Ab Initio Study of the Barrier to Planarity of Cyclobutane, Silacyclobutane and Germacyclobutane

The structures and barriers to planarity of cyclobutane (CB), silacylobutane (SiCB) and germacyclobutane (GeCB) have been computed using MP2, CCSD, MP4(STDQ) and CCSD(T) ab initio methods and the cc-pVTZ basis set in each case. These results were compared to experimental values previously reported. The reported experimental barrier to planarity of CB ranges from 448 to 518 cm$^{\mathrm{-1}}$. The CCSD/cc-pVTZ calculation gives a value of 586 cm$^{\mathrm{-1}}$ for CB, which is in closer agreement than the other ab initio results. For SiCB the reported experimental barrier is 440 cm$^{\mathrm{-1}}$. Our calculated CCSD/cc-pVTZ barrier is 472 cm$^{\mathrm{-1}}$, in very good agreement with the experimental value, but the other methods do more poorly. The experimental value of GeCB has not been determined, but we have calculated its CCSD/cc-pVTZ value to be 409 cm$^{\mathrm{-1}}$, which we consider to be a reasonable approximation. The trend to lower barrier heights from CB to SiCB to GeCB is expected since the torsional forces involving SiH$_{\mathrm{2}}$-CH$_{\mathrm{2}}$ interactions or GeH$_{\mathrm{2}}$-CH$_{\mathrm{2\thinspace }}$interactions are less than the CH$_{\mathrm{2}}$-CH$_{\mathrm{2}}$ interactions of CB.