Methyl rotor effects on acetone Rydberg spectra. II. The 1B2(3s←n)←1A1 transition

Philis, John G.; Goodman, Lionel
March 1993
Journal of Chemical Physics;3/1/1993, Vol. 98 Issue 5, p3795
Academic Journal
Both a2(ν12) and b1(ν17) methyl torsion fundamentals and their overtones are found to be active in two-photon resonance multiphoton ionization jet spectra of the acetone 1B2(3s←n)←1A1 Rydberg transition. The acetone 3s Rydberg state torsional fundamental frequencies determined from fundamental and sequence band measurements are 118 cm-1 (ν’12) and 175 cm-1 (ν’17) compared to ground state values 77.8 and 124.5 cm-1, respectively. Corresponding values in fully deuterated acetone are 83 and 132 cm-1 compared to 53.4 and 96.0 cm-1 in the ground state. Our measured frequencies differ significantly (greatly so for the gearing torsional fundamental ν’17) from published values for both acetone-h6 and -d6. The 3s state 2ν’12, 2ν’17, and ν’12 + ν’17 frequencies are also measured allowing determination of excited state methyl torsion potential constants. The important increase (∼250 cm-1) in the 3s state V3 constant from its ground state value leads to a large increase in the potential barrier height to internal rotation, i.e., ≊1250 cm-1, 50% greater than the ≊800 cm-1 ground state barrier. This result is similar to the large increase in barrier height for eclipsed–eclipsed→staggered–staggered synchronous rotation recently found for the 1A2(3px←n) Rydberg state. There are, however, important differences in potential curvature between the 3s and 3px state internal rotation potential functions.


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