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

Intermolecular charge-transfer (CT) complexes between a wide range of carbo nyl 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 basici ty 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 init io calculations at HF/LANL2DZ* and MP2(full)/LANL2DZ* were carried out in o rder to clarify the structures of these CT complexes. Two different conform ations, 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 perpendicu lar complexes are favored by bulky substituents for which the HOMO has a cl ear pi -character. Both kinds of complexes can be spectroscopically disting uished since they present the CT absorption at different wavelengths. In bo th kinds of complexes the carbonyl-ICl interaction is essentially electrost atic. The substituent effects were analyzed through the use of the Taft-Top som model. Experimental data in solution and theoretical estimates were fou nd to follow a good linear relationship.