Why are selenouracils as basic as but stronger acids than uracil in the gas phase?

The gas-phase basicity and acidity of 2-selenouracil (2SU), 4-selenouracil (4SU), and 2,4-diselenouracil (24SU) have been calculated at the B3LYP/6-311+G(3df,2p) level of theory. Our results showed that all these compounds behave as bases of moderate strength in the gas phase. As was found for uracil and for the thiouracil analogues, the most basic site is the heteroatom at position 4, and only for 2SU is there a certain ambiguity in assigning the basic site. More importantly, with the only exception of 2SU, selenouracils are as basic as or slightly less basic than uracil, because the replacement of the oxygen atom at position 2 by a selenium atom leads to an increase of the electron delocalization inside the six-membered ring, which decreases the intrinsic basicity of the heteroatom at position 4. As already reported for uracil and thiouracils, for selenouracils N1 is the most acidic site. However, selenouracils are predicted to be stronger acids than uracil. This acidity enhancement is essentially due to a specific stabilization of the anion when O is replaced by Se. Two factors are responsible for this stabilization: a significant aromatization of the ring upon deprotonation and a better dispersion of the excess electron density when the system contains third-row atoms.