Knowledge Bank

Microwave spectra have been observed in the 6-17-GHz region for trans-3,4-difluorocyclobutene, two $\mathrm{<mrow\; data-mjx-texclass="ORD">13</mrow>}C1$ modifications, two $d1$ modifications, and the perdeutero species. Watson-type Hamiltonians have been fit with a full set of quartic centrifugal distortion constants for all species except for the $d4$ species. Rotational constants for the normal species in MHz are $A=5664.02410(29),B=3366.07000(19)$, and $C=2345.58058(16)$. Complete structures fit with an $rs/r0$ treatment and an $r0$ treatment are in satisfactory agreement. Preferred $rs/r0$ results with Costain uncertainties in parentheses are $1.349(4)\backslash AA$ for the $C=C$ bond, $1.503(9)\backslash AA$ for the contiguous $C-C$ bonds, $1.534(4)\backslash AA$ for the unique $C-C$ bond, $1.398(6)\backslash AA$ for the $C-F$ bond, $1.081(3)\backslash AA$ for the $=C-H$ bond, and $1.102(5)\backslash AA$ for the $-C-H$ bond. Consistent with $C2$ symmetry, the ring is slightly puckered. Compared with cyclobutene, the $C=C$ bond is slightly longer and the $C-C$ bonds are shorter, adjustments which are familiar consequences of fluorine substitution on small rings.