The Millimeter‐ and Submillimeter‐Wave Spectrum of trans‐Ethyl Alcohol

The rotational‐torsional spectrum of the trans rotational isomer of ethyl alcohol was investigated in the 65–350 GHz frequency region. A total of 481 ground state transitions over a range of J and K a values up to 33 and 10, respectively, were measured and assigned. Doublets or triplets arising from the A and E torsional states of the v=0 torsional level of the three‐fold‐symmetric methyl internal rotation have been resolved in 168 of these transitions. Internal rotation theory predicts a significant number of c‐type E‐state transitions normally forbidden, but allowed when the rotational asymmetry operators mix E‐state rotational‐torsional levels. Over 40 of these transitions have been observed. The newly measured transitions, along with the results of many previous measurements, have been analyzed using an IAM internal rotation Hamiltonian and a Watson A‐reduced Hamiltonian to determine the rotational, centrifugal distortion, and torsional constants. The IAM analysis accounts for all of the analyzed transitions in the trans isomer, but for some applications the asymmetric rotor analysis is a satisfactory description of the molecule. The J and K a limits where the trans isomer can be analyzed without considering interactions with the gauche states are also discussed. Predicted frequencies for transitions unaffected by the gauche states are presented through 600 GHz using the constants determined by this work.