Electromigration of a heteroconjugated imidazole–acetate complex in ACN

Abstract

ACN is an extremely poor hydrogen bond donor and therefore the anions dissolved in it are solvated mainly by other hydrogen bond donors (e.g. uncharged acids) possibly present in the solution. Under properly selected experimental conditions stabilization via hydrogen bonding can be used for separation in CE as has been demonstrated for uncharged acids by several authors. Electromigration based on heteroconjugation can be of importance, e.g. when aqueous separation medium cannot be used due to stability reasons. It also allows CE to be used as a tool for solution chemistry measurements, if the required physicochemical properties of the studied system are known or they can be predicted with sufficient accuracy by existing theories. In the present work we showed that also an uncharged base can stabilize an anion via hydrogen bonding in ACN. In the setup imidazole was chosen as a model base and acetate ion as complexing anion in equimolar acetic acid-acetate buffer. The resulted hydrogen-bonded imidazole-acetate complex (i.e. heteroconjugate) possesses a charge and can thus migrate in CE. It was shown that the studied complexation in ACN is sensitive to competition by other hydrogen bond donors such as water and methanol. On the other hand, acetone, which is a poor hydrogen bond donor, did not have much effect on the complexation. To take the effect of ionic strength on mobility into account, mobilities of the imidazole-acetate complex measured at various ionic strengths were corrected to zero ionic strength by the aid of conductivity equation. A fit of the 1:1 binding isotherm to the ionic strength corrected mobility versus acetate concentration data led to rather good correlation. However, x-reciprocal linear transformation of the binding isotherm showed nonlinearity, which could be partly explained by homoconjugation of acetic acid and acetate ion. Since the homoconjugation constant for acetic acid under present experimental conditions was not available, theoretical simulations were used to demonstrate the effects of homoconjugation. The possibility of multiple complexation of imidazole was discussed as well.

A novel technique for NACE coupled with simultaneous electrochemiluminescence and electrochemical detection for fast analysis of tertiary amines

“…Concerning ion‐pair, it is caused by the interaction of two ions with opposite charge. Heteroconjugation and homoconjugation are the main effects in ACN, in that ACN is a dipolar‐aprotic solvent and it cannot donate hydrogen bond 30. Therefore, anions in ACN must be solvated by other hydrogen bond donor, which causes the formation of homoconjugation and heteroconjugation.…”

Non‐aqueous capillary electrophoresis 2005–2008

“…Porras and Jussila studied the electromigration of heteroconjugated complexes in ACN, using imidazole as the uncharged compound and acetate as small complexing anion 40. Ma et al demonstrated that the separation of four calix[4]pyrrole derivatives in NACE was possible as a result of heteroconjugation with chloride anion 36, 37.…”

Organic solvents in CE

“…The ionic strength of the solution is not changed, because for each reacting anion A − another anion is formed. Note that non‐ionic analytes can be transformed into ionic heteroconjugates (with the constituents of the BGE), a reaction that can be used to separate initially neutral analytes by CE 31–34.…”

“… One is the utilisation of heteroconjugation, which is hardly occurring in aqueous solution because of the excellent solvation properties of water. In solvents with low solvation ability, heterconjugation can be introduced as a vehicle to generate additional separation selectivity, and even to implement electrophoretic movement of initially non‐charged analytes 31–34. A convincing example was given by Okada (Fig.…”