Gold at crossroads of radical generation and scavenging at density functional theory level: Nitrogen and oxygen free radicals versus their precursors in the face of nanogold

In our previous report (J. Phys. Org. Chem., 2017), we discussed the dual behavior of gold nanocluster (Au3 NC), where it scavenged reactive oxygen species (ROS) while promoted their generation to a lesser extent. Continuing this quest, we investigate the effects of Au3 NC on common reactive nitrogen species (RNS: ON˙ and ON˙O) and their precursors (ONH and ONOH, respectively), at B3LYP/LACVP+* level of theory. We compare the results with those of prevalent ROS (HO˙ and HOO˙) and their precursors (HOH and HOOH, respectively). To this end, various parameters are probed such as binding energy (Eb), bond dissociation energy (BDE), bond lengths, Mullikan spin density (MSD), critical point (CP) eigenvalues (atoms in molecules [AIM] analysis), and natural bond orbital (NBO) analysis including natural population analysis (NPA) and change of charge transfer energy (ΔECT). Our results show that in the presence of Au3 NC, BDEs of the NH and OH bonds increase in both ONHAu3 (1) and ONOHAu3 (2) compared with their free forms (ONH and ONOH, respectively). In other words, the tendency to dissociate these bonds and form their corresponding RNS (1′ and 2′) decreases in the presence of gold. This is exactly the opposite of what is observed for the OH bonds in HOHAu3 (3), HOOHAu3 (4), and their corresponding ROS (3′ and 4′). Therefore, we can conclude that gold shows a different behavior in the face of RNS compared with ROS where it is a better scavenger of ROS than RNS.