Molecular complexes—IV : Effect of additional unspecific shielding (aus). Aus correction of NMR data applied to the 1:1 complex formation between 1,3,5-triacetyl benzene and benzene

Abstract

Shifts of the two proton NMR signals of 1,3,5-triacetylbenzene (TAB) both relative to external and internal reference have been measured in CCl 4 solutions as function of the donor (C 6D 6) concentrations. A very large range of donor concentrations [D o] i yielded plots of Δ i /[ D o] i vs Δ i (simple Scatchard plots) which were distinctly curved. External referencing yielded upward curvatures, internal referencing even stronger downward curvatures. The external referenced shifts were subjected to a linearizing procedure according to the modified Scatchard equation developed from the AUS concept. These independently from one another linearized Scatchard plots (for methyl and for aromatic signal) were exactly parallel, i.e. they gave exactly the same equilibrium quotient K. Performing the linearizing steps in the Cresswell-Allred procedure instead of the Scatchard procedure gave negligible deviations from the modified Scatchard K.