Title: Riddles of the structure and vibrational dynamics of HO ₃ resolved near the ab initio limit

Abstract

The hydridotrioxygen (HO ₃) radical has been explored in many previous theoretical and experimental studies over several decades, originally because of its possible relevance to the tropospheric HO _x 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 entire 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 barrier for cis → trans isomerization is a minute 0.27 kcal mol ^-1. 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 (α _i). The D ₀(HO–O ₂) dissociation energy converges to a mere 2.80 ± 0.25 kcal mol ^-1.more » Contrary to expectation for such a weakly bound system, vibrational perturbation theory performs remarkably well with the FPA anharmonic force fields, even for the torsional fundamental near 130 cm ^-1. Exact numerical procedures are applied to the potential energy function for the torsional reaction path to obtain energy levels, tunneling rates, and radiative lifetimes. The cis → trans isomerization occurs via tunneling with an inherent half-life of 1.4 × 10 ^-11 s and 8.6 × 10 ^-10 s for HO ₃ and DO ₃, respectively, thus resolving the mystery of why the cis species has not been observed in previous experiments executed in dissipative environments that allow collisional cooling of the trans-HO ₃ product. However, the pure ground eigenstate of the cis species in a vacuum is predicted to have a spontaneous radiative lifetime of about 1 h and 5 days for HO ₃ and DO ₃, respectively.« less

Authors:

Bartlett, Marcus A. ^[1]; Kazez, Arianna H. ^[1]; Schaefer, Henry F.  ^[1];

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Allen, Wesley D.  ^[1]

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• Search DOE PAGES for ORCID "0000-0002-4288-2297"
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+ Show Author Affiliations

1. Univ. of Georgia, Athens, GA (United States)

Publication Date:

2019-09-05

Research Org.:

Lawrence Berkeley National Laboratory (LBNL), Berkeley, CA (United States). National Energy Research Scientific Computing Center (NERSC); Univ. of California, Oakland, CA (United States); Univ. of Georgia, Athens, GA (United States)

Sponsoring Org.:

USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22); National Science Foundation (NSF)

OSTI Identifier:

1577604

Alternate Identifier(s):

OSTI ID: 1560353

Grant/Contract Number:  

AC02-05CH11231; SC0018412

Resource Type:

Accepted Manuscript

Journal Name:

Journal of Chemical Physics

Additional Journal Information:

Journal Volume: 151; Journal Issue: 9; Journal ID: ISSN 0021-9606

Publisher:

American Institute of Physics (AIP)

Country of Publication:

United States

Language:

English

Subject:

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY