A Chemical Shifts and Shielding Effects

The reverse mesomeric effect (p -p conjugation) is believed to be very favourable in the A -phospholen system (5). Compared with the corresponding A -phospholens (6), the conjugated system (5) shows both Y and the phosphorus atom to be deshielded, and the vinyl proton is shielded. The cyclic nature of the molecule is important because the analogous 

The chemical shifts, Sp, of substituted arylphosphonic acids (9) have been found to be linearly related to the Hammett a and Taft ojt and cti parameters. The shielding of the phosphorus nucleus increases with the electron-withdrawing properties of the substituents, which is analogous 

The P n.m.r, parameters have been tabulated for a wide range of P amino-compounds and P compounds. The value of Sp for compounds with four P—N bonds correlates with the hybridization of the nitrogen atom, moving to higher field in the order p sp sp sp. 

In contrast to the phosphines, the effect of angular restraint in phosphites is to cause shielding (see Table 2). However, the effect is not continuous because Sp decreases again when the restraint is severe. Possibly the increase in /-orbital occupation upon angular restraint goes through a maximum. 

The chemical shifts for P—F compounds have been reviewed.The compounds differ from most other organophosphorus compounds because Sp becomes more positive as the electronegativity of the atoms attached to phosphorus increases. The effect is at a maximum for P compounds. They behave normally with regard to an increase in shielding with increase in co-ordination number and therefore the P compounds are the least shielded. Thus the largest negative values (— 190 to — 250) are observed for compounds of the type YPFj. With the new value of Sp of + 80 for PFg, the variation of Sp with the number of fluorine atoms in P compounds is now shown to be fairly consistent. The value of Sp has also been reported for a series of aminohalogeno P compounds. - 

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