Restricted rotation of the phenyl group in β‐hydroxyphosphonates: Dynamic NMR and semiempirical studies

Abstract

AbstractFree energies of activation, ΔG, for the rotation of the phenyl group in the range of 46.06–63.25 kJ mol−1 were observed in some model β‐hydroxyphosphonates. These barriers were found to depend on the type of substituents at the phosphorus atom and on the substituents at the β‐position with respect to the phenyl ring. The steric origin of this observation is discussed. Dynamic NMR studies were performed in order to determine the activation parameters for the exchange process. For that purpose, variable‐temperature 1H and 13C NMR spectra were treated by non‐iterative, iterative and iterative double‐fit procedures. MNDO, AM1 and PM3 theoretical calculations of the barriers for three of the compounds studied are reported and results are compared with the experiments. It is found that the PM3 results are very close to the experimental data.