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

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 Kion-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.