Knowledge Bank

The $920cm-1$ combination band of acetaldehyde has been studied at 2 MHz (FWHM) resolution using an electric resonance optothermal spectrometer and a tunable microwave sideband laser. The spectrum was assigned by using microwave and radiofrequency infrared double-resonance and precise combination differences. Our study reveals a large torsional splitting of $-1.45cm-1$, with the E tunneling component below the A component. The splittings for the $K\prime =1$ and 2 levels for the E component are similar to the splittings found for the first excited torsional state $(\nu 15)$. These observations are consistent with a vibrational state having one quantum of torsional excitation, confirming previous low resolution assignment of the band to the $\nu 14+\nu 15$ vibration. The tunneling splitting of $1.45cm-1$ is $0.3cm-1$ smaller than that of the torsional fundamental due to the strong Fermi resonance between $\nu 14+\nu 15$ and $\nu 9$. Because of the sensitivity of tunneling splittings to contamination by vibrational mode coupling, care has to be taken before relating the observed splittings to a barrier height. A large number of forbidden transitions for the E state, caused by Coriolis interaction between a-axis rotation and the internal rotation angular momentum, have been observed. This fact suggest that caution should be used when invoking intramolecular vibrational relaxation as the source of spectral congestion in molecules with internal rotors.