Microwave Spectrum of Benzaldehyde

Abstract

The microwave absorption spectrum of C₆H₅CHO has been investigated in the region 17.5–40.0 GHz and that for C₆H₅CDO in the region 26.5–40.0 GHz. Rotational transitions in the ground and first three excited torsional states in both molecular species have been identified. There is evidence for interaction of the second torsional state of C₆H₅CHO with another mode of vibration of the molecule. Accurate relative intensity measurements give the first torsional frequency of 113.8 ± 5.0 cm⁻¹ for C₆H₅CHO and of 108.4 ± 4.25 cm⁻¹ for C₆H₅CDO. The barriers to internal rotation of the aldehyde group calculated from these frequencies are 4.90 ± 0.43 and 5.28 ± 0.42 kcal/mole for C₆H₅CHO and C₆H₅CDO, respectively. The discrepancy between the observed and calculated values of inertial coefficients suggests the importance of vibration–rotation interaction in the theory of internal rotors with twofold potential barriers.