TORSIONAL VIBRATIONS IN JET-COOLED ACETALDEHYDE

Two-photon and one-photon resonance multiphoton ionization spectra in the methyl torsional region were obtained for jet-cooled acetaldehyde- h4 and acetaldehyde-d4 3s <-- n Rydberg and pi <-- n transitions, res pectively. Up to three quanta of the ground-state torsion and two quan ta in the excited state were observed. There are significant modificat ions from published values. Precise infrared torsional frequencies3,22 determine a ground-state potential function (barrier 408.6 cm-1) with potential constants very close to that obtained from recent microwave measurements.25 The F constant, 7.5666, however, is substantially low er than the microwave value. The fundamental frequencies in acetaldehy de-d4 are predicted as 110.9 (A) and 110.8 cm-1 (E). This barrier is s ubstantially higher than the barrier obtained from recent ab initio sk eletal flexing model calculations at the HF and MP2 6-31G(d,p) levels for torsional vibrations. The 3s Rydberg state potential barrier (822 cm-1) calculated from the excited-state frequencies is substantially l owered from previous estimates.