John P. Hunt

Chlorine‐35 NMR Study of the Shifts and Line Shapes of Some Liquid Inorganic Chlorides

The chemical shifts and linewidths of a series of 18 liquid inorganic chlorides have been measured using 35Cl broadline NMR techniques. The chemical shifts vary from 120–950 ppm downfield from the chloride ion. Assuming the mean excitation energies of these compounds are equal, a rough correlation of shift with quadrupole coupling constants is obtained for the nontransitional metal chlorides. The shifts of the transitional metal chlorides do not correlate with the quadrupole coupling constants, suggesting dπ–pπ interactions. The full linewidths at half‐maximum intensity vary from 0.15–48 G. The observed widths, corrected for viscosity and molecular dimensions, correlate reasonably well with the quadrupole coupling constants. The observed correlation times τc are about a factor of 10 shorter than those calculated with the Debye–Einstein equation. The line shapes and widths were determined using a computer program that resolved the experimental spectra in terms of overlapping Lorentzian functions.

Variable pressure and temperature 1H NMR studies on exchange mechanisms in π-allylpalladium(II) complexes

Abstract High pressure 1H NMR spectroscopy on a mixture of [Pd(2-methallyl)Cl]2 (dimer) and Pd(2-methallyl)Cl(PPh3) (monomer) in CDCl3 at 22°C gives ΔV‡ = 0 ± 2 cm3/mol. At this temperature syn-anti exchange of the allyl hydrogens in Pd(2-methallyl)Cl(PPh3) occurs, catalyzed by the complex [Pd(2-methallyl)Cl]2. To explain this ΔV‡ value we propose an overall cancelling of opposite volume contributions from the initial partial bridge cleavage of dimer and its subsequent association with monomer. At 56° C ΔV‡ is 11 ± 2 cm3/mol. At this temperature, 2-methallyl exchange between Pd(2-methallyl)Cl(PPh3) and [Pd(2-methallyl)Cl]2 occurs by a process involving dimer cleavage followed by PPh3 transfer. We propose the bridge cleavage of dimer to Pd(2-methallyl)Cl is the dominant step causing the overall positive ΔV‡ change.