Ab initio CI study of the electronic spectrum of acetone

Abstract

SCF and CI calculations in a double-zeta basis are carried out in order to calculate the low-energy valence and Rydberg states of acetone (CH 3) 2 CO. The calculations support quantitatively the results of the most recent high-resolution electron impact studies but are at variance with a number of older assignments. The calculations assign the first band system as a 1(n, π * trasition with an underlying 3(n, π *) at lower energy, followed by a 3(π, π *) species with an intensity maximum around 5.8 eV; they characterize the second band system (B) at 6.3 eV primarily as a Rydberg 1(n, 3s) species. The relatively intense features between the B and D bands are calculated to result from the two allowed (n, 3p) members, as well as from the corresponding (n, 3d) tranitions; calculated oscillator strengths supplement the analysis and are found to be in good agreement with experimental results. The D band system is calculated to result from (n, 4s) transitions; higher-lying (n, 4p) species are also treated. In addition Rydberg states originating from an excitation out of the π orbital are calculated; together with the intensity of the dissociative 1(π, π *) these species are found to be responsible for the unresolved features of the acetone spectrum beyond 8.5 eV. Comparison with the formaldehyde analogue spectrum is made and it is shown that the difference between acetone and formaldehyde H 2CO can be traced primarily to the electron-donating effect of the CH 3 groups compared to that of hydrogen; calculated dipole moments for ground and excited states are also reported. The calculated energy levels are all within 0.2 eV of the most recently determined experimental values.