The Journal of Physical Chemistry, Vol. 83, No. 6,
Communications to the Editor
nonspecific solvent-solute interactions. The treatment proposed by Litt and Wellinghoff has several undesirable features. Mathematically, the correlation of five-seven experimental observations with four adjustable parameters is a poor practice, even with the restriction that the extinction coefficient of the complex ( E ) must be the same in all solutions. In the case of carbon tetrachloride as solvent, the conventional treatment (of all the data with two parameters ( K = 0.7 L/mol, E = 387) gives better agreement with the reported values of the normalized absorbance (in terms of root-mean-square deviations and the maximum deviation) than does the proposed treatment with four parameters. The obvious redundancy in the four-parameter fit must cast doubts on the physical significance of the parameters. This redundancy is clearly illustrated in the application of their equation with one of their parameters fixed (a = 1, eq 3, ref l),giving a linear relationship between the reciprocal of the normalized absorbancy and the ratio of concentrations of inert solvent and donor. From this linear relationship requiring only two independent parameters, they extract three parameters, each with stated limits of uncertainty. On a theoretical basis, their assumption of simple exchange equilibria between solvent and donor in the first coordination shell of the acceptor (an assumption which is not supported by any experimental evidence) should require the use of activity coefficients for the solvent and donor and for the various solvated fornis of the acceptor. Cancellation of these activity coefficients is a t best equivalent to similar assumptions which have been made in the usual treatments. In the study of weak charge-transfer complexes (or hydrogen bonds), it is often necessary to make measurements over the entire composition range from almost pure solvent (S)to pure donor (D), with the acceptor (A) and the complex (C) very dilute.' The NIBS (nearly ideal binary solvent) treatment3$4has been shown to be quite dependable for estimating thermodynamic properties of simple solutes at high dilution in binary solvent pairs which are free of association. The form of the NIBS equation which has been most successful for describing the excess chemical potential of solutes is based on the simple mixing model of a multicomponent ~ y s t e m ~ , ~
structure >As-C-C-As
