High-Resolution Infrared and Electron-Diffraction Studies of Trimethylenecyclopropane ([3]-Radialene)

Combined high-resolution spectroscopic,
electron-diffraction, and
quantum theoretical methods are particularly advantageous for small
molecules of high symmetry and can yield accurate structures that
reveal subtle effects of electron delocalization on molecular bonds.
The smallest of the radialene compounds, trimethylenecyclopropane,
[3]-radialene, has been synthesized and examined by these methods.
The first high-resolution infrared spectra have been obtained for
this molecule of D3h symmetry,
leading to an accurate B0 rotational constant
value of 0.1378629(8) cm–1, within 0.5% of the value
obtained from electronic structure calculations (density functional
theory (DFT) B3LYP/cc-pVTZ). This result is employed in an analysis
of electron-diffraction data to obtain the rz bond lengths (in Å): C–H = 1.072(17),
C–C = 1.437(4), and CC = 1.330(4). The results indicate
that the effects of rehybridization and π-electron delocalization
affects each result in a shortening of about 0.05 Å for the C–C
bond in radialene compared to ethane. The analysis does not lead to
an accurate value of the HCH angle; however, from comparisons of theoretical
and experimental angles for similar compounds, the theoretical prediction
of 117.5° is believed to be reliable to within 2°.