Absfract-Original mass spectra of uracil and thymine derivatives are presented with the corresponding fragmentation schemes.In the first series of spectra, the fragmentations of thymine derivatives, including 14C-2-thymine, dimers, and bromo thymines, confirm the basic retro Diels-Alder mechanism.The second series includes dihydro 5,6-derivatives of uracil (hydroxy and bromo substituents). The behaviour of these molecules is quite different; they are more sensitive to the substituents and a part of the fragmentation is often explained by protonated molecular ions.RBsum6-Nous presentom une serie de spectres de masse originaux, de derives de I'uracile et de la thymine, ainsi que les schemas de fragmentation correspondants.L'Ctude d'une premikre serie de mol&ules, & caractkre aromatique, dont une marquee au %, a permis de confirmer le mecanisme de base de la fragmentation comme &ant un 'retro Diels Alder'.Une deuxibme serie, saturee en position 5,6 du noyau uracile, montre le caractkre totalement different de ces mol6cules, plus fragiles, plus sensibles B la nature des substituants et donnant un ion protone [MH]+, responsable d'une partie des fragments observes.

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