Fluorinated Dimethyl Ethers, CH₂FOCH₂F, CHF₂OCHF₂, and CF₃OCHF₂:  Unusual Conformational Properties

Abstract

The geometric structures and conformational properties of bis(fluoromethyl) ether, CH2FOCH2F, bis(difluoromethyl) ether, CHF2OCHF2, and pentafluoromethyl ether, CF3OCHF2, have been studied with gas electron diffraction and quantum chemical methods (B3LYP/6-31G* and MP2 with 6-31G*, cc-pVTZ or 6-311G(2df) basis sets). IR(matrix) spectra have been recorded for CF3OCHF2. The most stable [sc,sc] conformer of CH2FOCH2F possesses C 2 symmetry with the C−F bonds of both CH2F groups in synclinal (sc, counterclockwise) orientation (φ(C−O−C−F) = 70(2)°). A small contribution (≤14%) of the [sc,−sc] form with C s symmetry cannot be excluded (−sc = synclinal, clockwise). CHF2OCHF2 exists in the gas phase as a mixture of two forms. In the main [ap,sp] conformer (82(8)%) one C−H bond is oriented antiperiplanar (ap), the other one synperiplanar (sp). This latter C−H bond nearly eclipses the opposite O−C bond with φ(C−O−C−H) = 18(2)°. Nearly eclipsed orientation of the C−H bond is also favored in CF3OCHF2 with φ(C−O−C−H) = 19(3)°. According to IR(matrix) spectra a small (8(3)%) contribution of the [ap] conformer with the C−H bond antiperiplanar relative to the opposite O−C bond is present at room temperture. The conformational properties of these fluorinated dimethyl ethers can be rationalized by orbital interaction energies derived from a natural bond orbital (NBO) analysis.

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“…We see that again bond lengths differ from the lowest basis set by less than 0.004 Angstroms while angles are within a half a degree and dihedral angles are within 0.5° for absolute differences. The B3LYP/6–31g* results of Ulic and Oberhammer [2004] are identical to those we found as well and their MP2/cc‐pVTZ results are extremely similar. In their work, they compared to experimental geometries and found very good agreement, indicating our results are comparable too.…”

“…In their work they reported only this structure with the lowest electronic energy. On the other hand, Ulic and Oberhammer [2004] identified three possible rotamers, with these two just described and another as (sc,‐sc) as shown in Figure 1g. In this work, we find yet another stable structure that is higher in energy for the (sc, sc) form.…”

“…More will be discussed shortly about rotamers. For nomenclature, we follow the convention of Ulic and Oberhammer [2004] where tetrahedral groups that contain one hydrogen or fluorine atom and two of the other atoms are designated as antiperiplanar (ap), synperiplanar (sp), synclinal (sc), or antisynclinal (−sc) as shown in Figure 1.…”

“…Of the four 1,1‐HFEs reported in this work, CHF 2 OCHF 2 is the most studied with four previous papers investigating structural properties. Suenram et al [1998], primarily performed experiments but also performed some MP2 calculations, Good and Francisco [1998] performed computations of molecular structures and frequencies, Horn et al [1999] did experiments and low‐level ab initio calculations, and Ulic and Oberhammer [2004] used both experiments and computational work. We compare the four different basis set results from our work to their prior work in Tables 6 and 7.…”

“…We see that rotamer 2 with the (ap, sp) form has the highest weighting in the gas phase, with a value of 77% with the 6–31g* results and 82% using information at the 6–31+g* level. Without accounting for hindered rotors or scaling their frequencies, Ulic and Oberhammer [2004] obtained a Gibb's free energy difference of 5.48 kJ/mol between the two most stable forms of CHF 2 OCHF 2 . When we include these corrections, the difference drops to 3.51 kJ/mol for results with the B3LYP/6–31g* level.…”