A proton magnetic resonance investigation of some weak interactions in solution

Abstract

The proton chemical shifts of cyclohexane, methyl iodide and iodoform are measured in a number of solvents. A complete calculation of the contribution to the solute proton chemical shift arising from the magnetic anisotropy of cylindrically symmetric solvents is given. Although the formula predicts the direction of the observed shifts, the observed values for non-polar solutes are always much larger than the calculated values. Some possible reasons for this are given and discussed. The variation of the proton chemical shifts of the polar solutes methyl iodide and iodoform in aliphatic solvents are shown to agree with present theories of these effects. However, in aromatic solvents considerable deviations from the theoretical line are found and these are postulated to arise from solute solvent complexes in which the dipole axis of the solute lies along the hexagonal axis of symmetry of the benzene ring with the protons towards the ring. From the variation of the proton chemical shifts of methyl iodide and iodoform in toluene solution with temperature the following parameters were obtained. For the methyl iodide toluene complex the energy and entropy of formation are 1·3 ± 0·5 kcals/mole and 4·9 ± 0·4 e.u. respectively. For the iodoform toluene complex the corresponding values are 1·6 ± 0·2 kcals/mole and 6·4 ± 0·2 e.u.