Iterative Analysis of Exchange-Broadened NMR Band Shapes. The Mechanism of Correlated Rotations in Triaryl Derivatives of Phosphorus and Arsenic

Article abstract of DOI:10.1021/ja00366a006

Tris<2,6-bis(difluoromethyl)phenyl>phosphine and tris<2,6-bis(difluoromethyl)phenyl>arsine as well as the corresponding oxides have been synthesized.The 19F NMR data extracted from the static spectra at low temperature rigorously establish that all four compounds adopt a chiral propeller equilibrium conformation in solution.A theoretical analysis of the conformational dynamics allows for seven differentiable permutational mechanisms (M1-M7), of which five (M1-M4, M7) could be experimentally excluded on the basis of the fast-exchange limit spectra.Iterative band shape analyses of the exchange-broadened spectra for the remaining two possibilities (M5, M6) and for two alternative relative assignments of the 19F chemical shifts simultaneously yielded the correct assignment as well as direct and quantitative proof for the exclusive operation of the permutational mechanism M5, which corresponds to a process commonly referred to in the literature as the "two-ring flip".The relative shift assignment was independently confirmed by a selective population inversion experiment on the arsenic compound.The skeleton of a density matrix theory for the latter experiment is outlined in the Appendix.