ACCEPTOR PROPERTIES OF SOLVENTS - THE USE OF ISOKINETIC RELATIONSHIPSTO ELUCIDATE THE RELATIONSHIP BETWEEN THE ACCEPTOR NUMBER AND THE SOLVATOCHROMISM OF N-PHENOLATE BETAINE DYES

The thermochromism of the pyridinium N-phenoxide betaine dyes, betaine 1 and betaine 2, has been studied in different solvents and over a ra nge of temperatures and shown to yield isoparametric (isokinetic) rela tionships (IKR). (The use of the more lipophilic betaine 2 enabled the range of solvents to be extended to cover very apolar solvents in whi ch betaine 1 is insoluble.) These IKRs led to the identification of fi ve different groups of solvents, each group showing different solute-s olvent and solvent-solvent mechanisms and being characterised by their functional groups. The corresponding T(iso) values are, as demanded b y theory, related to far-IR absorptions that are characteristic of rea ction heat baths, which in these systems are found to be the betaines themselves. These groups are reflected in the relationship between Rei chardt's E(T) values and Gutmann's acceptor numbers (AN) which is rein vestigated and each group shown to lie on one of a series of parallel lines in the E(T) vs. AN plot. It is demonstrated that acceptor parame ters depend strongly on the donor properties of the probe used to obta in them.