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Theoretical Studies on the Spectroscopy and Dynamics of Astrochemically Significant Species

Lin, Zhou
http://orcid.org/0000-0003-3271-5345
http://rave.ohiolink.edu/etdc/view?acc_num=osu1429633299

Abstract Details

2015, Doctor of Philosophy, Ohio State University, Chemical Physics.
Astrochemistry studies chemical reactions that occur in the interstellar medium. Most astrochemical studies focus on the detection of interstellar molecules via rovibrational spectroscopy or the interpretation of their abundance using reaction dynamics. This thesis is divided into two parts, and in each of them we address one of the two aspects from the perspective of theoretical chemistry. In the first part we propose the synthetic route for propene (CH3-CH=CH2), which has a surprisingly high abundance as the most saturated organic molecule in the extremely cold and thin gas-phase medium. Based on the reaction rates obtained from experiments and ab initio calculations for all three reactions in this route, we simulate the time-evolution for the abundance of propene and find that our synthetic route is able to reproduce part of the observed propene abundance. In the second part we discuss the spectroscopy and the dynamics of H5+, aiming to decipher its role as the intermediate of the astrochemically important proton transfer reaction, H3+ + H2 -> H5+ -> H2 + H3+. The large amplitude motions (LAM's) in H5+ allow the protons to permute between H3+ and H2 and result in products that have different nuclear spins and rovibrational states from the reactants. These LAM’s introduce challenges to the theoretical studies of H5+ because the conventional harmonic oscillator and rigid rotor approximation is no longer valid. Diffusion Monte Carlo and its extensions are used to capture the couplings between LAM's and other rovibrational modes in H5+. Specifically, we focus on three LAM's: the proton transfer vibration, the H2-H2 torsion, and the internal rotation of H3+ about its C3 symmetry axis. For selected states of H5+, we evaluate energies and wave functions as well as the reaction paths for the above-mentioned proton transfer process. In the spectroscopic studies, we find that the proton transfer vibration has a significant mixing with the dissociation vibration, which leads to strong absorptions for a series of transitions of v=0 -> 2n+1 in the infrared spectrum. We also find that the H2-H2 torsion is strongly coupled to the rotation of H5+ and causes the absence of half of the rotational-torsional states. Finally we predict the couplings between different rovibrational modes in the Hamiltonian in H5+ and the spectroscopic selection rules for its rovibrational transitions based on our group theoretical analysis. In the studies of chemical dynamics, we find that as H5+ dissociates into H3+ and H2, the energetically feasible permutations of protons achieved by LAM’s depend on dissociation coordinates. The tunneling along the internal rotation of H3+ is energetically feasible when the H3+-H2 distance is greater than 2.41 Å while the tunneling of the proton transfer vibration is only allowed when the H2-H2 distance is smaller than 2.59 Å. Interestingly, in a small range of molecular geometries the full permutations of protons are allowed. In addition, our group theoretical analyses about the geometries of H5+ that are sampled by the reaction paths provides the correlations between the energy levels of H5+ and those of H3+ + H2.
Anne McCoy (Advisor)
Terry Miller (Committee Member)
John Wilkins (Committee Member)
350 p.
Physical Chemistry
theoretical chemistry; astrochemistry; interstellar medium; kinetic studies; spectroscopy; large amplitude motions; diffusion Monte Carlo; propene; protonated hydrogen dimer; rotation-torsion coupling; reaction path; molecular symmetry; group theory

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Citations

  • Lin, Z. (2015). Theoretical Studies on the Spectroscopy and Dynamics of Astrochemically Significant Species [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1429633299

    APA Style (7th edition)

  • Lin, Zhou. Theoretical Studies on the Spectroscopy and Dynamics of Astrochemically Significant Species. 2015. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1429633299.

    MLA Style (8th edition)

  • Lin, Zhou. "Theoretical Studies on the Spectroscopy and Dynamics of Astrochemically Significant Species." Doctoral dissertation, Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1429633299

    Chicago Manual of Style (17th edition)

osu1429633299
836
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This open access ETD is published by The Ohio State University and OhioLINK.