Orthophosphate resonance structures

FIGURE 10 Alternative ways of showing the structure of inorganic orthophosphate, (a) In one (inadequate) representation, three oxygens are single-bonded to phosphorus, and the fourth is double-bonded, allowing the four different resonance structures shown, (b) The four resonance structures can be represented more accurately by showing... 

While benzene is the most celebrated example in carbon chemistry (3.46), the orthophosphate anion (3.47) is probably the most commonly encountered resonance structure in phosphorus chemistry. One of the simplest examples in carbophosphorus chemistry is phosphinine (see (3.48a) and (6.860a)). Cyclohexaphosphene (3.48b) may also exist under certain conditions (see (4.32a) and (8.259)). 

Two azaphosphorus anions (7,18c) and (7.18d) have been characterised in the solid state (see below). These orthophosphazate anions can be compared with the tetraamino phosphonium cation (7.18a) and the (hypothetical) phosphorimidic triamide (7.18b) to which they are related. Both the phosphorimidic anion (7.18c) (which is the imido analogue of the PO " anion) and the orthophosphazate anion (7.18d) appear to be symmetrical and tetrahedral with four equivalent bonds in each case. Resonating structures analogous to those of the orthophosphate anion (3.16a) are presumably involved. 

Phosphate (or, Orthophosphate) or PO/ . The conjugate base of HP04 , one of the 0x0 anions of phosphoric acid (the values of H3PO4 are, at 25°C, 2.15, 7.20, and 12.33). The structure of the resonance-stabi-hzed P04 is tetrahedral and can form complexes with metal ions and other cations. 

Other glasses of possible interest as possible biocompatible materials include those in the system Si02-P205-Ca0-Mg0, the structural aspects of which have been studied by - P MAS NMR (Oliveira et al. 2000). The NMR spectra show that the phosphorus is present in an orthophosphate environment and that replacement of Ca by Mg " in the structure is accompanied by a shift of the P resonance to lower frequencies. This replacement is reflected by the composition of the glass recrystallisation products, which contain mainly hydroxyapatite Caio(P04)6(OH)2 at lower MgO contents, but whitlockite (Ca7Mg2P6024) in glasses of higher MgO content (Oliveira et al. 2000). 

What is the structural basis of the high phosphoryl transfer potential of ATP Because AG° depends on the difference in free energies of the products and reactants, the structures of both ATP and its hydrolysis products, ADP and Pj, must be examined to answer this question. Three factors are important resonance stabilization, electrostatic repulsion, and stabilization due to hydration. ADP and, particularly, Pj, have greater resonance stabilization than does ATP. Orthophosphate has a number of resonance forms of similar energy (Figure 14.4), whereas the y-phosphoryl group of ATP has a smaller number. 

The solid-state chemical, optical, and physical properties of the RE, Y, and Sc orthophosphates have been extensively investigated by means of numerous techniques. Such studies include optical spectroscopy (Trukhin and Boatner 1997), x-ray absorption (Shuh et al. 1994), electron paramagnetic resonance (EPR) spectroscopy (Abraham et al. 1981, Boatner et al. 1981b), Mossbauer (Huray et al. 1982), Rutherford backscattering (Sales et al. 1983), and other techniques. Additionally, scanning ellipsometry has been used by Jellison and Boatner (2000) to determine the spectroscopic refractive indices of the xenotime-structure RE orthophosphates. The extensive range of studies of these orthophosphates was motivated initially by the potential application of the orthophosphates to radioactive waste disposal and subsequently by the other applications... 

Figure 19. X-band EPR spectra (at T = 4.2 K) showing the hyperfme structure due to Nd (isotopically enriched with " Nd). The spectra are shown for the apphed magnetic field oriented parallel and perpendicular to the c-axis of the Y(P04) host single crystal. The EPR spectrum of eE and lines from the spectrum of Gd are also present. This type of magnetic resonance spectroscopy has proven to be very useful in the study of the solid-state chemical properties of the rare-earth-, Y-, and Sc-orthophosphates (after Abraham et al. 1983).

Raman spectroscopy has been used in studies of the rare earth orthophosphates (table 4). The spectra are described by Richman (1966), and the normal coordination analysis calculations of the vibrations of free PO ion in xenotime structure are presented by Mooney and Toma (1967). Lazarev et al. (1978) have calculated the normal coordinates of YPO crystals and have shown that the resonance splitting of the frequencies of internal vibrations of the complex anion cannot be due to dipole-dipole interaction of the localized vibrators. Begun et al. 

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