Variable temperature study of the crystal structure of paracetamol (p-hydroxyacetanilide), by single crystal neutron diffraction

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Abstract
The crystal and molecular structure of paracetamol (p-hydroxyacetanilide) has been determined by single crystal neutron diffraction at seven temperatures between 20 and 330 K. Short data collection times were used in this neutron study, with the fastest data set collected in just 5.5 hours, the longest in 11 hours. The structure is monoclinic, P21/a, Z=4, with unit cell parameters over the temperature range 20-330K ranging from a=12.667(4)-12.872(3), b=9.166(3)-9.370(2), c=7.073(3)-7.085(2) Å, β=115.51(2)-115.62(2)°, V=741.2-770.5 Å3. The molecular geometry and hydrogen bonding are briefly discussed, along with the variation with temperature of non-bonded contacts. The refined anisotropic thermal displacement parameters in the structure are analysed using the TLS approach at each temperature, including those for the hydrogen atoms which are well determined in this neutron study. The large torsional motions of the terminal methyl groups are found to be adequately modelled within the harmonic approximation for atomic vibrations, and show values for the mean square torsional amplitude in the range 170 deg2 to 930 deg2, but with a large residual zero-point motion, whose lower limit is estimated as 120 deg2. The variable temperature experiment is also shown to be useful in estimating bond length corrections due to thermal vibration effects and an empirical approach for doing this is introduced in this work. Application of this empirical correction leads to an estimate for the unshortened C-H bond lengths of 1.103 Å.

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