New spectroscopic fits and ab initio study of the O2(Σg−3)-SO2 (1A1) open-shell dimer

We report the first accurate global fits for the rotation-spin-tunneling transitions in the microwave spectrum of the O₂(${}_{}{}^3{\Sigma }_g^{-}$)-SO₂ (¹A₁) weakly bonded open-shell complex. In addition, we present a new ab initio investigation of the potential energy surface of O₂(${}_{}{}^3{\Sigma }_g^{-}$)-SO₂, using the UCCSD(T)/aug-cc-pV(n + d)Z level of theory where n = 2 and 3. Analysis of the spectrum identified a-type and c-type transitions, frequencies of the a-type were not shifted while those of the c-type were shifted due to tunneling of the O₂ and the SO₂ moieties in the dimer. Only the A₁ symmetric tunneling state was detected because the antisymmetric A₂ state is not allowed by nuclear spin statistics in O₂-SO₂. Least squares fits with a standard deviation of 1 kHz were obtained using two computer codes incorporating semi-rigid rotor Hamiltonians that employ two different angular momenta coupling schemes. Results of the fits determined the effective tunneling frequency in the A₁ symmetric state as ${\nu }_{T_1}$= 2373.61134 $\pm$16 MHz, the electron spin coupling constant λ = 42,870.2186 $\pm$43 MHz, the rotational constants A = 7099.44 $\pm$33, B = 1528.886 $\pm$5, C = 1763.36 $\pm$6 MHz. The value of ${\nu }_{T_1}$ equals the tunneling splitting (${\Delta }_{T_1}$) between the$A_1^{+}$ and $A_1^{-}$ symmetric tunneling states in the dimer, where the$A_1^{+}$ and $A_1^{-}$ levels are shifted in energy by $-\frac{1}{2}{\nu }_{T_1}$ and $+\frac{1}{2}{\nu }_{T_1}$, respectively. The ab initio study identified a global minimum energy structure of C₁ symmetry and a metastable local minimum of C_s symmetry. We computed the optimized geometries of four equivalent configurations in the minimum energy isomer, which are due to rotation-tunneling motion of each monomer and concerted tunneling of the subunits in the dimer. A structure of C_2v symmetry was determined as the transition state between the concerted tunneling minima with a barrier height of 77.8 cm⁻¹. The binding energy in the global minimum was calculated with the BSSE correction as 2.676 kcal/mol. Results of the spectroscopic fits are in excellent agreement with those obtained from our ab initio calculations.