The puzzle of bond length variation in substituted cyclobutenes. A new example: molecular structure and conformations of 1,2-dimethoxy-3,3,4,4-tetrafluorocyclobut-1-ene.

The structure and composition of 1,2-dimethoxy-3,3,4,4-tetrafluorocyclobut-1-ene (DMCB) have been measured by electron diffraction from the gas at a temperature of 370 K with the help of auxiliary data from molecular orbital and normal coordinate calculations, the former at several levels of theory and basis-set size, most importantly B3LYP/cc-pVTZ. The compound was found to exist primarily as a rotamer of C(s) symmetry (ca. 98%; 2sigma = 11%) with the remainder one of C(2v) symmetry; theory predicts about 88% C(s). Values for some of the more important parameters (r(g)/A; angle(alpha)/deg) of the C(s) form are r(C=C) = 1.337(21), r(C1-C4) = 1.496(8), r(C2-C3) = 1.501(8), r(C3-C4) = 1.567(12), r(C1-O) = 1.318(12), r(C2-O) = 1.340(12), r(C3-F) = 1.375(4), r(C4-F) = 1.368(4), angle(ave)(C=C-C) = 94.4(4), angle(ave)(C=C-O) = 133.5(12), angle(ave)(C-O-C) = 119.6(13), and angle(ave)(F-C-F) = 104.4(7). Surprisingly, although electron-diffraction values for the fluorinated C3-C4 bond in other cyclobutenes are greater than that for cyclobutene itself, that is not the case for DMCB where it is found to be about the same. Details of the DMCB structure, together with possible reasons for the observed variations in the length of the C3-C4 bond in fluorinated cyclobutene-like molecules, are discussed.