Phosphorus-31 chemical-shift theory

More recently Gutowsky and Larmann and Letcher and Van Wazer ° have independently presented detailed quantum mechanical interpretations of phosphorus chemical shifts. Somewhat similar general conclusions were reached, although the former treatment was limited to (7-bonding effects, whereas the latter theory was based on the involvement of both a- and 7t-orbitals. 

Letcher and Van Wazer applying LCAO theory first developed by Pople and Karplus and Das, have expressed the shielding (which was dominated by the magnitude of (7 ), in terms of localized bond parameters, e.g., ionic character, hybridization and double bonding. Using expressions for (7 obtained by Jameson and Gutowsky and taking account of both phosphorus p and d orbital occupation, the phosphorus chemical shift, <5 —was related to four parameters and/by equation (11)—... 

Up to the submission time of this manuscript there appears to have been no clear separation of these three in terms of their relative importance for phosphine complexes. This fact, combined with the paucity of calculations in this area, has led to the elevation of a group of empiricisms to the level of "theory in the question of the interpretation of phosphorus chemical shifts. This situation will undoubtedly change however, until such time the empirical approach, often strongly supported by X-ray structure determinations, must suffice. To appreciate how these relationships were developed it is useful to begin with a brief discussion of the phosphine ligands themselves. 

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