Molecular Complexes, V [1, 2] 1H NMR Investigation of the Caffeine Benzene Complex Followed by AUS Correction of Experimental Data Structural Requirements for Substances to Serve as Internal Reference in Investigations of Weak Complexes

Abstract

Abstract The 4 proton signals of caffeine, dissolved in carbon tetrachloride, were investigated as functions of benzene concentrations in a very large concentration range. Shifts of the 4 signals were measured relative to both internal and external reference. Scatchard plots of the internally referenced data were curved downwards, the degree of curving increasing from 7-methyl/8-H to 3-methyl to 1-methyl. The 1-methyl plot even showed a maximum in the observed relative shifts ⊿oi which turned to negative values for high benzene concentrations. Scatchard plots of the externally referenced data were curved upwards. Applying AUS corrections [1, 8] to both sets of data furnished linear Scatchard plots for ah signals. The plots for the 4 protons were parallel to one another in case of the externally referenced data and not parallel for the internally referenced data. Equilibrium quotients K calculated from the externally referenced data for each of the 4 different protons were essentially the same. This gives evidence that the system behaves in accord with the pure 1:1 complex model and does not demand the assumption of simultaneous occurence of higher complexes. The values for K (= .115 l/mol), for the 4 apparent complex shifts (Icpt/K) and for the 4 nuclei specific AUS coefficients a 2 [1, 8] could be confirmed and somewhat improved by a Creswell-Allred processing of the data including AUS correction. The failure of the AUS correction in producing parallel Scatchard plots from internally referenced shifts is caused by the nonlinear floating with the donor concentration of the internal reference signal. The cause of this nonlinear behaviour is analyzed, and arguments are presented that it can be prevented by use of reference substances with only one single reference group per molecule, i.e. the signal of such an internal reference must not have its origin in two or more equivalent nuclei groups within the reference molecule.