Correlated ab initio investigations on the intermolecular and intramolecular potential energy surfaces in the ground electronic state of the O₂^-(X²Π_g)- HF (X ¹Σ⁺) complex

This work reports the first highly correlated ab initio study of the intermolecular and intramolecular potential energy surfaces in the ground electronic state of the O₂^-(X²Π_g) - HF (X¹Σ⁺) complex. Accurate electronic structure calculations were performed using the coupled cluster method including single and double excitations with addition of the perturbative triples correction [CCSD(T)] with the Dunning's correlation consistent basis sets aug-cc-pVnZ, n = 2-5. Also, the explicitly correlated CCSD(T)-F12a level of theory was employed with the AVnZ basis as well as the Peterson and co-workers VnZ-F12 basis sets with n = 2 and 3. Results of all levels of calculations predicted two equivalent minimum energy structures of planar geometry and C_s symmetry along the A'' surface of the complex, whereas the A' surface is repulsive. Values of the geometrical parameters and the counterpoise corrected dissociation energies (Cp-D_e) that were calculated using the CCSD(T)-F12a/VnZ-F12 level of theory are in excellent agreement with those obtained from the CCSD(T)/aug-cc-pV5Z calculations. The minimum energy structure is characterized by a very short hydrogen bond of length of 1.328 Å, with elongation of the HF bond distance in the complex by 0.133 Å, and D_e value of 32.313 Kcal/mol. Mulliken atomic charges showed that 65% of the negative charge is localized on the hydrogen bonded end of the superoxide radical and the HF unit becomes considerably polarized in the complex. These results suggest that the hydrogen bond is an incipient ionic bond. Exploration of the potential energy surface confirmed the identified minimum and provided support for vibrationally induced intramolecular proton transfer within the complex. The T-shaped geometry that possesses C_2v symmetry presents a saddle point on the top of the barrier to the in-plane bending of the hydrogen above and below the axis that connects centers of masses of the monomers. The height of this barrier is 7.257 Kcal/mol, which is higher in energy than the hydrogen bending frequency by 909.2 cm^-1. The calculated harmonic oscillator vibrational frequencies showed that the H-F stretch vibrational transition in the complex is redshifted by 2564 cm^-1 and gained significant intensity (by at least a factor of 30) with respect to the transition in the HF monomer. These results make the O₂^- - HF complex an excellent prototype for infrared spectroscopic investigations on open-shell complexes with vibrationally induced proton transfer.