The hydridotrioxygen (HO3) radical has been investigated in many previous studies over several decades, originally because of its possible relevance to the tropospheric HOx cycle but more recently because of its fascinating chemical bonding, geometric structure, and vibrational dynamics. We have executed new, comprehensive research on this vexing molecule via focal point analyses (FPA) to approach the ab initio limit of optimized geometric structures, relative energies, complete quartic force fields, and the reaction path for cis-trans isomerization. High-order coupled cluster theory was applied through the CCSDT(Q) and even CCSDTQ(P) levels, and CBS extrapolations were performed using cc-pVXZ (X = 2-6) basis sets. The cis isomer proves to be higher than trans by 0.52 kcal mol–1, but this energetic ordering is achieved only after the CCSDT(Q) milestone is reached. The FPA central re(O-O) bond length of trans-HO3 is astonishingly long (1.670 Å), consistent with the semiexperimental re distance we extracted from microwave rotational constants of 10 isotopologues using FPA vibration-rotation interaction constants. The D0(HO-O2 ) dissociation energy is only 2.7 kcal mol–1! Contrary to expectation for such a weakly bound system, vibrational perturbation theory (VPT2) performs remarkably well, even for the torsional fundamental near 130 cm–1 that we also treated using the Cooley-Numerov method. Finally, cis-trans isomerization occurs via tunneling with a half-life of 7.2 × 10−11 s and 6.5 × 10−9 s for HO3 and DO3, respectively, thus explaining the mystery of why the cis species has not been observed in previous experiments.

Riddles of the structure and vibrational dynamics of HO3 resolved near the ab initio limit

2019 Fall National Meeting

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