A Simple Strategy to Design Polycyclic Superhalogens

Polycyclic hydrocarbon (PH) radicals and anions such as C₉H₇^-, C₁₁H₇^-, C₁₃H₉^-, and C₁₅H₉^- generally possess low electron affinity (EA) and vertical detachment energy (VDE), respectively, due to their aromaticity and, consequently, enhanced stability. In this work, we propose a simple strategy to design polycyclic superhalogens (PSs) by replacing all hydrogens by cyano (CN) groups. Superhalogens can be defined as the radicals having higher EA than halogens or anions having higher VDE than halides (3.64 eV). Our density functional calculations suggest that the EA (VDE) of PS radicals (anions) exceeds 5 eV. All these PS anions are aromatic, except C₁₁(CN)₇^-, which is anti-aromatic. The superhalogen property of these PSs can be attributed to the EA of CN ligands, leading to the delocalization of extra electronic charge significantly as explained using prototype C₅H_5-(CN) systems. We also notice that the 'superhalogenity' (superhalogen behavior) of C₅H_5-(CN)^- is directly related to their aromaticity. We have also shown that the substitution of CN is energetically favorable, which confirms their experimental viability. Our findings should motivate experimentalists to synthesize these superhalogens for further exploration and future applications.