Phosphorus atom, deshielding

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... 

It is not clear why the sp2-hybridized carbon atom of a fluorenylidene germene (entry 3, Table II) is so shielded (8 79.8), unless this chemical shift is from the spectrum of the THF adduct.28 The chemical shift of the silicon atom in the germasilene (entry 4, Table II) and the phosphorus atom in germaphosphenes (entries 6-10, Table II) is deshielded compared to the corresponding silicon analogs. For example, for Mes2M=SiMes2... 

Hydroxy(alkoxy)phosphonium Ions. Olah and McFarland560 studied the protonation in HS03F or HS03F-SbF5 solution of varied phosphorus oxyacids and derivatives. Treatment of tetravalent phosphoms compounds (phosphorus, phosphonic, and phosphinic acid and their trialkyl and triaryl derivatives) results in (9-protonation and the formation of hydroxyphosphonium ions. Trivalent phosphites, in turn, are protonated at the phosphorus atom. The 31P shifts observed for the latter ions are significantly deshielded, which was attributed to significant oxonium ion character. 

Complexes of type A with phosphane are determined by the tr-donor bond of the phosphorus atom. On the contrary, the (d-d)n back donation of the phosphorus is poor, due to much lower x-acceptor ability, and the balance of electrons cannot take place within the phosphorus metal bond. With respect to 31P NMR the phosphorus molecule is deshielded, exhibiting a low-field shift for the i 1-coordinated P atom of about 30-60 ppm compared to the uncoordinated phosphane phosphorus. 

A selection of AN values has already been given in Table 2-5 of Section 2.2.6 cf. also Table 6-6 in Section 6.5.1. The observed solvent-dependent P chemical shifts result mainly from the polarization of the dipolar P=0 group, induced by the interaction with electrophilic solvents A, particularly HBD solvents. The decrease in electron density at the phosphorus atom results in a deshielding proportional to the strength of the probe/solvent interaction. In solutions of protic acids, the P chemical shift of the 0-protonated triethyl hydroxyphosphonium salt is observed. Since Et3PO is very hygroscopic and therefore not very suitable from an experimental point of view, the use of (n-Bu)3PO instead of Et3PO as probe molecule has been recommended [250]. 

The positive value found for PCI3 (and for other haldgenophosphines) can be understood as an inductive effect or as a very strong t bond which deshields the phosphorus atom. Many authors prefer the first explanation. In fact, in the case of PF3 the Ni-P force constant was 2.7 (206) or 2.37 mdyne/A. (123)—a rather low value which does not indicate the presence of a double bond. On the other hand, the examination of the force constants in the series Ni(CO)4 n(PF3)n (n = 0, 1, 2, 3, and 4) indicates that PF3 has about the same properties as CO (59, 123). This is supported also by the statistical distribution of the products in the exchange reaction between Ni(CO)4 and PF3 and by the little variation of NMR chemical shift on these products (56). Loutellier and Bigorgne (123) prefer to consider trifluorophosphine as a weak a donor and weak 7T acceptor for the following reasons. 

So far, we have discussed the nucleus in terms of a specific isotope in our case, P. NMR spectroscopy is only useful to us because there is a difference between phosphoms atoms in different local enviromnents. Each phosphorus atom in its own local enviromnent is assigned its own chemical shift value, measured in ppm. We know from C- and H-NMR that the origin of the chemical shift lies in the influence (shielding and deshielding) that the neighbouring atoms assert on the phosphoms atom. Mathematically, we can describe this phenomenon by the following equation ... 

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