C-13 NMR, HOSE calculations and structural analyses of 3,5,5-trimethyl-4-oxo-imidazolidine-2-thione and -4-thioxoimidazolidine-2-one.

The crystal structures of 3,5,5-trimethyl-4-oxoimidazolidine-2-thione (1) and 3,5,5-trimethyl-4-thioxoimidazolidine-2-one (2) are reported and compared with the previously determined structures of 5,5-dimethylimidazolidine-2,4-dithione, 5,5-dimethyl-2-selenoxoimidazolidin-4-one, 5,5-dimethyl-4-sele-noxo-2-thione and 2-thiohydantoin. (1) is orthorhombic (a = 11.310(10)angstrom, b = 7.326(2)angstrom, c = 10.675(6)angstrom, Z = 4, space group Pnma; R = 0.033). (2) is orthorhombic (a = 8.349(4)angstrom, b = 9.719(8)angstrom, c = 20.337(11)angstrom, Z = 8, space group Pbca; R = 0.038). In all these molecules the changes of the bond lengths and angles are coherent with the influence of the different exo-chalcogen atoms bonded to C(2) and C(4). The structural data are used for a simple harmonic oscillator calculation (HOSE model) in order to evaluate the contributions of the resonating forms and to relate them with the C-13 NMR chemical shifts. The changes in angles around the C(sp2) atoms are discussed in terms of the Valence-Shell Electron-Pair Repulsion (VSEPR) model.