ANION-MOLECULE COMPLEXES IN SOLUTION .I. NUCLEAR MAGNETIC RESONANCE AND INFRARED STUDIES OF HALIDE ION-TRIHALOMETHANE ASSOCIATION

In weakly solvating media, the trihalomethanes associate specifically with the anions of tetraalkylammonium salts. The concentration and temperature dependence of the trihalomethane proton shielding has been analyzed to yield shieldings in the complex, equilibrium constants, and thermodynamic parameters. The values of ΔH suggest that complex formation is a process of differential solvation. CHF3, CHCl3, and CHBr3 are deshielded by 1-3 ppm in the complex; this and the infrared evidence indicate primarily hydrogen bonding to the ion. CHl3, on the other hand, shows increased shielding in the complex, and infrared spectra suggest a polarization or charge-transfer interaction via the iodine atoms. Since the field strength of a monopolar ion is known, the value of the coefficient a, in the Buckingham-Musher equation, [formula omitted], may be evaluated from the complex shifts; the commonly used value of a = 3 esu-1 appears to be a lower limit. Interpretation of the stabilities of the complexes in terms of an electrostatic model indicates that both hydrogen- and halogen-bond energies are dominated by the polarizability of the carbon-halogen bond in the molecule. © 1968, American Chemical Society. All rights reserved.