High Resolution Nuclear Magnetic Resonance Spectroscopic Studies of Some Aspects of Molecular Interaction in Solution

Abstract

The investigations described herein concern the proton magnetic resonance studies of complexes transiently formed by nitroform and the haloforms with benzene and its methyl homologues. The association constant for each of the complex equilibria in cyclohexane, and the difference between the solute proton screening in free and fully complexed situations are determined at a number of temperatures from the solute proton shift dependence on solution composition. The chemical shift difference is used to estimate the average position of the proton in each complex which is found to be in contact with the aromatic ring on its six-fold symmetry axis. The distance between the solute proton and the aromatic ring for most of the complexes is found to be slightly less than expected from the sums of the proton and aromatic ring Van der Waals radii. Parallel compression of the π-system is independently indicated. The nature of the associative bond between solute and aromatic molecules is found to be consistent with a dipole-induced dipole interaction. Two models for describing the nature of the complex are suggested but that favoured has the solute proton fixed in the position mentioned above with the remainder of the solute "wobbling" relative to the aromatic. ΔGº, ΔHº, and ΔS° for complex formation are deduced. The enthalpy changes are found to be independent of temperature within the range investigated and indicate the exothermic nature of the formation of each complex. The entropy changes indicate in each case the expected increased order on complex formation. The free energy changes are correlated with calculated interaction energies for the favoured model.(ii)Published dipole moment data are used to determine the solute-aromatic inter-dipole distances. From these and the complex geometries, the location of the solute equivalent point dipole is deduced.