Basicity of some carbonyl compounds towards iodine monochloride:: experimental and theoretical study

Intermolecular charge-transfer (CT) complexes between a wide range of carbonyl compounds and iodine monochloride were spectroscopically studied in the UV-visible region. Equilibrium constants and Gibbs energy changes of 1 : 1 charge transfer complexes were determined in CCl4 solution. The ICl basicity scale in CCl4 of the set of carbonyl derivatives included in this study is well correlated with the I-2 basicity scale in the same solvent. Ab initio calculations at HF/LANL2DZ* and MP2(full)/LANL2DZ* were carried out in order to clarify the structures of these CT complexes. Two different conformations, depending on the characteristics of the substituents, may be found. In one of them the ICl moiety lies in the plane of the carbonyl group, in the other the ICl subunit is perpendicular to the C=O group. The perpendicular complexes are favored by bulky substituents for which the HOMO has a clear pi -character. Both kinds of complexes can be spectroscopically distinguished since they present the CT absorption at different wavelengths. In both kinds of complexes the carbonyl-ICl interaction is essentially electrostatic. The substituent effects were analyzed through the use of the Taft-Topsom model. Experimental data in solution and theoretical estimates were found to follow a good linear relationship.