Theoretical insights on electron donor–acceptor interactions involving carbon dioxide

Modeling Solvation in Supercritical CO2

In recent decades, a microscopic understanding of solute-solvent intermolecular interactions has been key to advances in technologies based on supercritical carbon dioxide. In many cases, computational work has provided the impetus for new discoveries, shedding new light on important concepts such as the local structure around the solute in the supercritical medium, the influence of the peculiar properties of the latter on the molecular behavior of dissolved substances and, importantly, CO₂ -philicity. In this Review, the theoretical work that has been relevant to these developments is surveyed and, by presenting some crucial open questions, the possible routes to achieving further progress based on the interplay between theory and experiments is discussed.

Rotational Spectrum, Structure, and Interaction Energy of the Trifluoroethylene···Carbon Dioxide Complex

Rotational spectra for four isotopologues of the 1:1 weakly bound complex between trifluoroethylene (HFC═CF₂) and carbon dioxide (CO₂) were recorded using 480 MHz bandwidth chirped-pulse and resonant cavity Fourier transform microwave spectroscopy between 5.0 and 18.5 GHz. Two planar forms are possible: experimental rotational constants, planar moments, and dipole moment components are consistent with the form in which CO₂ is positioned at the CHF end of the TFE subunit and is approximately perpendicular to the C═C bond; the other form, with CO₂ aligned roughly parallel to the C═C bond, is not observed, consistent with ab initio relative energy predictions. Symmetry-adapted perturbation theory (SAPT) calculations provided interaction energies for possible structural forms of this complex, and comparisons are made with this and other members of the series of carbon dioxide complexes with fluorinated ethylenes (vinyl fluoride, 1,1-difluoroethylene, cis- and trans-1,2-difluoroethylene, and trifluoroethylene).

Structure-Property Relationships in CO2-philic (Co)polymers: Phase Behavior, Self-Assembly, and Stabilization of Water/CO2 Emulsions

This Review provides comprehensive guidelines for the design of CO2-philic copolymers through an exhaustive and precise coverage of factors governing the solubility of different classes of polymers. Starting from computational calculations describing the interactions of CO2 with various functionalities, we describe the phase behavior in sc-CO2 of the main families of polymers reported in literature. The self-assembly of amphiphilic copolymers of controlled architecture in supercritical carbon dioxide and their use as stabilizers for water/carbon dioxide emulsions then are covered. The relationships between the structure of such materials and their behavior in solutions and at interfaces are systematically underlined throughout these sections.

Solvation of Esters and Ketones in Supercritical CO2

Vibrational Raman spectra for the C═O stretching modes of three esters with different functional groups (methyl, a single phenyl, and two phenyl groups) were measured in supercritical carbon dioxide (scCO2). The results were compared with Raman spectra for three ketones involving the same functional groups, measured at the same thermodynamic states in scCO2. The peak frequencies of the Raman spectra of these six solute molecules were analyzed by decomposition into the attractive and repulsive energy components, based on the perturbed hard-sphere theory. For all solute molecules, the attractive energy is greater than the repulsive energy. In particular, a significant difference in the attractive energies of the ester-CO2 and ketone-CO2 systems was observed when the methyl group is attached to the ester or ketone. This difference was significantly reduced in the solute systems with a single phenyl group and was completely absent in those with two phenyl groups. The optimized structures among the solutes and CO2 molecules based on quantum chemical calculations indicate that greater attractive energy is obtained for a system where the oxygen atom of the ester is solvated by CO2 molecules.

Driving Forces Controlling Host-Guest Recognition in Supercritical Carbon Dioxide Solvent

The formation of supramolecular host-guest complexes is a very useful and widely employed tool in chemistry. However, supramolecular chemistry in non-conventional solvents such as supercritical carbon dioxide (scCO2 ), one of the most promising sustainable solvents, is still in its infancy. In this work, we explored a successful route to the development of green processes in supercritical CO2 by combining a theoretical approach with experiments. We were able to synthesize and characterize an inclusion complex between a polar aromatic molecule (benzoic acid) and peracetylated-β-cyclodextrin, which is soluble in the supercritical medium. This finding opens the way to wide, environmental friendly, applications of scCO2 in many areas of chemistry, including supramolecular synthesis, reactivity and catalysis, micro and nano-particle formation, molecular recognition, as well as enhanced extraction processes with increased selectivity.

Enhanced Solubility of Polyvinyl Esters in scCO2 by Means of Vinyl Trifluorobutyrate Monomer

The novel monomer, vinyl trifluorobutyrate (VTFBu), when polymerized in a controlled fashion by RAFT/MADIX polymerization with a xanthate transfer agent, yields poly(vinyl ester)s with improved solubility in supercritical carbon dioxide. The thermodynamic parameters controlling the solubility of VTFBu/vinyl acetate statistical copolymers are discussed based on ab initio calculations, glass transition temperatures of the copolymers, and surface tension measurements. The enhanced solubility of this new class of CO₂-philic polymer combined with its good chemical stability render it attractive for the preparation of next-generation macromolecular surfactants for the formation of water-scCO₂ emulsions.