ELECTRIC RESONANCE OPTOTHERMAL SPECTRUM OF THE 920-CM(-1) NU(14)+NU(15) TORSIONAL COMBINATION BAND OF ACETALDEHYDE

The 920 cm(-1) vibrational band of acetaldehyde (CH3CHO) has been studied at about 2 MHz (FWHM) resolution using an electric resonance optothermal spectrometer and a tunable microwave-sideband CO2 laser. Microwave and radiofrequency infrared double resonance and precise combination differences are used to assign the spectrum and verify that it originates from the vibrational ground state of the molecule. The band is observed to have a large A-E torsional tunnelling splitting of 145 cm(-1), with the A state higher in energy than the E state. The K' = 1 and 2 levels for the E state have large K splittings of 28.8 GHz and 54.5 GHz, respectively, which are similar to the 37.3 GHz and 58.4 GHz splittings found for the first excited torsional vibration of the ground vibrational state. The above observations demonstrate that the 920 cm(-1) vibrational state has one quantum of torsional excitation, confirming previous low-resolution assignments of the band to the A' nu(14) + nu(15) combination vibration, consisting of the CH out-of-plane bend (nu(14) approximate to 764 cm(-1)) and the CH3 torsion (nu(15) M 143 cm(-1)). An effect of the previously proposed Fermi-resonance interaction between nu(14) + nu(15) and nu(9) is seen in the much smaller torsional splitting in the nu(14) + nu(15) state compared with the nu(15) fundamental. The observation of a large number of forbidden transitions for the E state suggests that caution should be used when invoking intramolecular vibrational redistribution as the source of spectral congestion in molecules with internal rotors.