Principles of Phosphorus Chemistry: Molecular States of Phosphorus Compounds - A Report on Gasphase Investigations ^1-3

Abstract

An up-to-date concept of bonding in phosphorus compounds has to be based on the reality of molecular states. Molecules, which change their structure with energy, at present are best rationalized in terms of topology and symmetry, effective nuclear potentials and charge distribution. To reduce the complexity of the resulting manifold, comparison of equivalent states of chemically related molecules, based advantageously on quantum chemical calculations, is strongly recommended. Adding the time-scale, molecular dynamics within the numerous degrees of freedom become important, also as a basis to gain some understanding of the rather complex microscopic reaction pathways of medium-sized molecules. Examples are presented to illustrate the use of spectroscopic ‘fingerprints’ for the analysis and optimization of gasphase reactions as well as the benefit of inherent information on molecular states for the preparative phosphorus chemist. The catalytic dehydrochlorination of alkyldichlorophosphanes RH₂C-PCl₂ → RHC = PCI → R-C ≡ P and their dechlorination on magnesium metal surface are discussed in some detail as well as the generation of other unsaturated phosphorus molecules like Cl-P = O, Cl-P = S, Cl-P(= O)₂, Cl-P(= S)₂ or H₃C-P =CH₂. Approximate energy hypersurface calculations for the gasphase equilibrium P₄ ⇄ 2 P₂ or for the unexpected dehydration (H₃C)₂HP=O → H₂O + H₃C-P = CH₂, which includes chemical activation, provide some insight into microscopic reaction pathways of phosphorus compounds.