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

Abstract

Abstract The complexation of tetramethyluric acid (TMU) with the donors (D) benzene (B), biphenyl (BP), naphthalene (N), and phenanthrene (P) in CCl4 is investigated by 1H 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 (CDCl3) and in part by methodical errors.