This review shows a short compendium of the structures of cytosine and its tautomeric forms and the effect of hydration on their geometries. The homo-and hetero-dimer forms were also included, with special attention to the guaninecytosine pair. The tautomerism on monosubstituted 5-haloderivatives of cytosine was also studied. The importance and main applications of these molecules are also briefly indicated, together with their geometries.

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Spectroscopic and structural studies of uracil and its derivatives have been reported both theoretically and as well experimentally by many authors. Uracil and its thio analogue 2-thiouracil (2-TU) affect the growth of plants. To understand biochemically the mode of inhibitory or malforming actions of uracil and 2-TU on the growth of rice and wheat plants, the knowledge of their structures is essential. Hence, in the present work an attempt has been made to study their spectra and structures using vibrational spectroscopy as technique. The replacement of an oxygen atom by sulphur leads to a shift of the experimental ν(N-H) bands to lower wavenumbers, 27 cm -1 for the N1-H mode and 20 cm -1 for N3-H. The effect of sulphur substitution on ν(N3-H) wavenumber and intensity reflects changes in proton abilities of this group as well as the hydrogen bonding in which they participate. Compared to uracil, the sulfur atom in 2-TU results mainly in a significant change of the bond-length at the substitution site: S=C ~1.66 Å, as compared to C=O ~1.22 Å. This fact leads to a slightly reduction in the neighboring bond lengths N1-C2 and N3-C2, but the N-H and C-H bonds are little affected.

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