Complete Structure of trans-3,4-Difluorocyclobutene from Microwave Spectroscopy

Microwave spectra have been observed with Fourier transform and cold jet expansion techniques in the 6−17 GHz range for trans-3,4-difluorocyclobutene, its two 13C1 modifications, two d 1 modifications, and the perdeutero species. Watson-type Hamiltonians have been fit with a full set of quartic centrifugal distortion constants for all species except the d 4 species. Complete structures fit with an r s/r 0 treatment and an r 0 treatment are in satisfactory agreement. Preferred r s/r 0 results with Costain uncertainties in parentheses are 1.349(4) Å for the CC bond, 1.503(9) Å for the contiguous C−C bonds, 1.534(4) Å for the distant C−C bond, 1.398(6) Å for the C−F bond, 1.081(3) Å for the C−H bond, and 1.102(5) Å for the −C−H bond. Consistent with its C 2 symmetry, the ring is slightly puckered, and the C−F bonds are roughly equatorial. Compared with the structure of cyclobutene, the structure of trans-3,4-difluorocyclobutene shows a modest lengthening of the CC bond and more significant decreases in the C−C bond lengths. These effects are consistent with observations for other fluorine-substituted rings in which contiguous fluorine atom substitution causes CC bond shortening and distant fluorine atom substitution causes CC bond lengthening. The CC bond adjustments in trans-3,4-difluorocyclobutene are, however, surprisingly similar to the adjustments observed for 3,3,4,4-tetrafluorocyclobutene.