Crystal structure of 1-methyluracil from neutron diffraction at 15, 60 and 123 K.

The crystal structure of 1-methylpyrimidine-2,4-dione (1-methyluracil, C5H6N2O2) has been determined at 15, 60 and 123 K from neutron diffraction data. Molecules lie in the eightfold special positions (symmetry m) of space group Ibam, with a = 13.213 (2), b = 13.172 (2), c = 6.163 (1) A at 15 K. Full-matrix least-squares refinements based on 807 (15 K), 805 (60 K) and 815 (123 K) reflections with sin theta/lambda less than 0.69 A-1 converged with R(F2) = 0.023, 0.024 and 0.024 respectively. Bond lengths and angles have e.s.d.'s less than 0.002 A and 0.2 degrees. Detailed analysis of the anisotropic thermal parameters gives estimates of the zero-point internal vibrations for all nuclei and also for molecular rigid-body thermal vibrations at 15, 60 and 123 K. With thermal-vibration corrections there is good agreement in bond lengths and angles at the three temperatures.