Doctor of Philosophy, The Ohio State University, 2015, 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to the theoretical studies of H5+ because the conventional harmonic oscillator and rigid rotor approximation is no longer valid.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, (open full item for complete abstract)

Committee: Anne McCoy (Advisor); Terry Miller (Committee Member); John Wilkins (Committee Member) Subjects: Physical Chemistry

Doctor of Philosophy, The Ohio State University, 2012, Physics

We examine the role cosmic rays, X-rays and ultra-violet (UV) photons play in the chemical evolution of the interstellar medium, and how astrophysical processes like massive star formation can change the fluxes of these energetic particles. We connect star formation rates to interstellar chemistry. We first explore the basic effects of cosmic-ray and X-ray ionization and UV photodissociation on the chemistry. For cosmic-ray and X-ray ionization, increasing the ionization rates enriches the chemistry, up to a value of 10(-14) s-1, whereupon molecules and ions are quickly destroyed due to the high electron fraction. Isolated from other effects, the UV field tends to dissociate species much more efficiently than ionizing them, and generally reduces molecular abundances, especially those of complex molecules. The combination of a high ionization rate and a high UV field can enhance the production of some molecular species, such as small hydrocarbons. We investigate the role of cosmic rays and UV photons in the Horsehead Nebula, and determine the impact a column-dependent cosmic ray ionization rate makes on photodissociation region (PDR) chemistry. The column-dependence of cosmic rays is solved using a three-dimensional two-fluid magnetohydrodynamics model, treating the cosmic rays as a fluid governed by the relativistic Boltzmann Transport Equation, and treating the interstellar medium as a second fluid, governed by the standard non-relativistic magnetohydrodynamics equations. We then utilize a modified version of the Morata-Herbst time-dependent PDR model, incorporating our function for cosmic ray ionization. Our results help solve a chemical mystery concerning high abundances of small hydrocarbons at the edge of the nebula. We discuss predictions the model makes for species currently unobserved in the Horsehead Nebula. Finally, we examine the role of star formation on interstellar astrochemistry in the Orion KL region. We develop a new astrochemical gas-grain PDR mode (open full item for complete abstract)

Committee: Richard Freeman (Advisor); Eric Herbst (Advisor); Pierre Agostini (Committee Member); John Beacom (Committee Member); Gregory Lafyatis (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2011, Physics

Models of the abundances of interstellar molecules in various sources in our galaxy, and outside of the Milky Way are discussed. L1527 is a low-mass protostellar region in Taurus, and it was thought to be unusual because the envelope of the protostar contains observable abundances of unsaturated carbon-chain molecules as well as negative ions C4H- and C6H-. These molecules are associated with prestellar cold cores before the heat up phase. It was suggested that these molecules are formed in L1527 from the chemical precursor methane, which evaporates from the grains heated by the protostar (Sakai et al., 2008). We model the chemistry that occurs following their methane evaporation scenario with the OSU gas-phase network with anion reactions, and we are able to reproduce most of the observed molecular abundances in L1527 including anions. The anion-to-neutral ratio in our calculation is in good agreement with observation for C6H- but exceeds the observed ratio by more than three orders of magnitude for C4H- . Further study is needed on the rate coefficients for electron attachment and other reactions regarding anions to resolve this discrepancy. In order to model high-temperature regions influenced by energetic processes, we constructed a high-temperature network, an extension of the OSU gas-phase reaction network for time-dependent kinetics. The additional reactions include processes with significant activation energies, reverse reactions, proton exchange reactions, charge exchange reactions, and collisional dissociation. Rate coefficients already in the OSU network were modified for H2 formation on grains, ion-neutral dipole reactions, and some radiative association reactions. Given that efficient production of water at high temperature forces much of the elemental oxygen to be in the form of water at T ≥ 300 K, effective carbon-rich conditions are created, which can efficiently produce carbon-chain species such as C2H2. At higher temperatures, HCN and NH3 are (open full item for complete abstract)

Committee: Eric Herbst (Advisor); Todd Thompson (Committee Member); John Beacom (Committee Member); Gregory Lafyatis (Committee Member); Moira Konrad (Committee Member) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 2009, Chemical Physics

There are billions of stars in our galaxy, the Milky Way Galaxy. In between the stars is where the so-called “interstellar medium” locates. The majority of the mass of interstellar medium is clumped into interstellar clouds, in which cold and hot dense cores exist. Despite of the extremely low densities and low temperatures of the dense cores, over one hundred molecules have been found in these sources. This makes the field of astrochemistry vivid. Chemical modeling plays very important roles to understand the mechanism of formation and destruction of interstellar molecules. In this thesis, chemical kinetics models of different types were applied: in Chapter 4, pure gas phase models were used for seven newly detected or confirmed molecules by the Green Bank Telescope; in Chapter 5, the potential reason of non-detection of O2 was explored; in Chapter 6, the mysterious behavior of CHNO and CHNS isomers were studied by gas-grain models. In addition, effects of varying rate coefficients to the models are also discussed in Chapter 3 and 7.

Committee: ERIC HERBST Prof (Advisor); Frank De Lucia Prof (Committee Member); Anil Pradhan Prof (Committee Member) Subjects: