Microwave studies on 1,4-pentadiene: CH2═CH-CH2-CH═CH2; transformations among the three rotational isomers.

In order to examine significant roles of conformations played in various research fields, a molecule with two internal-rotation axes of high symmetry, 1,4-pentadiene, was studied in detail through the observation of its rotational spectra by using various types of microwave spectroscopy, Stark modulation and Fourier transform in the centimeter-wave region, direct absorption in the millimeter-wave region, and centimeter-/millimeter-wave combinations for double resonance, along with ab initio molecular orbital calculations. The molecule was confirmed to exist in three rotameric forms: skew-skew, cis-skew, and skew-skew'. For the cis-skew form, rotational spectra not only in the ground vibrational state, but also in three excited C-C torsional states were detected.