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Broadband microwave spectra have been recorded for weakly bound clusters formed from mixtures containing fluoroethylene (FE) or 1,1-difluoroethylene (DFE) and varying percentages of CO2. These clusters serve as simple models for behavior of supercritical CO2 as a solvent. Analysis of changes in rotational transition intensities as CO2 concentration is systematically varied allows identification and assignment of spectra for a multitude of different clusters from within a single data set, with at least 15 species identified in FE/CO2 mixtures, and 4 species so far identified in DFE/CO2 mixtures. Several analysis approaches have been implemented, ranging from simple correlation of intensity variations between two spectra recorded with different sample concentrations, to more sophisticated principal component analysis based representations of the spectroscopic data from multiple scans. These techniques provide a range of tools that, in addition to facilitating assignment of spectra of complex mixtures, also assist in identifying the chemical species that give rise to the spectra within each mixture. The visual aspects of the research process, and the accessibility of coding tools such as Python, Excel and Mathcad, make this work particularly appropriate to undergraduate students, including those with little or no previous physical chemistry experience. 

Enhancement of the σ–hole on the halogen atom of aryl halides due to perfluorination of the ring is demonstrated by use of the Extended Townes−Dailey (ETD) model coupled to a Natural Atomic Orbital Bond analysis on two perfluorinated aryl halides (C 6 F 5 Cl and C 6 F 5 Br) and their hydrogenated counterparts. The ETD analysis, which quantifies the halogen p-orbitals populations, relies on the nuclear quadrupole coupling constants which in this work are accurately determined experimentally from the rotational spectra. The rotational spectra investigated by Fourier-transform microwave spectroscopy performed in supersonic expansion are reported for the parent species of C 6 F 5 Cl and C 6 F 5 Br and their 13 C, 37 Cl or 81 Br substituted isotopologues observed in natural abundance. The experimentally determined rotational constants combined with theoretical data at the MP2/aug-cc-pVTZ level provide precise structural information from which an elongation of the ring along its symmetry axis due to perfluorination is proved. 

The need for a deep understanding of CO2 interactions with other mols. is significant given the importance of supercrit. CO2 (s.c.-CO2) as a green solvent, and interest in design of novel materials for CO2 capture and storage. Soly. of fluorinated compds. in s.c.-CO2 is generally higher than their hydrocarbon analogs and fluorination of hydrocarbon compds. improves "CO2-philicity". Although dissoln. of a compd. in s.c.-CO2 (or any solvent) is a complex process, much can be learned by systematic examn. of solute-solvent interactions as a function of solute mol. properties in dimers and other small clusters. In the present study, Fourier-transform microwave spectroscopy and Symmetry Adapted Perturbation Theory (SAPT) calcns. have been used to examine intermol. interactions between CO2 and a series of fluorinated ethylene mols. with varying degree and position of fluorination (1-3 F atoms). While 1-fluoro-, 1,1-difluoro- and 1,1,2-trifluoroethylene...CO2 complexes are planar (with two isomers obsd. for 1-fluoroethylene), cis-1,2-difluoroethylene...CO2 is nonplanar. Although lowest energy structures predicted by MP2/6-311++G(2d,2p) calcns. are not always in agreement with obsd. configurations, SAPT calcns. provide binding energies consistent with the obsd. structures. We are now extending these studies to trimers contg. one fluorinated ethylene with two CO2 "solvent" mols. in order to gain further insight into structural and energetic changes as a solvation shell is formed.