Electrophilic and oxidative chemistry of pyrene and its non-alternant isomers: theoretical (DFT, GIAO-NMR, NICS) study of protonation carbocations and oxidation dications from pyrene, azupyrene (dicyclopenta[ef,kl]heptalene) and dicyclohepta[ed,gh]pentalene

Abstract

Mono- and diprotonated carbocations and the two-electron oxidation dications derived from parent pyrene 1 and its nonalternant isomers “azupyrene” (dicyclopenta[ef,kl]heptalene) (DCPH) 2 and dicyclohepta[ed,gh]pentalene (DCHP) 3 were studied by DFT at the B3LYP/6-31G(d) level. The most likely site(s) for mono- and diprotonation were determined based on relative arenium ion energies and the structures of the energetically most favored carbocations were determined by geometry optimization. The NMR chemical shifts for the protonated mono- and dications and the oxidation dications were computed by GIAO-NMR at the B3LYP/6-31G(d)//B3LYP/6-31G(d) level and their charge delocalization paths were deduced based on magnitude of the computed Δδ¹³C values and the NPA-derived changes in charges. Relative aromaticity/antiaromaticity in various rings in the energetically favored mono- and dications was estimated via NICS and ΔNICS. Calculated NMR chemical shift data for 1H⁺+ and 1²⁺ were compared with the available experimental NMR values. The available data on chemical and physical properties of DCPH 2 and DCHP 3 are extremely limited and biological activity data are non-existent. The present study provides the first glance into their carbocations and oxidation dications, while augmenting and reinforcing the previous stable ion data on the pyrenium cations.