Basicity of lactones and cyclic ketones towards I 2 and ICl. An experimental and theoretical studyy A. El Firdoussi, a M. Esseffar,* a W. Bouab, a A. Lamsabhi, a J.-L. M. Abboud,* b O. Mo ´ c and M. Ya ´n ˜ez* c a De ´partement de Chimie Faculte ´ des Sciences Semlalia, Universite ´ Cadi Ayyad, Marrakesh, Morocco. E-mail: esseffar@ucam.ac.ma b Instituto de Quimica Fisica, ‘‘ Rocasolano ’’, CSIC, Serrano, 119, E-28006, Madrid, Spain. E-mail: jlabboud@iqfr.csic.es c Departamento de Quimica C-9, Universidad Autonoma de Madrid, Cantoblanco, E-28049, Madrid, Spain. E-mail: manuel.yanez@uam.es Received (in Montpellier, France) 10th May 2003, Accepted 2nd July 2003 First published as an Advance Article on the web 24th September 2003 Intermolecular charge transfer (CT) spectra of several complexes between cyclic ketones and lactones and molecular iodine and iodine monochloride were studied in the UV-visible region. Equilibrium constant and free energy changes of the formed complexes were determined in solution. Ab initio calculations at HF/LANL2DZ* and MP2(full)/LANL2DZ* were carried out to establish the nature of the complexation site in the case of lactones, to determine the complex structures and to examine the ring size effect. Although range of the basicity towards I 2 and ICl of compounds studied was small, it was found that cyclic ketones are more basic than lactones. This basicity difference decreases from small to large cycles and practically vanishes for six- and seven- membered rings. A comparative analysis between basicities of lactones and aliphatic esters towards I 2 and ICl has also been carried out. Experimental data in solution were found to be linearly correlated with theoretical results in the gas phase. Proton affinities of cyclic ketones and aliphatic carbonyl compounds that do not present any secondary interaction with the Lewis acid correlate very well with their gas-phase basicity towards I 2 and ICl. Introduction Over half a century has elapsed since Mulliken’s seminal the- oretical work on the electronic structure of charge-transfer (CT) complexes. 1,2 These studies were triggered by the experimental determination of the existence, stoichiometry and thermodynamic stability of 1 : 1 complexes between aro- matic hydrocarbons and diiodine in solution. 3 Soon after, X-Ray geometries for benzene–dibromine, 4 benzene–dichlor- ine 5 and acetone–dichlorine 6 complexes in the solid state were reported. The successful interplay between theory and experiment in the field of CT complexes has continued ever since. In 1969 a substantial body of experimental and theo- retical data was already available 7 that allowed a rather satis- factory overview and rationalization of structural and energetic aspects of CT complexes involving both p and n-donor bases. 7 Over the following years, the basic concepts of Mulliken’s theory have played a key role in the develop- ment of Drago’s classical quantitative empirical model of reactivity. 8 Most importantly, they have provided quanti- tative tools and a conceptual framework (e.g., the electron- transfer paradigm) 9,10 for the study of an impressive array of reactions. Very recently, complexes between diiodine or iodine monochloride and n or p bases have been used to probe at the femtosecond time-scale the dynamics and structure of dative bonding in bimolecular electron-transfer reactions. 11,12 Current computational techniques provide extremely accurate geometrical structures and thermody- namic stabilities of charge-transfer adducts. 13 This informa- tion is nicely completed by the structural information on CT complexes in their electronic ground state obtained by state- of-the-art rotational spectroscopy. 14,15 Consider now reactions (1) and (2), corresponding to the CT association between a base B and diiodine or iodine mono- chloride in solution in an ‘‘ inert ’’ solvent (e.g. saturated hydrocarbon): B þ I 2 ! B  I 2 K c ðI 2 Þ D r G  m ð1; solnÞ ð1Þ B þ ICl ! B  ICl K c ðIClÞ D r G  m ð2; solnÞ ð2Þ Some years ago we explored the possibility for these reac- tions to provide a ‘‘ solution basicity ranking ’’ 16 based on the standard Gibbs energy changes for these reactions, D r G  m (1, soln) or D r G  m (2, soln), that could be closely related to the ‘‘ gas-phase basicity ranking ’’ 17 defined through D r G  m (3, g), the standard Gibbs energy change for reaction (3, g), the protonation in the gas phase: BðgÞþ H þ ðgÞ! BH þ ðgÞ D r G  m ð3; gÞ ð3Þ This relationship between D r G  m (1, soln) or D r G  m (2, soln) and D r G  m (3, g) was expected because of the similarity between these processes and the ‘‘ dative bonding ’’ mesomeric structure A–D postulated in Mulliken’s donor(D)–acceptor (A) model (1) of charge-transfer complexes in their electronic ground state. The basic concept was that the quantitative pattern of structural effects on aqueous solution basicity, reaction (4), is often different from that found in the gas phase because the energetics of this reaction heavily depend on the solvation of y Electronic supplementary information (ESI) available: total energies (E h ), zero point energies (ZPE) and entropy values (S) of ketones and ketones-I 2 /ICl and of lactones and lactones-I 2 /ICl. See http:// www.rsc.org/suppdata/nj/b3/b305387n/ DOI: 10.1039/b305387n New J. Chem., 2003, 27, 1741–1747 1741 This journal is # The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2003 Downloaded by Centro de Química Orgánica "Lora Tamayo" on 08/05/2013 12:43:52. Published on 24 September 2003 on http://pubs.rsc.org | doi:10.1039/B305387N View Article Online / Journal Homepage / Table of Contents for this issue