Ab initio Predictions of the Structures, Reactivities, Spectroscopic and Energetic Properties of Some Chemical Systems

Abstract

In this thesis, the first part focuses on high-level theoretical predictions for the benchmark dissociation of [HCCH]²⁺ into H₂⁺ and C₂⁺, which involves H migration, H-H combination, and C-H bond cleavage, using the state-average complete active-space self-consistent field (SA-CASSCF) and the internally contracted multi-reference configuration interaction (MRCI) approaches with the zero-point vibrational energy corrections (ZPVE). This unusual dissociation channel is unambiguously identified by measuring the time-of-flight (TOF) of both fragment ions in coincidence. This channel serves as an important prototype for understanding the H₂⁺ / H₃⁺ formation reactions which are widely observed in the dissociation of organic di-cation in interstellar media. The second part is devoted the reaction mechanism of: i) concerted proton-N atom transfer in the reaction of HSO₃^- with a ruthenium (VI) nitrido complex, [(L)Ru^VI(N)(OH₂)]⁺ (Ru^VIN, L=N,N'-bis(salicylidene)-o-cyclohexyldiamine dianion) in aqueous solution; ii) proton-coupled O-atom transfer in the oxidation of HSO₃^- by the ruthenium oxo complex trans-[Ru^VI(TMC)(O)₂]²⁺ (TMC = 1,4,8,11-Tetramethyl-1,4,8,11-tetraazacyclotetradecane); using density functional theory (DFT). The theoretical calculations combining with the experimental results provide important insights to the chemistry of transition metal containing complexes, specially the reaction mechanisms, which are promising their potential uses in catalysis. In the third part, the single photon ionization processes of gas-phase 9H-adenine (9A), thymine (T), and 2-quinolone (2Q) are theoretically studied by DFT, coupled-cluster, and multi-reference methods. The ionization energies (IEs) required to form ground and different excited states of 9A, T and 2Q are determined by explicitly correlated computation using the coupled-cluster level with single, double excitations plus a perturbative triple excitation [CCSD(T)] with ZPVE, the core-valence electronic corrections (CV) and the scalar relativistic effect corrections (SR). The CASSCF and MRCI approaches have been employed to predict the energetics of excited states and their vibrational spectra. The slow photoelectron spectrum (SPES) of 9A, T and 2Q are assigned based on the equilibrium geometries, electronic states patterns, and anharmonic vibrational frequencies. The last part presents an efficient composite theoretical method for the predications of the reduction potential of several important redox couples: i) ferrocene/ferrocenium [Fe^II(C₅H₅)₂/Fe^III(C₅H₅)₂⁺, Fc/Fc⁺]; ii) cobaltocene/cobaltocenium [Co^II(C₅H₅)₂/Co^III(C₅H₅)₂⁺, Cp₂Co/Cp₂Co⁺]; iii) iron(II)/iron(III) (Fe²⁺/Fe³⁺) and iv) manganate/permanganate (Mn^VIO₄^2-/Mn^VIIO₄^-) in aqueous and nonaquaous solution using the wavefunction-based CCSD(T) method and complete basis set (CBS) approach. The CCSD(T)/CBS calculations presented in this part involve the approximation to the CBS limit at the coupled cluster level up to full quadruple excitations with ZPVE, CV and SR corrections. Comparisons between the CCSD(T)/CBS and the precise experimental values suggest that the composite procedure is capable of reliably predicting reduction potential in chemical accuracy (less than 120 mV). Besides the theoretical predictions on reduction potentials, the AIEs of Fc and Cp₂Co in gas phase are also presented.

Date of Award	31 Jul 2019
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kai Chung LAU (Supervisor)