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, trimethylenecydopropane, [3]-radialene, has been synthesized and examined by these methods. The first high-resolution infrared spectra have been obtained for this molecule of D-3h symmetry, leading to an accurate B-0 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 r(z) bond lengths (in angstrom): 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 pi-electron delocalization affects each result in a shortening of about 0.05 angstrom 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 degrees is believed to be reliable to within 2 degrees.