The S1(n, π*) states of acetaldehyde and acetone in supersonic nozzle beam: Methyl internal rotation and C=O out-of-plane wagging

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Abstract
Fluorescence excitation spectra of CH3CHO, CH3CDO, (CH3)2CO, and (CD3)2CO have been observed in an Ar supersonic nozzle beam. Vibrational analyses have been performed for vibronic bands in the region at wavelengths longer than 313 nm. The 0–0 bands of the S1(n, π*) states were located at 29 771, 29 813, 30 435, and 30 431 cm−1, respectively. The spectra could be analyzed taking the C=O out‐of‐plane wagging and the CH3 internal rotation as active modes. By fitting a double minimum potential function to the observed vibrational levels, it has been shown that these molecules are pyramidally distorted in the S1(n, π*) state with barrier heights to inversion of 541, 578, 468, and 480 cm−1, respectively. Similar analyses using the Mathieu function gave threefold potential functions for the methyl internal rotation with barrier heights to rotation of 691, 645, 740, and 720 cm−1 for CH3CHO, CH3CDO, (CH3)2CO, and (CD3)2CO, respectively. High resolution measurements of rotational envelopes have shown that the out‐of‐plane polarization dominates in the acetone spectrum. This result, together with a detailed investigation of the vibronic intensity borrowing mechanism, has led us to conclude that the second order interaction dominates in which the methyl torsion and the C=O out‐of‐plane wagging are active. The origin of the methyl rotational barrier in the S1 state is discussed on the basis of our recent ab initio calculations. The hyperconjugative interaction is suggested to be important in determining the barrier.