The Diiodine Basicity Scale

J Is the Diiodine Basicity Scale a General Halogen-Bond Basicity Scale ... 

Such family-dependent behaviour is also observed for the correlations between the diiodine basicity scale and (i) IR and UV spectroscopic shifts upon halogen bonding (see below) and (ii) halogen-bond lengths [14]. However, a set of NMR chemical shifts... 

Table 5.5 Equilibrium constants (I mol 0 and diiodine basicity scales pKgo 3/7 J AG° (kj mol 0 for the complexes of diiodine with six-membered N-heteroarenes.

Table 5.9 Equilibrium constants (I ) and diiodine basicity scales pKgi and AC° (kj moh ) for the complexes of diiodine with five-membered N-heterocycles (mainly heteroarenes).

Table 5.12 Equilibrium constants Kc (I mol j and diiodine basicity scales pKsi2 3-nd AC° (kj mol j for the complexes of diiodine with carbonyl bases.

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. 

These differences between halogen-bond and hydrogen-bond basicities, together with the growing importance of the halogen bond in chemistry, advocate the construction of a halogen-bond basicity scale. Because there are hundreds of equilibrium constants for the formation of diiodine complexes with Lewis bases (Equation 5.1) ... 

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. 

Haloalkanes are weak Lewis bases towards all Lewis acids. For this reason, they have attracted little attention in the construction of Lewis basicity scales. In this experiment, their basicity towards diiodine is studied by measuring the complexation constant of diiodine with lodocyclohexane in cyclohexane and comparing the result with those for other haloalkanes. 

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. 

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