Theoretical Infrared Frequencies of the trans-HOCO Radical

Details Theoretical Infrared Frequencies of the trans-HOCO Radical Theoretical Infrared Frequencies of the trans-HOCO Radical

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011agufm.p23e..11f&link_type=abstract

American Geophysical Union, Fall Meeting 2011, abstract #P23E-11

Biology

[5210] Planetary Sciences: Astrobiology / Planetary Atmospheres, Clouds, And Hazes

Scientific paper

It has been proposed that the retention of large amounts of carbon dioxide in the Martian atmosphere (over 95%) results from the reaction of the hydroxyl radical with carbon monoxide. Even for such a simple process, the full chemical mechanism has not been elucidated. Recently, it has been proposed that the highly reactive hydroxyformyl, or HOCO, radical may be a necessary intermediate step in this reaction in the Martian atmosphere. The upcoming ESA/NASA ExoMars Trace Gas Orbiter will have instruments sensitive enough to detect the infrared signatures of various relatively rare Martian gases including, potentially, the HOCO radical. Its detection in the Martian atmosphere would establish how the ultra-high concentrations of carbon dioxide could be maintained and clearly how the fundamental process of OH + CO ultimately creates carbon dioxide and hydrogen gas which in the case of Mars subsequently escapes the atmosphere. Unfortunately, only two of the six infrared-active, fundamental vibrational frequencies of the trans-HOCO radical have been measured in the gas phase. This form of the radical is the most stable isomer and the one most likely to exist in the Martian atmosphere. Using state-of-the-art quantum chemical computational tools including exclusive use of coupled cluster theory (one of the most accurate chemical theories to date) and the established approach of quartic force fields for describing the nuclear motion, we have been able to predict rotational constants to better than 0.01 cm-1 (0.3 GHz) of experiment and vibrational frequencies for the two known fundamental frequencies to within 4 cm-1 (0.01 kcal/mol) of experiment as well as gas phase fundamental vibrational frequencies for the other four previously unknown vibrational modes of the trans-HOCO radical. These data are the only reference points for true gas phase fundamental vibrational frequencies, and they may help to establish whether HOCO is present in the Martian sky and what role it may play in the retention of a CO2-rich atmosphere.

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