Combined Electron-Diffraction and Spectroscopic Determination of the Structure of Spiropentane, C₅H₈

Gas-phase electron-diffraction (GED) data have been combined with recent spectroscopic rotational constants to determine the r_α⁰ structural parameters for spiropentane, C₅H₈. The structure has D_2d symmetry, and the results yield values of 1.105(2) Å for the CH bond length, 1.557(3) Å for the distal CC bond length, and a smaller value of 1.482(1) Å for the four lateral CC bonds that connect to the central carbon atom. The HCH angle is 113.7(13)°, and the HCH flap angle, defined as the angle of the HCH bisector and the distal CC bond, is 150.2(16)°. Corresponding r_g values are 1.122(2) Å, 1.560(3) Å, 1.485(1) Å, 115.1(13)°, and 148.9 (16)°. The results are in good accord with values from density functional calculations (B3LYP/cc-pVTZ) and resolve some questions about the structure reported in an earlier GED study, in particular about the HCH angle and anomalous rotational constants calculated for the structure.