Why Are Selenouracils as Basic as but Stronger Acids than Uracil in the Gas Phase?

Abstract

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.

On the Bonding of Selenocyanates and Isoselenocyanates and Their Protonated Derivatives

“…This method, which includes Becke's three parameter nonlocal hybrid exchange potential 49 nonlocal correlation functional of Lee, Yang, and Parr, 50 has been successfully used for the treatment of other Secontaining compounds. [51][52][53] To get reliable relative energies, final energies were obtained in single-point calculations using a 6-31++G(3df,2p) basis set. Harmonic vibrational frequencies were evaluated at the same level of theory used for the geometry optimizations to classify the stationary points found as local minima and to evaluate the thermal corrections necessary to obtain the free energies at 298.2 K. The bonding was analyzed primarily by using the Becke and Edgecombe electron localization function 54 (ELF) topological approach.…”