Zero point motion of the librating methyl group in p-hydroxyacetanilide

Single crystal pulsed neutron diffraction of p-hydroxyacetanilide (the analgesic paracetamol; C8H9NO2) has been undertaken at ten temperatures between 20 K and 330 K. The librations of the methyl group have been analysed in terms of a torsional oscillator model assuming harmonic motion of the hydrogen atoms about the equilibrium configuration. Determination of the significant zero point motion by this method has allowed improved estimates of the force constant and barrier height for the methyl group oscillation to be obtained.