Stable ion and electrophilic chemistry of fluoranthene-PAHs

Abstract

The first examples of persistent carbocations derived from parent fluoranthene (four fused rings), benz[e]acephenanthrylene (benzo[b]fluoranthene) (five fused rings) and its C-10 substituted derivatives (X = OMe, F), as well as indeno[1,2,3-cd]pyrene (six fused rings) by protonation with FSO₃H–SO₂ClF are reported. NMR characteristics (500 MHz) of the resulting carbocations, their charge delocalization mode and tropicity are examined.

Relative arenium ion energies for all possible protonation sites were calculated by AM1 for comparison with the NMR-based assignments. The NMR chemical shifts for the observed arenium cations were computed by GIAO-DFT calculations at the B3LYP/6-31G(d)//B3LYP/6-31G(d) level (their energies and those of their neutral precursors were also calculated at this level of theory). Relative aromaticity in various rings in the resulting PAH-arenium ions was gauged via nucleus independent chemical shift (NICS) calculations.

It has been shown that the site of protonation and nitration in 10-methoxybenz[e]acephenanthrylene are the same (C-9). Quenching of the superacid solutions of fluoranthene and 10-methoxybenz[e]acephenanthrylene produced the dimers 3,3′-bifluoranthenyl and 10,10′-dimethoxy-9,9′-biacephenanthrenyl as minor products (ca. 10% and ca. 33% respectively) in addition to the intact PAHs. It has also been demonstrated that fluoranthene-PAHs and their derivatives are easily protonated with [NH₄][NO₃] and observed in the gas phase via electrospray mass spectrometry (ES-MS).