Molecular structure and conformations of 1-methoxy-2,3,3,4,4-pentafluorocyclobut-1-ene

The structure of 1-monomethoxy-2,3,3,4,4-pentafluorocyclobut-1-ene (F5MCB) has been studied as a part of a series of investigations of the structures of substituted cyclobutenes. The experimental method was gas-phase electron diffraction (GED) augmented by molecular orbital calculations at the HF and B3LYP levels of theory with basis sets 6-31G(d), 6-311+G(3df,2p), and cc-pvtZ. The matter of most interest in these compounds is the length of the C3-C4 bond, which has been found to be significantly longer than the corresponding bond in cyclobutene itself when both ligands to the double bond are fluorine or chlorine atoms, but about the same as in cyclobutene when both substituents are methoxy groups. This result has been verified by the additional data provided by this study of F5MCB: the C3-C4 bond is about the same as in the dimethoxy compound and in cyclobutene. In the gas-phase at 25 °C, F5MCB exists as a mixture of two conformers, one with the methyl group cis to the double bond (about 70%) and one with it trans (about 30%), in good agreement with theory that predicts 63-70% cis. Some values (with estimated 2σ uncertainties) of the more important parameters (rg/Å; ∠α/°) are for the cis/trans forms r(〈C-H〉) = 1.101 (17)/1.121, r(CC) = 1.329 (36)/1.328, r(C1-C4) = 1.498 (19)/1.504, r(C2-C3) = 1.483 (19)/1.485, r(C3-C4) = 1.581 (21)/1.576, r(C2-F6)=1.346 (12)/1.339, r(C3-F7) = 1.347 (10)/1.349, r(C4-F9) = 1.344 (10)/1.350, r(C1-O5) = 1.337 (26)/1.343, r(O5-C11) = 1.469 (37)/1.493, ∠(C4C1C2) = 93.3 (8)/93.1, ∠(C1C2C3) = 96.4 (8)/96.6, ∠(C2C3C4) = 84.4 (8)/84.4, ∠(C3C4C1) = 85.0 (9)/85.9, ∠(C2C1O5) = 141.0 (14)/135.2, ∠(C1C2F6) = 135.4 (14)/135.1, ∠(C1O5C11) = 117.3 (33)/117.8, ∠(F9C4F10) = 108.3 (13)/107.7, ∠(F7C3F8) = 107.7 (13)/107.6, ∠(XC4C3) = 134.4 (15)/134.4, and ∠(X′C3C4) = 134.2 (15)/133.9, where X and X′ are, respectively, the bisectors of the F9C4F10 and F7C3F8 angles. The structure is discussed with reference to the structures of similar molecules.