Spectroscopic Studies of Model Carbonyl Compounds in CO 2 : Evidence for Cooperative C-H‚‚‚O Interactions Marc A. Blatchford, Poovathinthodiyil Raveendran, and Scott L. Wallen* Department of Chemistry, CB#3290, Kenan and Venable Laboratories, and the NSF Science and Technology Center for EnVironmentally Responsible SolVents and Processes, The UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 ReceiVed: October 14, 2002; In Final Form: March 25, 2003 Acetylated carbohydrates have extremely high solubilities and miscibilities in CO 2 and form the basis of a new approach toward the development of renewable CO 2 -philes. Ab initio computational studies relevant to this system indicate that in CO 2 complexes with simple, model carbonyl compounds the Lewis acid-Lewis base interaction between the carbon atom of CO 2 and the carbonyl oxygen is accompanied by a cooperative, intermolecular C-H‚‚‚O interaction between the CO 2 oxygen and the solute’s hydrogen atom. The results show that this may provide an additional stabilization mechanism for solvation complex formation. Spectroscopic studies provide the best approach to study these interactions and the validation of either computational or theoretical models. The present study focuses on room temperature, gaseous, liquid, and supercritical condition spectroscopic data to evaluate the extent that such complexes are relevant to CO 2 solvation. An examination of the temperature- and density-dependent changes in the vibrational spectra and the NMR shielding constants in both interacting (CO 2 ) and noninteracting (N 2 and He) systems supports the existence of the C-H‚‚‚O interaction. 1. Introduction Liquid and supercritical CO 2 has attracted attention as an environmentally benign solvent because of its nontoxicity, low cost, ease of removal from solutes, and recyclability. However, a large number of compounds have low solubility in CO 2 , which limits the usefulness of this solvent in many applications. This has led to various molecular-level approaches to solubilize CO 2 - phobic materials. 1-4 Recent ab initio calculations on the interaction of model carbonyl compounds with CO 2 indicate that these systems may offer quite interesting possibilities in terms of CO 2 -philicity. In fact, the functionality predicted to have the highest interaction energy with CO 2 , acetate, has been shown to allow the dissolution of carbohydrates in CO 2 upon acetylation of the hydroxyls. 5 The solvation of these compounds in CO 2 is thought to be governed principally by the interaction between CO 2 and the acetate moieties. The interaction energy for the individual acetate-CO 2 complex interaction is 2.82 kcal/ mol, which presumably promotes an enthalpy-driven solvation mechanism. Understanding the fundamental solvation structures and mechanisms of these molecules is important because it will provide a systematic framework from which to approach the problem of the design of other CO 2 -philes. Although CO 2 does not possess a permanent dipole moment, it is not strictly nonpolar because there is a clear charge separation caused by the opposing bond dipoles, leaving a partial positive charge on the carbon atom and partial negative charges on the oxygen atoms. 6 CO 2 also has a quadrupole moment that causes it to interact more strongly with dipolar solutes than predicted on the basis of its dielectric constant alone. 7,8 The charge separation and quadrupole moment enable CO 2 to serve as both a Lewis acid 9,10 and a Lewis base, 5 which is recognized as being important in the CO 2 -based solvation. 3,4,11,12 Energy- minimized calculations of these systems reveal that in all of the various CO 2 -carbonyl interaction geometries (Figure 1) there is the possibility of a weaker, previously unknown C-H‚‚‚O interaction that acts cooperatively with the Lewis acid-Lewis base interaction. The optimized structures and binding energies suggest the existence of cyclic six-membered (CO 2 -acetate) or five- membered (CO 2 -aldehyde) interaction geometries involving a weak, cooperative C-H‚‚‚O interaction between one of the negatively polarized CO 2 oxygens and a nearby solute proton. 5 * Corresponding author. E-mail: wallen@email.unc.edu. Figure 1. Optimized geometries (MP2/6-31+G*) and interaction energies (MP2/aug-cc-pVDZ) for CO2 complexes of acetaldehyde: (A) methyl side approach (ΔE )-2.52 kcal/mol), (B) aldehyde side approach (ΔE )-2.69 kcal/mol) and methyl acetate, (C) methyl side approach (ΔE )-2.82 kcal/mol), and (D) ester side approach (ΔE ) -2.64 kcal/mol). 5 10311 J. Phys. Chem. A 2003, 107, 10311-10323 10.1021/jp027208m CCC: $25.00 © 2003 American Chemical Society Published on Web 11/07/2003