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

Abstract

The one color REMPI and two color ZEKE-PFI spectra of acetone-d3 have been recorded. The 3px Rydberg state of acetone-d3 lies at 59 362.3 cm−1 and both of the torsional modes are visible in this spectrum. The antigearing Rydberg (a2) mode, v12*, has a frequency of 62.5 cm−1, while the previously unobserved gearing (b1) mode, v17*, is found at 119.1 cm−1. An ionization potential of 78 299.6 cm−1 for acetone-d3 has been measured. In acetone-d3 n-radical cation ground state, the fundamentals of both of the torsional modes have been observed, v12+ at 51.0 cm−1 and v17+ at 110.4 cm−1, while the first overtone of v12+ has been measured at 122.4 cm−1. Deuterium shifts show that v12+ behaves like a local C3v rotor, but that v17+ is canonical. Combining this data with that for acetone-d0 and aacetone-d6 has allowed us to fit the observed frequencies to a torsional potential energy surface based on an ab initio C2v cation ground state geometry. This potential energy surface allows for prediction of the v17 vibration in acetone-d0 and acetone-d6. The barrier to synchronous rotation is higher in the cation ground state than in the neutral ground state, but significantly lower than in the 3s Rydberg state. The 3px Rydberg and cation ground state potential energy surfaces are found to be very similar to each other, strongly supporting the contention that the 3px Rydberg state has C2v geometry and is a good model for the ion core. The altered 3s Rydberg state potential surface suggests this state has significant valence character.