FHF

Single-crystal X-ray studies indicate that Rb2 [Pt(CN)4 ] (FHF)o.38 is body-centered tetragonal with lattice constants a = 12.66 A and c = 5.58 A, as compared to Rbj., [Pt(CN)4 ]-2H2 O, which is monoclinic. Thermogravimetric analyses indicate that Rb2 [Pt(CN)4](FHF)0.38 is anhydrous. 

Elements Percentage Composition Average of Pour Samples Theoretical 

Total halogen trace or trace or trace or trace or trace or - 

After 24-36 hours, bronze crystals fill the bottom of the beaker. They are filtered (plastic funnel), washed with two 2-mL portions of ice cold water and allowed to dry in air. Yield 0.80-050 g (78-88%), based on Cs2 [Pt(CN)4] H20. For analyses see Table I. 

for Cs2 [Pt(CN)4](FHF)0.23 Cs, 46.34 Pt, 33.97 C, 8.36 N, 9.76 H, 0.05 F, 1.52. Thermogravimetric analysis6 of Cs2 [Pt(CN)4](FHF)0j3 showed that the compound contained no water. A platinum oxidation state of +2.27 was determined by iodine-thiosulfate titrations.10 The Pt oxidation state reported here was that determined from the fluorine analyses. 

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We note that the virial theorem is automatically fulfilled in the Hartree-Fock approximation. This result follows from the fact that the single Slater determinant (Eq. 11.38) built up from the Hartree-Fock functions pk x) satisfying Eq. 11.46 is the optimum wave function of this particular form, and, since this wave function cannot be further improved by scaling, the virial theorem must be fulfilled from the very beginning. If we consider a stationary state with the nuclei in their equilibrium positions, we have particularly Thf = — Fhf, and for the correlation terms follows consequently that... 

Pimentel employed this three-center, four-electron (3c/4e) MO model to discuss the bonding in triiodide (I3-), bifluoride (FHF-), and other prototypical hypervalent species. In triiodide and other trihalides, for example, the relevant AOs are the (pa, Pb, Pc) orbitals along the bonding axis,... 

Figure 3.84 An illustration of the Pimentel-Rundle three-center MO model of hypervalency, showing equilibrium valence AO (xa-/b-Xc) overlap patterns for (a) 2pF—2pF—2pF NAOs of the trifluoride ion, F3 and (b) 2pF—lsp—2pF NAOs of the bifluoride ion, FHF-.

Tables 3.28-3.30 summarize the geometry, binding energies, and NBO/NRT descriptors for a variety of linear triatomic XYZ- species. These include representatives of the p-p-p orbital motif (such as symmetric trihalides [X3-, X = F, Cl, Br] and the mixed chlorofluorides [FFCl-, C1FC1-, FC1F-, I CICI-]) as well as the p-s-p (FHF-) and s-s-s (HLiH-, HJ) orbital motifs. (Examples of transition metal species manifesting the s-d-s and p-d-p motifs will be considered in Section 4.10.)...

Because FHF- epitomizes the limit of strong hydrogen bonding in a particularly simple geometrical form, let us examine some further aspects of its potential-energy surface. The triatomic species can generally be described in terms of three variables,... 

In summary, we may say that the NBO/NRT description of partial proton transfer in the equilibrium H-bonded complex(es) is fully consistent with the observed behavior along the entire proton-transfer coordinate, including the transition state. At the transition state the importance of partial co valency and bond shifts can hardly be doubted. Yet the isomeric H-bonded complexes may approach the TS limit quite closely (within 0.2 kcal mol-1 in the present example) or even merge to form a single barrierless reaction profile (as in FHF- or H502+). Hence, the adiabatic continuity that connects isomeric H-bond complexes to the proton-transfer transition state suggests once more the essential futility of attempting to describe such deeply chemical events in terms of classical electrostatics. 

It should be emphasized that n.. and JPS, and therefore c and T, refer to the condition at the pore tip. The dissolution valence and the temperature can be assumed to be independent of pore depth. This is not the case for the HF concentration c. Because convection is negligible in macropores, the mass transport in the pore occurs only by diffusion. A linear decrease in HF concentration with depth and a parabolic growth law for the pores according to Pick s first law is therefore expected, as shown in Fig. 9.18 a. The concentration at the pore tip can be calculated from the concentration in the bulk of the electrolyte c, the pore length l, the diffusion coefficient DHf (Section 1.4) and the flow of HF molecules FHf. which is proportional to the current density at the pore tip ... 

This quantity was the subject of several indirect attempts at calculation based on physical properties as well as ab initio computations. A direct measurement was finally achieved by ion cyclotron experiments and gave a value of 163kJmol (McMahon and Larson, 1982a) for A [HF(g)+ F (g) -> FHF (g)]. Shortly after, ab initio calculations with the basis set [(14s) (9p) (2d)/(10s) (lp)] fell in line with a value of 169 kJ mol" (Emsley et ai, 1983). Prior to this, a wide range of values for the bond energy had been canvassed, often with theoretical computations providing support (see Table 8). 

Table 8 Experimental, empirical and theoretical estimates of the hydrogen-bond energy of the bifluoride ion, A [HF(g) -F - FHF,g) ]/kJ mol". ...

The isotopic ratio v(FHF )/v(FDF ) has a value of unity for Vj since the proton does not move during the course of the symmetric stretching vibration. For Vj and V3, this ratio should be 1.396 if the motion of the hydrogen is perfectly harmonic, with both H and D moving in a single... 

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