Tris electronic-spectroscopic structural studies

A"-Ray structure determinations (see Chapter 11 for details) have been reported for triphenylphosphine oxide, tri-o-tolylphosphine oxide, sulphide, and selenide, and for cw-2,2,3,4,4-pentamethyl-l-phenylphos-phetan-1-oxide (5). Electron spectroscopic studies of phosphorus oxychloride and thiophosphoryl chloride in the gaseous state, and n.m.r., i.r., and u.v. spectra of phosphine sulphides have appeared. Dipole moments have been used to define the stereochemistry of 2-cyanoethylphosphine oxides, such as (6), which is shown in its preferred conformation. 

Rudd, M.D., Lindeman, S.V and Husebye, S., Three-centre, four-electron bonding and structural characteristics of two-coordinate iodine(I) complexes with halogen and chalcogen ligands. Synthesis, spectroscopic characterization and x-ray structural studies of (triiodo) [tris(dimethylamino)phosphaneselenide]iodine and bis (triiodo) [tri(iV-morpholyl)phosphaneselenide]iodine(I) /diiodine molecular complex, Acta Chem. Scand. 51 (6-7), 689-708 (1997). 

It is true that in some cases, the spectroscopic data on a reactive intermediate are so persuasive that the connection between structure and spectroscopic features is firm. However, in general this will not be the case, and additional spectroscopic or preparative criteria will have to be provided. So we are faced with the question How can we connect the information obtained, for example, from observations in matrices or in solution-phase fast kinetic studies, to molecular structure How do we know that the results of these experiments, using what we hopefully call direct methods, really pertain to the species we are trying to characterize I attempt to deal with this issue in what follows. Since the methods used vary from one class of non-Kekule species to another, specific classes are individually discussed, and special techniques are introduced as needed. Electron spin resonance spectroscopy has played such a pervasive role that it will be useful to give first a brief outline of that method. 

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