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.