D-ions

This teclnhque can be used both to pennit the spectroscopic detection of molecules, such as H2 and HCl, whose first electronic transition lies in the vacuum ultraviolet spectral region, for which laser excitation is possible but inconvenient [ ], or molecules such as CH that do not fluoresce. With 2-photon excitation, the required wavelengdis are in the ultraviolet, conveniently generated by frequency-doubled dye lasers, rather than 1-photon excitation in the vacuum ultraviolet. Figure B2.3.17 displays 2 + 1 REMPI spectra of the HCl and DCl products, both in their v = 0 vibrational levels, from the Cl + (CHg) CD reaction [ ]. For some electronic states of HCl/DCl, both parent and fragment ions are produced, and the spectrum in figure B2.3.17 for the DCl product was recorded by monitoring mass 2 (D ions. In this case, both isotopomers (D Cl and D Cl) are detected. 

You are responsible for determining the amount of KI in iodized salt and decide to use an D ion-selective electrode. Describe how you would perform this analysis using... 

In an ociiihcOraJ field (he free-ion ground F lerm of a d ion is split into an A and two T terms which, along with the excited T(P) term (Fig. A), give rise to the possibility of three spin-allowed d-d transitions of which (he one of lowest eneigy is a direct measure of the ciystal field splitting, A or 10 Dq ... 

Figure A Energy Level diagram for a d ion in an octahedral crystal held.

Although Fc304 is an inverse spinel it will be recalled that Mn304 (pp. 1048-9) is normal. This contrast can be explained on the basis of crystal field stabilization. Manganese(II) and Fe" are both d ions and, when high-spin, have zero CFSE whether octahedral or tetrahedral. On the other hand, Mn" is a d and Fe" a d ion, both of which have greater CFSEs in the octahedral rather than the tetrahedral case. The preference of Mn" for the octahedral sites therefore favours the spinel structure, whereas the preference of Fe" for these octahedral sites favours the inverse structure. 

Figure A The variation with temperature and spin-orbit coupling constant, of the magnetic moments of octahedral, low-spin, d" ions, (The values of at 300 K are marked for individual ions).

Figure A Simplified Energy Level diagram for d ions showing possible spin-allowed transitions in complexes of low-spin cobalt(lll).

In tetrahedral fields the splitting of the free ion ground term is the reverse of that in octahedral fields so that, for d ions in tetrahedral fields A2g(F) lies lowest but three spin-allowed bands are still anticipated.In fact, the observed spectra usually consist of a broad, intense band in the visible region (responsible for the colour and often about 10 times as intense as in octahedral compounds) with a weaker one in the infrared. The only satisfactory interpretation is to assign these, respectively, as, wj = 7 i (P)-i A2(F) and ut = i(F)- A2(F) in which case U = ) should be... 

More comprehensive examination of speetroseopie and magnetic properties of d ions followed which provided an explanation for the different types of Lifsehitz salts (p. 1160) and led to studies of systems exhibiting anomalous properties. Rational explanations of these properties were eventually forthcoming. 

Nickel(ll) is the only common d ion and its spectroscopic and magnetic properties have accordingly been extensively studied. 

In a cubic field three spin-allowed transitions are expected because of the splitting of the free-ion, ground term and the presence of the term. In an octahedral field the splitting is the same as for the octahedral d ion and the same energy level diagram (p. 1029) can be used to interpret the spectra as was used for octahedral Cr Spectra of octahedral Ni usually do consist of three bands which are accordingly assigned as ... 

For d ions in tetrahedral fields the splitting of the free-ion ground term is the inverse of its splitting in an octahedral field, so that ig(F) lies lowest. In this ca.se three relatively intense bands are to be expected, arising from the transitions ... 

Figure 27.7 The splitting of d orbitals in fields of different symmetries, and the resulting electronic configurations of the Ni" d ion.

Nevertheless, crystal fields cannot be completely ignored. The intensities of a number of bands ( hypersensitive bands) show a distinct dependence on the actual ligands which are coordinated. Also, in the same way that crystal fields lift some of the orbital degeneracy (2L -)- 1) of the terms of d" ions, so they lift some of the 2J -)- 1 degeneracy of the sates of P ions, though in this case only by the order of 100cm This produces fine structure in some bands of Ln spectra. 

As with most other metals, the anodic behaviour of nickel is influenced by the composition of the solution in which measurements are made, particularly if the solution is acidic. Acidic solutions containing d ions or certain sulphur compounds in particular have a pronounced influence both in increasing the rate of anodic dissolution in the active range and in preventing passivation, and in stimulating localised corrosion . Thiourea and some of its derivatives have a complex effect, acting either as anodic stimulators or inhibitors, depending on their concentration . 

Thus in the reaction between A+ ions and D ions, the A + ions are replaced by C+ ions during the titration. As the titration proceeds the conductance increases or decreases, depending upon whether the conductivity of the C + ions is greater or less than that of the A + ions. 

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