Diiodine affinity scale

There have been many determinations of the enthalpy of complexation of diiodine with Lewis bases. The data measured in alkanes and/or CCU are collected in Table 5.25. It is difficult, however, to assemble a reliable diiodine affinity scale from these values. The reasons are that many results appear inaccurate and determinations have been made in... 

Vmin, -Pvmin and 7s,min are closely related to the electrostatic, polarization and charge-transfer components, respectively, of the Morokuma decomposition of proton affinity. It has been shown [197] that the spectroscopic scale Ziv(OH) (related to methanol), the phenol affinity, the diiodine affinity and the proton affinity of 42 nitrogen, oxygen and sulfur bases can be correlated by the triple-scale Equation 1.133 ... 

The discovery of charge-transfer bands in the UV spectra of diiodine complexes (Benesi and Hildebrand, 1949) [202] and the development of the underlying theory (Mulliken, 1952) [31] initiated a wealth of thermodynamic and spectroscopic measurements on diiodine complexes, mainly in the period 1949-1980. Complementary measurements by Berthelot, Guiheneuf, Laurence et al. (1970-2002) and Abboud et al. (1973-2004) enabled a homogeneous scale of diiodine basicity to be constructed. In addition, recommended values of diiodine affinity have been compiled from the literature (Laurence, 2006), for comparison with the SbCls, BF3 and 4-FC6H4OH affinity scales. UV and/or IR shifts upon complexation of the acids h, ICl and ICN have also been systematically measured by Berthelot, Laurence, Nicolet et al. (1981-1985). These thermodynamic and spectroscopic scales will allow the recent concept of a halogen bond to be treated quantitatively. They can be found in Chapter 5. 

Studies of the X—Y stretching vibration in complexes of XY with different Lewis bases reveal a characteristic decrease in frequency as the strength of the base increases [35, 36]. Hence spectroscopic scales of halogen-bond basicity can be built [37] in the manner described in Chapter 4 for the O—H stretching vibration in hydrogen-bonded complexes. Spectroscopic scales based on the shifts of the v(I—I) band of diiodine at 211 cm , the u(I—Cl) band of iodine monochloride at 376 cm and the v(I—CN) band of iodine cyanide at 485 cm will be presented and compared with thermodynamic basicity and/or affinity scales. 

Nevertheless, the values given in Table 5.25 may be useful for comparison with other Lewis affinity scales and with the spectroscopic scales presented later. Their comparison with theoretical diiodine affinities may also help in choosing the best calculation method(s) in halogen-bonding studies. 

The relationships between the IR spectroscopic shifts and the affinity scales are more difficult to study. There is a lack of reliable ICN and ICl affinity values. The diiodine affinity values are numerous and diverse (Table 5.25), but their reliability is difficult to assess. The largest set of data is found for the comparison of diiodine affinity and 2 v(I—CN) scales. A correlation has previously been claimed for 41 bases [37]. A careful distinction between affinity values measured in heptane and in CCI4 and the extension of the Av(I—CN) scale to many new bases [65] enable the enthalpy-frequency shift relationships 5.19 and 5.20 to be proposed ... 

This chapter is intended to provide detailed examples of the spectroscopic and thermodynamic determination of most Lewis basicity scales presented in the previous chapters, namely the BF3 affinity scale, 4-fluorophenol basicity and affinity scales, the methanol infrared (IR) shift scale, the 4-nitrophenol solvatochromic shift scale, diiodine basicity and affinity scales, the iodine cyanide IR shift scale, the diiodine blue shift scale and the lithium cation basicity scale. With these examples, it is hoped that professional chemists, and also students of physicochemical sciences, will be able to supplement the scales for the molecules in which they are interested. 

The relationship 5.20 is shown in Figure 5.13. It is difficult to assess the relative importance of model and experimental errors in these correlations. However, it is clear that the Ai7(Bl2)-Av(I—CN) correlation is less family dependent than the pAlBi2- i (I I) and p7fBicN-Av(I—CN) relationships shown in Figures 5.12 and 5.10. Hence the relationships 5.19 and 5.20 may support the use of Av(I—CN) as a spectroscopic scale of soft affinity. Indeed, diiodine is the archetype of soft Lewis acids in the Pearson classification since it has a very low absolute hardness (rj = 3.4 eV). Moreover, Av(I—CN) values obey the HSAB principle (soft acids prefer soft bases) since they decrease with the absolute hardness of the donor atom (in a given column of the periodic table), as shown in Table 5.28. 

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