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Complexities in Nonadiabatic Dynamics of Small Molecular Anions

Opoku-Agyeman, Bernice
http://orcid.org/0000-0002-6493-6655
http://rave.ohiolink.edu/etdc/view?acc_num=osu1503094708588515

Abstract Details

2018, Doctor of Philosophy, Ohio State University, Chemistry.
The studies in this thesis utilize theoretical approaches to investigate the dynamics involved in the photodissociation of small anionic systems in which nonadiabatic interactions result in charge transfer during the dissociation process. The aim of these studies is to gain insights into the nature of the electronic states involved in the dissociation and to understand the role of intramolecular motions and the energy distributions among the various degrees of freedom. In this thesis, the dynamics of the photodissociation of XCN- (X = I or Br) following electronic excitations to states that dissociate to X- + CN and X* + CN- have been investigated. Based on previous quantum dynamics studies of the photodissociation of ICN-, the two electronic states are accessed by visible or ultraviolet excitation and are found to be coupled, leading to the observation of both photoproducts in the dissociation process. Due to the small energy difference between the two asymptotic channels (roughly 0.14 eV for ICN- and -0.05 eV for BrCN-), a nonadiabatic interaction similar to that observed in ICN- plays an important role in the dynamics involving BrCN-. The dissociation processes in BrCN- are studied using a similar quantum dynamics approach that was applied in the studies of ICN-. The calculations are performed on diabatic models of the potentials developed for the two relevant excited states. The calculated branching ratios indicate that the nonadiabatic effects are more pronounced in BrCN- than in ICN-. The energy distribution among the various degrees of freedom is also found to have a large influence on the branching ratios of the photoproducts. The solvation of ICN- by argon atoms interestingly leads to the experimental observation of a small fraction of recombination products following the excitation of clusters as small as ICN-(Ar) or ICN-(Ar)2 in the visible or ultraviolet region, respectively. Argon solvation is therefore expected to alter the interactions between ground state and excited states in ICN-, and perhaps BrCN-, and thus affect the branching ratios of the photo-products compared to the isolated XCN-. Hence, performing theoretical calculations on XCN-(Ar)n clusters can provide insights into the differences in the dynamics of these dissociation processes. In order to simulate the dynamics, a model is proposed for the development of the potential energy surfaces for the argon clusters using the results obtained from electronic structure calculations of the XCN- and the fragments in the clusters. The potential energies are approximated as the interaction in the bare anion and pair-wise interactions between the argon and the dissociation products. The dynamics can then be carried out using a semi-classical method that incorporates Tully's surface hopping algorithm to treat the nonadiabatic effects. To assess the accuracy of the surface hopping approach, the method is applied to the dissociation of the bare XCN-. The calculated branching ratios and the partitioning of the energies among the various degrees of freedom in the dynamics are then compared with the quantum dynamics of XCN-. The results for the excitation of the bare XCN- show dependence on the type of calculation (quantum or classical) performed and the representation (adiabatic or diabatic) used in the classical dynamics calculations. The photodissociation processes for BrCN- are also found to be more sensitive to the type of dynamics calculations than ICN-. The results for the semi-classical calculations on the ICN- show that during the dissociation process, the anions that are described by some of the trajectories live longer and undergo recombination. Furthermore, the quantum dynamics calculations on XCN- also show evidence of recombination in the dynamics. As reported in previous work, the presence of the argon atoms may stabilize the high energy complexes, which will result in longer lifetimes. Perhaps the dynamics of these long-lived bare anions may provide a step toward understanding the dynamics processes that lead to recombined products in the XCN-(Ar)n.
Anne B. McCoy (Advisor)
Claudia Turro (Committee Chair)
Vicki Wysocki (Committee Member)
Cosmin S. Roman (Committee Member)
249 p.
Chemistry; Physical Chemistry
Potential energy surfaces; Excitation energies; Wave functions; Ground states; Excited states; Photodissociation; Surface Hopping; Semi-classical; energy distribution; argon solvation; charge transfer; nonadiabatic

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Citations

  • Opoku-Agyeman, B. (2018). Complexities in Nonadiabatic Dynamics of Small Molecular Anions [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1503094708588515

    APA Style (7th edition)

  • Opoku-Agyeman, Bernice. Complexities in Nonadiabatic Dynamics of Small Molecular Anions. 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1503094708588515.

    MLA Style (8th edition)

  • Opoku-Agyeman, Bernice. "Complexities in Nonadiabatic Dynamics of Small Molecular Anions." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1503094708588515

    Chicago Manual of Style (17th edition)

osu1503094708588515
126
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