Thermal effects in urea: the crystal structure at –140°C and at room temperature

The crystal structure of urea has been analyzed on the basis of (hO1) and (hk0) data obtained at room temperature and at -140 °C.Fourier and least-squares methods were used.Results generally agree well with those previously reported by Vaughan & Donohue (1952), but several of the atomic temperature factors differ significantly from those reported by Worsham, Levy & Peterson (1957) on the basis of neutron diffraction measurements.Coefficients of thermal expansion have been measured, and changes in atomic coordinates, bond lengths and angles with temperature have been computed.The only major dimensional change with temperature involves the intermolecular hydrogen bond lengths which decrease considerably with decreasing temperature, resulting in a corresponding decrease in unit cell dimensions.