Reactivity of the O2+.(H2O)n and NO+.(H2O)n cluster ions in the D-region of the ionosphere

Sharma, Sainish (2018) Reactivity of the O2+.(H2O)n and NO+.(H2O)n cluster ions in the D-region of the ionosphere. MSc(Res) thesis, University of Nottingham.

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The structure, spectroscopy and reactivity of hydrated dioxygenyl cluster ions O2+.(H2O)n with n = 1 – 5 has been theoretically investigated for the first time to understand potential reaction pathways that may convert O2+, present in high abundance in the upper layers of the ionosphere (E-region), into protonated water clusters, which are the predominant positively charged ion species in the lower D-region. The reactivity of hydrated nitrosonium as NO+.(H2O)n with n = 4 – 5 has also been reinvestigated to clarify the potential intramolecular reaction for the formation of nitrous acid and a protonated water cluster.

Lowest energy geometries, binding energies and harmonic vibrational frequencies have been obtained for O2+.(H2O)n (n = 1 – 5) cluster ions. Through performing geometry optimisation calculations on O2+.(H2O) using coupled cluster theory (CCSD(T)), second order Møller-Plesset perturbation theory (MP2) and density functional theory (DFT) with a wide range of exchange-correlation functionals, it was found that the MP2/6-311++G** level of theory was reliable for the calculation of minimum energy geometries and harmonic vibrational frequencies. In particular, methods that include 100% exact Hartree-Fock exchange such as MP2 and M06-HF were found to accurately describe the charge distribution in the clusters. MP2-based Born-Oppenheimer ab initio molecular dynamics (AIMD) simulations of the reactions of the O2+.(H2O)n and NO+.(H2O)n cluster ions to form protonated water clusters reveal different mechanisms for the O2+ and NO+ based ions. AIMD simulations of O2+.(H2O)n (n = 2 – 5) with initial velocities of the atoms sampled from the Maxwell-Boltzmann distribution at 220 K show that following charge transfer, a reaction to form a protonated water cluster and OH radical occurs rapidly where the neutral O2 molecule is just a spectator. In contrast, the reaction of NO+.(H2O)n (n = 4 – 5) has been hypothesised to involve an intracluster reaction, but no reaction is observed in the AIMD simulations using thermal initial velocities. AIMD simulations of a water molecule colliding with NO+.(H2O)4 to form NO+.(H2O)5 were performed to establish whether the resulting activated complex, with excess energy, would be more susceptible to forming a protonated water cluster and nitrous acid. The simulations showed that an intramolecular reaction could occur and was dependant on the impact direction and velocity of the incoming water molecule.

Item Type: Thesis (University of Nottingham only) (MSc(Res))
Supervisors: Besley, Nicholas
Wright, Timothy
Keywords: Quantum Chemistry, Atmospheric Chemistry, Theoretical Chemistry; Ionosphere
Subjects: Q Science > QC Physics > QC811 Geomagnetism. Meteorology. Climatology
Faculties/Schools: UK Campuses > Faculty of Science > School of Chemistry
Item ID: 55573
Depositing User: Sharma, Sainish
Date Deposited: 05 Apr 2019 13:54
Last Modified: 07 May 2020 14:00

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