Single- and double-proton-transfer in the aggregate between cytosine and guaninediazonium ion

[GRAPHICS] The structure of "guaninediazonium ion" in its aggregate with cytosine has been explored with abinitio and density functional methods. The hydrogen bonded aggregate between cytosine and guaninediazonium ion, 1, is a stable minimum, 3, While the isolated enol tautomer of guaninediazonium ion, 2, is significantly more stable than 1, the tautomeric aggregate 4 that results from double-proton-transfer in 3 is almost isoenergetic with 3. Most interesting and entirely unexpected is the finding that neither 3 nor 4 is predicted to be the thermodynamically predominant structure. Instead, single proton transfer to cytosine results in the most stable cytosinium-guaninediazo complex, 5.