Nature of the Interaction between Natural and Size-Expanded Guanine with Gold Clusters: A Density Functional Theory Study

In this paper, we study the interaction of natural and size-expanded guanine molecules with small gold clusters, to shed light on the nature of the N/O–Au bonds and of the unconventional NH···Au hydrogen bonds, as well as on the dependence of these bonds on the charge state of the systems. Based on density functional theory results, it is found that the nature of the N/O–Au bonds between both guanine and its size-expanded form and three- and four-atom Au clusters is covalent in the neutral systems. In the −1 charged systems, the binding energy decreases by almost 50% with a significant change of geometry. Although the NH site in the spacer ring of size-expanded guanine may supply a new acceptor opportunity for forming an additional NH···Au hydrogen bond, this hardly emerges because of the nonplanarity and the large steric effect. The introduction of a spacer ring in guanine decreases the highest occupied molecular orbital–lowest unoccupied molecular orbital gap and expands the spatial distribution of electron wave functions, which make size-expanded guanine appealing for charge transfer performance. At the same time, it increases the steric hindrance, making the adsorption process more orderly, which is also good in view of molecular electronic devices