Ion—molecule chemistry of pyrene in an ion‐trap mass spectrometer

Abstract

AbstractThe quadrupole ion‐trap mass spectrometer is used to investigate aspects of the ion‐molecule chemistry of the pyretic molecular radical cation (M+), the [M  H]+ ion and the [M  H2]+˙ ion. Cycloaddition and alkyl halide addition reactions with the reagents pyrene, methyl iodide, ethyl iodide, isopreue and 2‐iodopropane are investigated. The pyrene molecular radical cation is remarkably unreactive while the closed shell [M  H]+ ion is highly reactive and the [M  H2]+˙ ion shows intermediate reactivity. Analogies are found between the behaviour of the pyrene molecular radical cation and the corresponding [M  H]+ ion on the one hand, and the benzene radical cation and the phenylium ion on the other. The total lack of reactivity of the M+˙ ion is attributed to π‐delocalization of the radical and the cation over the large aromatic system. Addition of isoprene to the [M  H]+ and [M  H2]+˙ ions of pyrene occurs via cycloaddition reactions. A characteristic fragmentation of each adduct ion is methyl radical loss and evidence obtained using pyrene‐d10 indicates that the product in the case of [M  H]+ is an ionized indene. Addition of methyl iodide, ethyl iodide and 2‐iodopropane to the [M  H]+ ion of pyrene occurs to give an iodonium product (C16H9–I+–R) which fragments by loss of I˙ or CH3˙ to give the methylpyrene or iodopyreue product ions. Reactions of ethyl iodide and 2‐iodopropane with the dehydropyrene ion, [M  H2]+˙, give primarily an HI addition product, together with small contributions from the intact adducts. Evidence is provided suggesting that in this case the HI and alkyl halide molecules add across a CC bond rather than forming an iodine‐bound adduct. An experiment in which nine consecutive isolation and activation steps (MS10) are used to couvert the [2M  H]+ adduct, C32H, to C32H˙ exemplifies the intriguing ion chemistry and the stability of the carbon skeleton of the adduct ions.