The origin of unequal bond lengths in the C̃1B2 state of SO2: Signatures of high-lying potential energy surface crossings in the low-lying vibrational structure

Abstract

The C̃ (1)B2 state of SO2 has a double-minimum potential in the antisymmetric stretch coordinate, such that the minimum energy geometry has nonequivalent SO bond lengths. The asymmetry in the potential energy surface is expressed as a staggering in the energy levels of the ν3(') progression. We have recently made the first observation of low-lying levels with odd quanta of v3('), which allows us-in the current work-to characterize the origins of the level staggering. Our work demonstrates the usefulness of low-lying vibrational level structure, where the character of the wavefunctions can be relatively easily understood, to extract information about dynamically important potential energy surface crossings that occur at much higher energy. The measured staggering pattern is consistent with a vibronic coupling model for the double-minimum, which involves direct coupling to the bound 2 (1)A1 state and indirect coupling with the repulsive 3 (1)A1 state. The degree of staggering in the ν3(') levels increases with quanta of bending excitation, which is consistent with the approach along the C̃ state potential energy surface to a conical intersection with the 2 (1)A1 surface at a bond angle of ∼145°.

Observation of b2 symmetry vibrational levels of the SO2 C̃ 1B2 state: Vibrational level staggering, Coriolis interactions, and rotation-vibration constants

“…In the observed vibrational origins given in Tables VII and VIII (and pictured in Fig. 1 of the third part of this series 11 ) it is evident that levels with a single quantum of ν 3 are significantly depressed in frequency. However, the degree of odd-even staggering rapidly decreases with increasing v 3 , indicating a low barrier at the C 2v geometry.…”

“…The observation of b 2 vibrational levels allows us to determine much more accurate rotational constants for both the b 2 and a 1 vibrational levels, because it enables direct deperturbation of the strong Coriolis interactions. In Part II of this series, 9 we describe an internal force field determination of the potential energy surface for the C state around equilibrium, and in Part III 11 we model the vibronic mechanism for the observed staggered vibrational level structure.…”

“…Ref. 5 (0,0,3) (0,1,2) (0,2,1) (1,0,0) (0,1,2) (0,2,1) (1,0,0) T 0 43464.393 (11) 1 [(0,0,3):(0,1,2)] = 0.3250(89) a t…”

“…We characterize anharmonically-induced interference effects in the vibronic transition intensities, similar to those recently characterized in the à → X emission spectrum of acetylene. 10 In part III of the series, 11 we discuss the vibronic mechanisms for the distortion in the SO 2 C state. We propose a three-state interaction model, which suggests that the observed level pattern is sensitive to interaction with both a higher bound electronic state and a repulsive state.…”

Ionization Energies and Dyson Orbitals of the Iso-electronic SO2, O3, and S3 Molecules from Electron Propagator Calculations

“…In the case of the former, experimental evidence has been provided that the C ̃B2 1 state has a double minimum along the antisymmetric stretching vibrational mode, leading to unequal bond lengths, i.e., to a C s , rather than C 2v , structure as a minimum. 39 For the A ̃B2 2 state of SO 2 + , Chang et al found analogous behavior theoretically by carrying out vibrational frequency calculations for this state with densityfunctional theory (DFT) and complete active space selfconsistent field (CASSCF) methods. 30 Are these theoretical approaches reliable enough for describing such a subtle, doubleminimum situation?…”