Computational and Experimental Binding Energy Study of Non-covalent Interactions of Polyaromatic Dimers and Trimers

Abstract

Non-covalent interactions play an important role in the stabilisation and structure of various organic and biological molecules and are therefore of importance in astrochemistry, biochemistry, and material science. The work in this thesis focuses on pi-pi and/or CH-pi interactions, i.e. the interaction of aromatic rings. Resonant two-photon ionisation of the benzene dimer was performed experimentally. The binding energy of neutral benzene, naphthalene, anthracene, pyrene, and coronene dimers and their heterodimers and trimers were successfully calculated theoretically. The binding energy of heterodimer and trimer radical cations of benzene, naphthalene, and anthracene clusters were also calculated. The experimental set-up was upgraded and extended with two new nanosecond laser systems for excitation and ionisation, coupled to a very substantially modified ultrahigh- vacuum apparatus with a reflectron and linear Time-of-Flight Mass Spectrometer (TOF-MS) and a ZEKE electron analyser, in order to study molecular clusters based on their mass detection, using the supersonic free-jet expansion technique. The TOF of the benzene monomer appeared at 57.976 μs and that of the benzene dimer at 79.893 μs. The DFT-D3 results for the binding energy were identical to the high level ab initio SCS-MP2. The difference in binding energies of aromatic dimers between the stacked and T-shaped geometries increased with the increasing number of aromatic rings. The binding energy for the benzene dimer was very flat, so different ab initio method calculations were performed for its T-shaped and parallel-displaced geometries. The geometry of Polycyclic Aromatic Hydrocarbon (PAH) dimers shows two different structural preferences depending on their size. Large PAHs such as naphthalene, anthracene, pyrene and coronene favour a stacked geometry, whereas the theoretical work on the benzene dimer indicates a TT-shaped geometry as the global minimum based on the most accurate computational methods: QCISD(T) and CCSD(T), but the PD isomer was confirmed to be the global minimum based on the DFT functionals and SCS-MP2 method. The sandwich isomer was found to be the global minimum structure of cation dimer and trimer complexes.

Date of Award	31 Dec 2021
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Nicholas Lockyer (Supervisor)