Association between polar molecules. Part 2.—Equilibrium and thermodynamic studies on the dipole association of benzonitrile derivatives with hexamethylphosphoramide, di-n-butyl sulphone, di-n-butyl sulphoxide and tetramethylurea in non-polar solvents

Abstract

Dipole association between p-substituted benzonitriles and polar substances such as hexamethylphosphoramide(HMPA), di-n-butyl sulphone(DBSN), di-n-butyl sulphoxide(DBSX) and tetramethylurea (TMU) is investigated in non-polar solvent by means of n.m.r. spectroscopy assuming the formation of a 1 : 1 associate between the polar molecules. Association parameters (association constant and association shift) are derived and discussed in relation to the dipole moment and the substituent constant. The association constant is, as expected, dependent upon the dipole moment of HMPA, DBSN, DBSX and TMU, and also upon the substituent constants of the p-substituted benzonitriles. However, the correlation between this constant and the dipole moment of the benzonitriles is the reverse of that expected. Polarization in the Ph—X fragment in benzonitriles which opposes that in the CN bond is thought to cause this phenomenon.

The association shift, which is the change in shift on association, cannot be explained simply by the dipole moment of HMPA, DBSN, DBSX and TMU, and so the magnetic anisotropy of these compounds is considered to be responsible. However, when a comparison is made within each benzonitrile + polar substance system, the anisotropy effect seems to be cancelled out and a correlation is established with the substituent constants.

Thermodynamic parameters are also estimated for the association of HMPA with substituted benzonitriles and for association with substituted nitrobenzenes and several aliphatic nitro and nitrile compounds. These are in the range expected for the dipoles under consideration, and the model adopted to interpret the concentration and temperature dependences of the shift is proved to be sound.