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

Abstract

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⁺˙ + Cu⁺, although it is metastable with respect to the coulomb explosion yielding [uracil-Cu]⁺ + uracil⁺˙. 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)₂Ca²⁺, 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.