmetric, hydrogen‐bonded dimers in the unit cell. The bond lengths (corrected for rigid‐body motion) are $\mathrm{C}–\mathrm{O}$ , 1.311(4) Å; $\mathrm{C}=\mathrm{O}$ , 1.216(4) Å, $\mathrm{C}–\mathrm{C}$ , 1.546(4) Å, $\mathrm{C}–\mathrm{Cl}$ (mean), 1.768(3) Å; $\mathrm{O}–\mathrm{H}$ , 1.013(11) Å. The $\mathrm{O}\mathrm{···}\mathrm{O}$ distance of the hydrogen bond is 2.666(6) Å, and the $\mathrm{O}–\mathrm{H}\mathrm{···}\mathrm{O}$ angle is 176.5(5)°. A rigid‐body analysis of the thermal motion indicates a librational amplitude of 10° approximately about the long axis of the dimer, smaller librations about the other two axes, and rms translational amplitudes of 0.20 Å in all directions. The apparent vibrational amplitude of the hydrogen atom along the hydrogen bond is 0.12 Å. This exceeds the value to be expected for a harmonic oscillator with the observed vibrational frequency; substantial anharmonicity is suggested. There is no evidence of disorder.