Studies on nucleobases and their derivatives: crystal structures of 1-((S)-2,3-dihydroxypropyl)uracil (C7H10N2O4), 5-bromo, 1-((S)-2,3-dihydroxypropyl) uracil (C7H9N2O4Br) and arprinocid (9-(2-chloro-6-fluorobenzyl)adenine, C12H9N5FCl)