Advances in the theoretical determination of molecular structure with applications to anion photoelectron spectroscopy
Keolopile, Zibo G.
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This Dissertation is focussed primarily on development of methods aiming at the determination of molecular structures with application to systems with intra and intermolecular hydrogen bonds. I have developed and demonstrated usefulness of Potential Energy Surface Scanning Tool (PESST) by performing a systematic search for the most stable structures of neutral and anionic phenylalanine and tyrosine molecules using electronic structure methods. I have found out that tautomers resulting from the proton transfer from the carboxylic OH to phenyl ring determine the structure of the most stable anions of phenylalanine, but double proton transfer from the carboxylic and hydroxyl groups determine structures of the most stable anions of tyrosine. The most stable conformer of these valence anions remained adiabatically unbound with respect to the canonical neutral in case of phenylalanine but bound in case of tyrosine. Valence anions identified in this report have recently been observed experimentally. Acetoacetic acid (AA), equipped with neighbouring carboxylic and keto groups, is a promising system for studies of intramolecular proton transfer. The results of my computational search for the most stable tautomers and conformers of the neutral and anionic AA were used to interpret anion photoelectron and electron energy-loss spectroscopy measurements. The valence anion was identi ed in photoelectron spectroscopy experiments and the measured electron vertical detachment energy is in good agreement with my computational predictions. My computational results allow rationalizing these experimental findings in terms of the co-existence of various conformers of AA. I considered stability of dimers formed by molecules that can exist in different conformational states. I have developed a protocol that allows the dissection of the total stabilisation energy into one-body conformational and deformational components and the two-body interaction energy term. Interplay between these components determines the overall stability of the dimer. The protocol has been tested on the dimers of oxalic acid. The global minimum stability results from a balancing act between a moderately attractive two-body interaction energy and small repulsive one-body terms. I have analysed zero-point vibrational corrections to the stability of various conformers of oxalic acid and their dimers. I have found that minimum energy structures with the most stabilising sets of hydrogen bonds have the largest zero-point vibrational energy, contrary to a naive anticipation based on red shifts of OH stretching modes involved in hydrogen bonds. My computational results demonstrated an unusual electrophilicity of oxalic acid (OA), the simplest dicarboxylic acid. The electrophilicity results primarily from the bonding carbon-carbon interaction in the SOMO orbital of the anion, but it is further enhanced by intramolecular hydrogen bonds. The well-resolved structure in the photoelectron spectrum has been reproduced theoretically, based on Franck-Condon factors for the vibronic anion!neutral transitions. The excess electron binding energies in the dimer and trimer of OA become very signi cant due to intermolecular proton transfer, with the corresponding vertical detachment energy (VDE) values of approximately 3.3 and 4.6 eV. I have postulated a mechanism of excess electron mobility along molecular linear chains supported by cyclic hydrogen bonds. Searches for the most stable molecular conformer are frustrated by energy barriers separating minima on the potential energy surface (PES). I have suggested that the barriers might be suppressed by subtracting selected force field terms from the original PES. The resulting deformed PES can be used in standard molecular dynamics (MD) or Monte Carlo simulations. The MD trajectories on the original and deformed PESs of ethanolamine differ markedly. The former gets stuck in a local minimum basin while the latter moves quickly to the global minimum basin.