The FTIR spectroscopic study of isophorone solvation in various media using the CdO group as the microenvironmental probe shows that in aprotic medium (cyclohexane) at least two forms are present, even at low concentrations: the monomolecular form and the aggregated forms (dimers and larger). In hydroxylated organic media (butanol), only monosolvated forms are present. In aqueous solutions, solvated species with various degrees of aggregation and hydration can be identified. Molecular mechanics studies show that isophorone, which is an R-enone, has a preference for an aggregated form over the monomolecular form. Further, the hydrogen atoms of a water molecule can take part in a stabilizing interaction with the carbonyl oxygen atoms of two isophorone molecules. The information on the solvation states (derived from FTIR spectroscopy) and on the nature of the supramolecular structures (calculated via molecular mechanics) have been correlated to the results of photodimerization. Finally, we propose a mechanism of photoreaction, based on the supramolecular structures which are flexible and therefore have a modifiable molecular structure and whose geometry is determined by the interactions with the medium. Experimental SectionReagents. The solvents for spectroscopy, cyclohexane, n-butanol, and methanol (Merck, spectroscopically pure), D 2 O 99.8% (Spin et Techniques), were used as supplied. The water was distilled or double distilled. Isophorone, 1 (Aldrich), was distilled to purity as checked by gas chromatography (GC) and by 1 H NMR. Deuterated isophorone, 2, 12 was prepared by isotope exchange using the method described in ref 15.Photodimerization. The concentration of 1 in the reaction mixture was varied from 0.02 to 0.06 M in water; it was constant at 0.1 M in the other media. The reaction mix (after bubbling with nitrogen or argon for 30 min) was irradiated for 24 h (ambient temperature 25-30 °C) in quartz tubes in a RPR 100 † In memory of my mother.

No abstract

No abstract