Internal conversion calculated for nucleobases yield erroneous results using the energy gap law ; it cannot explain the experimentally observed subpicosecond internal conversion due to the minimal correlation between the energy gap of the bases and fluorescence lifetimes. Cadet & Vigny report that the adiabatic S 1 energies for each nucleoside are as Adenosine > Uridine > Thymidine > Cytidine > Guanosine. Interestingly, nucleosides Uridine and Adenosine, with the shortest S 1 fluorescence lifetime, also yield the largest S 1 → S 0 energy gaps.