NUCLEAR MAGNETIC RESONANCE MEASUREMENTS OF COMPLEXES OF CHLOROFORM WITH AROMATIC MOLECULES AND OLEFINS

Abstract

In order to study the effect of association with π-donors, the proton resonance signal of chloroform in the aromatic solvents benzene, toluene, mesitylene, chlorobenzene, o-dichlorobenzene, bromobenzene, nitrobenzene and in the olefinic solvents 1-hexene, cyclohexene, and cyclohexadiene was measured as a function of concentration. The observed shifts of the resonance signal are indicative of specific complex formation. In the aromatic solvents benzene, toluene, and mesitylene, complex formation results in an anomalous shift of the chloroform resonance to high field, which can be attributed to a large induced diamagnetism resulting from the circulation of π-electrons in the aromatic ring. An approximate calculation indicates that the chloroform molecule in the complex is oriented with the hydrogen atom near the six-fold axis above the plane of the aromatic ring and the chlorines directed away from the ring. In nitrobenzene this type of association with chloroform gives way to n-donor association with the oxygen atom of the nitro group. This type of association also occurs to some extent in the halogenated benzenes. A small shift of the chloroform signal to low field is observed with the olefinic solvents, which is attributable to weak π-donor association. Analysis of the freezing-point diagrams of the binary systems with chloroform provide further evidence of complex formation.