Knowledge Bank

The phasing out of the chlorofluorocarbons (CFCs) commonly used as refrigerants, solvents, and propellants for the last 50 years is causing the search for ozone- and greenhouse-acceptable alternatives to be accelerated. Among the compounds being considered for substitutes are families of hydrogenated fluorocarbons (HCFCs), hydrogenated chlorofluorocarbons (HCFCs) and fluorinated ethers. When several conformers are present, the dielectric measurements that are normally used to determine the dipole moments and relative abundances of the various conformers are insufficient due to a lack of knowledge of the number of degrees of freedom. Both quantities are needed for accurate thermodynamic modeling calculations. Unique interpretation of the isomeric composition from the dielectric measurements is possible if the dipole moments of the various conformers present can be independently determined. Thus a few benchmark experimental studies are imperative to develop reliable models for predicting unknown mixture properties based on known molecular properties. Two compounds of interest are the partially fluorinated ethers: $CF2HOCF2H,CF2HOCH2CF3$. The rotational spectra of these two species have been observed and analyzed using a combination of conventional Stark-modulated and pulsed Fourier-transform microwave spectrometers. Broad-banded scans of the spectra were recorded using a conventional spectrometer which provided an overall view of the rotational spectrum and a rough assignment of the spectrum and the number of conformers present in the gas phase. In addition to these broad-banded scans, high resolution spectra were obtained over selected frequency ranges using the NIST pulsed-molecular-beam Fourier-transform microwave spectrometer. With this spectrometer, only the spectrum of the lowest energy conformer is populated in the cold molecular beam. Thus, this spectrum enables us to determine the lowest energy conformer. The electric dipole moment of the observed lowest energy conformer was obtained using the pulsed molecular beam instrument. Energies of the observed conformations will be compared with conformational energies obtained at different levels of theory.