Acetone n-radical cation internal rotation spectrum: The torsional potential surface

The one color REMPI and two color ZEKE-PFI spectra of acetone-d(3) have bee n recorded. The 3p(x) Rydberg state of acetone-d(3) lies at 59 362.3 cm(-1) and both of the torsional modes are visible in this spectrum. The antigear ing Rydberg (a(2)) mode, v(12)*, has a frequency of 62.5 cm(-1), while the previously unobserved gearing (b(1)) mode, v(17)*, is found at 119.1 cm(-1) . An ionization potential of 78 299.6 cm(-1) for acetone-d(3) has been meas ured. In acetone-d(3) n-radical cation ground state, the fundamentals of bo th of the torsional modes have been observed, v(12)(+) at 51.0 cm(-1) and v (17)(+) at 110.4 cm(-1), while the first overtone of v(12)(+) has been meas ured at 122.4 cm(-1). Deuterium shifts show that v(12)(+) behaves like a lo cal C-3 upsilon rotor, but that v(17)(+) is canonical. Combining this data with that for acetone-d(0) and aacetone-d(6) has allowed us to fit the obse rved frequencies to a torsional potential energy surface based on an ab ini tio C-2 upsilon cation ground state geometry. This potential energy surface allows for prediction of the v(17) vibration in acetone-d(0) and acetone-d (6). The barrier to synchronous rotation is higher in the cation ground sta te than in the neutral ground state, but significantly lower than in the 3s Rydberg state. The 3p(x) Rydberg and cation ground state potential energy surfaces are found to be very similar to each other, strongly supporting th e contention that the 3p(x) Rydberg state has C-2 upsilon geometry and is a good model for the ion core. The altered 3s Rydberg state potential surfac e suggests this state has significant valence character. (C) 2000 American Institute of Physics. [S0021-9606(00)01206-X].