The trans-HOCO radical: quartic force fields, vibrational frequencies, and spectroscopic constants.

In the search for a full mechanism creating CO(2) from OH + CO, it has been suggested that creation of the hydroxyformyl or HOCO radical may be a necessary step. This reaction and its transient intermediate may also be responsible for the regeneration of CO(2) in such high quantities in the atmosphere of Mars. Past spectroscopic observations of this radical have been limited and a full gas phase set of the fundamental vibrational frequencies of the HOCO radical has not been reported. Using established, highly accurate quantum chemical coupled cluster techniques and quartic force fields, we are able to compute all six fundamental vibrational frequencies and other spectroscopic constants for trans-HOCO in the gas phase. These methods have yielded rotational constants that are within 0.01 cm(-1) for A(0) and 10(-4) cm(-1) for B(0) and C(0) compared with experiment as well as fundamental vibrational frequencies within 4 cm(-1) of the known gas phase experimental ν(1) and ν(2) modes. Such results lead us to conclude that our prediction of the other four fundamental modes of trans-HOCO are also quite reliable for comparison to future experimental observation, though the discrepancy for the torsional mode may be larger since it is fairly anharmonic. With the upcoming European Space Agency/NASA ExoMars Trace Gas Orbiter, these data may help to establish whether HOCO is present in the Martian sky and what role it may play in the retention of a CO(2)-rich atmosphere. Furthermore, these data may also help to clear up questions built around the fundamental chemical process of how exactly the OH + CO reaction progresses.