The 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 

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 changes in the vibrational spectrum of iodine cyanide, iodine monochloride and diiodine as these molecules form halogen-bonded complexes B- ICN, B- -ICl and B- I2 with series of Lewis bases B have been extensively studied [35, 36, 273-277]. The main perturbation is a lowering of the frequency of the I—CN, I—Cl and I—I stretching motions. These frequency shifts are greater for strong bases, such as pyridines, than for weak bases, such as benzene or dioxane. Hence correlations were attempted between frequency shifts 

4-Chloropyridine /V-oxide Miscellaneous oxygen bases CCI4 19.5 [170] 

Since that time, additional spectroscopic measurements have been performed under standard conditions on an extended sample of bases [65, 274]. They are collected in Table 5.26. These are  

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