Molecular structure of phosphabenzene: Analysis combining electron diffraction and microwave data

Abstract

The average (ra) structure of vapor phase C5H5P, the phosphorus congener of pyridine, was deduced. Vibrational corrections of diffraction data and rotational constants were made via a normal coordinate analysis based on an approximate quadratic force field. The heterocyclic molecule was found to be planar with principal structure parameters and uncertainties (2.5σ) of rg(C�P) = 1.733±0.003 Årg(C�P)=1.733±0.003Å, rg(C2�C3) = 1.413±0.010 Årg(C2�C3)=1.413±0.010Å, rg(C3�C4) = 1.384±0.012 Årg(C3�C4)=1.384±0.012Å, rg(C�H) = 1.122±0.015 Årg(C�H)=1.122±0.015Å, ∡CPC = 101.1°±0.3°∡CPC=101.1°±0.3°, ∡PCC = 124.4°±0.7°∡PCC=124.4°±0.7°, and ∡C2C3C4 = 123.7°±0.8°∡C2C3C4=123.7°±0.8°. Amplitudes of vibration were determined in the diffraction analysis as well as in the normal coordinate computation. A variety of different types of error matrices were computed to assess the effect of data correlations and the possible coupling, over and above scale factor shifts, between systematic errors in intensities and derived parameters. Conventional treatments of data correlations appear to lead to overoptimistic error estimates in the case of the longer internuclear distances. The structural characteristics of phosphabenzene are those expected for an aromatic molecule. Semiempirical molecular orbital calculations, adjusted to fit the experimental structure, suggest a small but significant involvement of phosphorus d orbitals in the π bonding.