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The complexation of tetramethyluric acid (TMU) with the donors (D) benzene (B), biphenyl (BP), naphthalene (N), and phenanthrene (P) in CCl 4  is investigated by  1 H NMR shift measure­ments relative to an external reference in a series of solutions containing a constant concentration of TMU and varying concentrations of D under conditions of a great excess of D. Reduction of data according to the AUS concept (additional nonspecific shielding) provided formation con­stants K (in 1/mol) and Icpt/K as approximations of the complex shifts. The increase in K from .17 (TMU-B) to .75 (TMU-N) to 2.08 (TMU -P) is compatible with a face-to-face complexation and with a preferred contact between the donor and TMU along the area of positive charge in the TMU molecule. The estimated Icpt/K figures indicate the centers of the complexes to lie between N-3 and N-9 of TMU, quite in the center of the positive charge area of TMU. K and Icpt/K figures for TMU-BP indicate this complex to have the same topology as TMU-B. the second ring of BP rotating in plane around the complexing ring of BP. The previous rejection of a face-to-face topology for such complexes was based on a misinterpretation of the ring current calculations. Deviating complex parameters in the previous report are shown to be caused by a solvent effect (CDCl 3 ) and in part by methodical errors.

zeitschrift für naturforschung b

Molecular Complexes, 8  Formation Constants and Topology of Complexes of Tetramethyluric Acid. An  1 H NMR Reinvestigation Based on the AUS Concept and on the Shift Difference Method. A Revised Complex Topology