On the stability of [(uracil)2-Cu]2+ complexes in the gas phase. Different pathways for the formation of [(uracil-H)(uracil)-Cu]+ monocations

The association of uracil dimers and copper(II) has been studied through the use of B3LYP/6-311+G-(3df,2p)//6-31+G(d,p) calculations. Although uracil-Cu2+ complexes have never been experimentally detected, our results show that [(uracil)(2)-Cu](2+) is thermodynamically stable with regard to both the proton loss and the fragmentation into (uracil)(2)(+center dot) + Cu+, although it is metastable with respect to the coulomb explosion yielding [uracil-Cu](+) + uracil(+center dot). Importantly, a proton transfer from [(uracil)(2)-Cu](2+) to a third neutral uracil molecule is very exothermic. This is consistent with the fact that when electrospray mass spectrometry techniques are used [(uracil-H)(uracil)-Cu](+) and uracil-H+ monocations are detected, but not the [(uracil)(2)-Cu](2+) doubly charged species. In the most stable conformers of [(uracil)(2)-Cu](2+) the two uracil monomers are held together through the metal cation which forms a linear bridge between two carbonyl groups each belonging to a different monomer. This is at variance with what has been found for complexes involving alkaline-earth dications, such as (uracil)(2)Ca2+, in which the metal dication association preserves the network of hydrogen bonds which stabilize the free (uracil)(2) dimers. The formation of [(uracil-H)(uracil)-Cu](+) complexes is accompanied by the enolization of the uracil units. All possible mechanisms to reach the experimentally detected [(uracil-H)(uracil)-Cu](+) singly charged ions, either by direct association of Cu2+ to uracil dimers and posterior deprotonation of the formed complex or through the interaction of Cu2+ with uracil followed by its deprotonation and subsequent association with a second uracil molecule, have been investigated.