Halogen-bond basicities

Thermodynamic and Spectroscopic Scales of Halogen-Bond Basicity and Affinity... 

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) ... 

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. 

Even more characteristic is the blue shift observed for the Ttg- Ou transition of diiodine (at 521 nm) upon complexation. This blue shift explains the various colours of diiodine in solvents of different basicities. There appears to be a correlation between the strength of the complex and the blue shift of the diiodine visible spectrum [38, 39]. The generality of the diiodine blue shift as a spectroscopic indicator of halogen-bond basicity (affinity) will be analysed. 

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

Thermodynamic and Spectroscopic Scales of Halogen-Bond Basicity and Affinity 285 Table 5.24 Summary of linear Gibbs energy Equation 5.17 results. 

Table 5.26 Spectroscopic scales (cm ) of halogen-bond basicity.

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