The effect of tetramethylammonium ion on the voltammetric behavior of polycyclic aromatic hydrocarbons: computations explain a long-standing anomaly

Abstract

Cyclic voltammograms of several polycyclic aromatic hydrocarbons (PAH's) in highly purified N,N-dimethylformamide are known to exhibit two reversible reduction waves. To a good approximation, the potential of the first wave is independent of the nature of the supporting electrolyte, but the potential of the second wave is highly dependent upon the nature of the electrolyte. The spacing ΔE° between the first and second waves increases as the size of the cation of the electrolyte is increased from Et₄N⁺ through Pr₄N⁺ to Bu₄N⁺. This is typically interpreted as due to decreasing strength of ion-pairing between the cation and the dianion of the PAH with increasing size of the electrolyte cation. However, it has been known for many years that Me₄N⁺ exhibits anomalous behavior: even though Me₄N⁺ is much smaller than Et₄N⁺, ΔE° is greater with Me₄N⁺ than with Et₄N⁺ for anthracene and in fact greater than any of the larger electrolytes with perylene. It is now shown that this behavior arises out of the fact that Me₄N⁺ ion is present in solution as a tetrasolvate [Me₄N⁺/(DMF)₄]. The PAH dianion (Ar⁻²) reacts with Me₄N⁺/(DMF)₄ to displace a molecule of DMF and produce the species Me₄N⁺/(DMF)₃/Ar⁻². The computed pairing association constant K_ion-pairing for the anthracene species is 35 M⁻¹, compared with a value of 50 000 M⁻¹ for association of the bare Me₄N⁺ ion with the dianion; the corresponding values for perylene are computed to be 4400 and 3.5 M⁻¹, respectively.