| 研究実績の概要 |
Radical species in the interstellar medium (ISM) play important roles in the formation of complex organic molecules. Chemical reactions in the ISM may occur through accumulation of the radicals and molecules on the grain surfaces at very low temperatures, giving rise to complex organic molecules. However, the mechanistic details of these chemical processes are not fully understood.
During the 2019-2020 academic year, I focused on the OH radical on ice, which is a primary radical in the ISM. My objectives were two-fold; (1) study OH radical binding on ices, and (2) study the mechanisms for the desorption of OH radical on ices. For these purposes, I have used quantum mechanics/molecular mechanics methods.
A range of binding energies (0.06 - 0.74 eV) were found. Binding energy is sensitive to the number of dangling-H or dangling-O atoms at the binding site. Weak binding energy (0.06 eV) was observed when the OH radical did not interact with dangling atoms. In such cases, OH radical may desorb though a mild thermal process (e.g. phonons).
Strong binding energy (0.74 eV) was observed when the OH radical interacts with three water molecules on ice surface. These strongly bound OH radicals on ice would not be desorbed through a mild thermal process. Thus, a photochemical process would be possible. My calculations suggested that the strongly bound OH radicals on ice absorb 500-600 nm radiation. Then, the excited OH radical can be entered to a dissociation channel, where quantum mechanism tunneling plays a key role.
These findings are in agreement with the experimental studies.
|
| 現在までの達成度 (区分) |
現在までの達成度 (区分)
1: 当初の計画以上に進展している
理由
In collaboration with Prof. Naoki Watanabe at the Institute of Low Temperature Science in Hokkaido University, experimental studies have been performed. As we have performed theoretical and experimental studies side-by-side, interpreting the research outcomes were straightforward.
|
| 今後の研究の推進方策 |
During the 2020-2021 academic year, my plan is to calculate potential energy surfaces for the radical reactions between OH radicals on ice.
Once the OH radical is formed on ice, it can react with another OH radical. I plan to calculate the potential energy surfaces for possible radical reactions on ice; e.g. OH + OH -> H2O2 and OH + OH -> H2O + O. The quantum mechanics/molecular mechanics methods will be used for calculating the local minima and transition states on the potential energy surfaces. Then, reaction barriers will be calculated using the transition state theory.
Depending on the dangling hydrogen or dangling oxygen atoms at the binding site, I have observed a range of binding energies for OH radical on ice (0.06 eV to 0.74 eV). Thus, the reaction barriers for the radical reactions may be also sensitive to the dangling atoms at the binding site. In order to address this issue in broad sense, reaction paths for the radical reactions will be calculated at a number of binding sites.
As the reactions occur at 10 K, quantum tunneling may play a role on the reaction rates. In order to address this, I will be testing several tunneling corrected versions of the transition state theory, specifically the Wigner method, Eckart method, and Harmonic quantum transition state theory.
|
