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Physisorption of CO and N2O on surfaces of ceria (CeO2) was investigated by means of high-level quantum-mechanical embedded cluster calculations. Both systems have high relevance in the field of environmental chemistry and heterogeneous catalysis. The CO/CeO2 system, has been investigated in a couple of both experimental and theoretical studies, but for the N2O/CeO2 system, this is the first study in the literature, experimental or theoretical. In physisorption, the interaction relies entirely on classical electrostatic interactions and electron dispersion forces. No covalent bond is formed between the molecule and the surface. A proper description of the dispersion requires some of the most accurate quantum-mechanical methods available, such as MP2 or CCSD(T). Moreover, even the most sophisticated methods cannot heal errors anywhere else in the theoretical treatment. Standard periodic models cannot be used with methods such as CCSD(T), but embedded cluster models can, and have been thoroughly explored in this thesis. In this thesis, embedded cluster models were constructed for the CeO2(110) and (111) surfaces. Using a range of assessment tests, it was verified that the electronic structure of the central region of a large and fully embedded surface cluster agrees well with the corresponding region in a periodic system. CO physisorption was investigated at the CCSD(T) level. Due to the prohibitively large expenses (in computer time) for standard CCSD(T) calculations, the method of increments, previously used in the literature for bulk systems, was extended to adsorption problems. It was found that, electron correlation contributes by 30 - 80% to the molecule-surface interaction and that the contribution depends on the topology of the surface. The calculated CO-ceria interaction energy is 20 kJ/mol for the (111) surface and 27 kJ/mol for the (110) surface. In low temperature TPD experiments for the N2O/CeO2(111) system, one surface species was found with an adsorption energy of about 29 kJ/mol. IR measurements showed stretching frequencies that are typically assigned to N2O adsorption with the O-end directed towards surface cations. However, theoretical calculations up to the MP2 level predicted two equally favorable adsorption species. Improvements in the structural model (larger clusters, consideration of molecule-induced relaxation) or the computational method (larger basis sets) did not affect this result. Only at the CCSD(T) level was one dominating surface species found, namely N2O adsorbed over a Ce ion, with the O-end of the molecule directed towards the surface. The calculated stretching vibrational frequency shifts (with respect to the gas phase) for this adsorbed species agree well with the measured IR spectra.

Reactions of singlet (1S and 1D) and triplet (3P) carbon atoms with water, and 1D and 3P carbon atoms with methanol were studied computationally. In the water and methanol systems, the carbon vapor containing a mixture of C(1S), C(1D), and C(3P) atoms, is predicted to react by primarily interacting with the oxygen, OH bond and CH bond of the substrate mainly with the 1D state. While C(1S) was proven to be unreactive C(3P) can hardly be supported to be reactive, and can safely be defined as unreactive. The major product, CO forms as a result of oxygen abstraction, which is observed as a fast, energetically quite favorable process. The scheme of this oxygen abstraction is promising to be applicable to substrates with the general formula R1-O-R2 i.e. water, alcohols, and ethers. OH insertion, both for water and methanol, yields trappable carbenes / the carbene being a key species on the distribution of the end products. Water matrix trapping the carbene opens the path to the formation of formaldehyde / and exhibits a prototype reaction for the formation of dialkoxymethanes. Gas phase product spectrum from the reactions are broader, due to the accessibility of the routes originating from the otherwise trapped intermediates / and the excess energy of the reactions being carried by them. In the condensed phase the very early and rapid reactions seem to have chance, the subsequent rearrangements are hard to occur. The conclusions thus far apply to the reactions in the gas phase as well as in condensed phases involving inert matrices / and the experimental isolation of the species is highly dependent on the ability of the medium to trap the intermediates via effective transfer of excess energy. Due to the large excess energies of intermediates involved, subsequent reactions are fast / of the order 1013 s-1 from kinetic rate calculations. In the absence of efficient transfer of non-fixed energies to the surrounding medium, all of the reaction paths will conclude with irreversible dissociation reactions. Plausible mechanisms for all the experimentally observed products are predicted. The results are in agreement with the available experimental data.

Abstract The thesis consists of seven studies dealing with high resolution vibration-rotation spectra of planar asymmetric tops. Six studies deal with D212CO and D213CO species of the formaldehyde molecule and one study is from DCOOH specie of the formic acid molecule. The measurements were carried out at high accuracy and the rotational analyses of the recorded spectra were performed. The observed anharmonic and Coriolis resonances were taking into account in the analyses. The rotational constants of the present and literature studies were used in evaluation of the planarity defects of formaldehyde and formic acid molecules in the summary part of the thesis. Finally, a semi-experimental structure for formaldehyde was obtained by employing experimental and theoretical data.

Computational chemistry lies at the intersection of chemistry, physics, mathematics, and computer science, and can be used to explain the behavior of atoms and molecules, as well as to augment experiment. In this work, computational chemistry methods are used to predict structural and energetic properties of small molecules, i.e. molecules with less than 60 atoms. Different aspects of computational chemistry are examined in this work. The importance of examining the converged orbitals obtained in an electronic structure calculation is explained. The ability to more completely describe the orbital space through the extrapolation of energies obtained at increasing quality of basis set is investigated with the use of the Sapporo-nZP-2012 family of basis set. The correlation consistent Composite Approach (ccCA) is utilized to compute the enthalpies of formation of a set of molecules and the accuracy is compared with the target method, CCSD(T,FC1)/aug-cc-pCV∞Z-DK. Both methodologies are able to produce computed enthalpies of formation that are typically within 1 kcal mol-1 of reliable experiment. This demonstrates that ccCA can be used instead of much more computationally intensive methods (in terms of memory, processors, and time required for a calculation) with the expectation of similar accuracy yet at a reduced computational cost. The enthalpies of formation for systems containing s-block elements have been computed using the multireference variant of ccCA (MR-ccCA), which is designed specifically for systems that require an explicit treatment of nondynamical correlation. Density functional theory (DFT) has been used for the prediction of the structural properties of a set of lanthanide trihalide molecules as well as the reaction energetics for the rearrangement of diphosphine ligands around a triosmium cluster.

The aim of this thesis is to investigate strength and origin of the stabilization for various types of noncovalent interactions. As this knowledge could lead to a deeper understand- ing and rationalization of the binding phenomena. Further, to participate on the de- velopment of new noncovalent data sets, which are nowadays inevitable in the process of parametrization and validation of new computational methods. In all the studies, different binding motifs of model complexes, which represent usually crystal structures, structures from unrelaxed scans or the local minima, were investi- gated. The calculations of the reference stabilization energies were carried out at ab initio level (e.g. CCSD(T)/CBS, QCISD(T)/CBS). Further, the accuracy of more ap- proximate methods (e.g. MP2.5, DFT-D or SQM methods) toward reference method, was tested. In order to obtain the nature of the stabilization the DFT-SAPT decompo- sition was frequently utilized. In the first part of the thesis, the importance and basic characteristics of different types of noncovalent interactions (e.g. halogen bond, hydrogen bond, π· · · π interaction etc.), are discussed. The second part provides the description of computational methods which were essential for our investigation. The third part of the thesis provides an overview for part...

Research Doctorate - Doctor of Philosophy (PhD) / (**Note: this abstract is a plain text version of the author's abstract, the original of which contains characters and symbols which cannot be accurately represented in this format. The properly formatted abstract can be viewed in the Abstract and Thesis files above.**) The electronic structure and rovibrational spectroscopy of MH2, MHn+2, HMHen+ and MHen+2 (M = Li, Be, Na, Mg, K, Ca; n = 1, 2) have been investigated using correlated ab initio ansatz. In order to determine the efficacy of various electronic structure methods with respect to Group-I and II hydrides and helides, atomic properties of Li, Be,Na, Mg, K and Ca were calculated. Relativistically-corrected UCCSD(T) and ICMRCI(+Q) were deemed to be the most suitable ansatz with respect to both efficiency and accuracy. The lowest 2A1 and 2Σ- states of MH2 were found to be purely repulsive, in agreement with previous predictions. The main factor determining the structure and stability of the excited states of MH2 was the relative orientations and occupations of the valence p atomic orbital of M and the H2 1Ou orbital. The ground states of MHn+2 were found to be the result of the charge-quadrupole interaction between Mn+ and the H2 molecular subunit. The structures of the ground states of HMHe+ were extremely uxional with respect to the central bond angle co-ordinate. The ground state PESs of MHe+2 were also extremely sensitive to the ab initio ansatz by which they are modelled. The respective bonding of the H and He in both HMHe+ and HMHe2+ appeared to be charge-dependent in the case of Be, Mg and Ca. Despite the weak bonding observed for the Group-II hydrohelide and helide monocations, the corresponding dications each exhibit thermodynamically stable equilibria. The solution algorithm of von Nagy-Felsobuki and co-workers was employed in the calculation of vibrational and rovibrational spectra. This algorithm employed an Eckart-Watson Hamiltonian in conjunction with rectilinear normal co-ordinates. Vibrational and rovibrational Hamiltonian matrices were diagonalised using variational methods. This algorithm was extended so that the vibration transition moment integrals, and hence vibrational radiative properties, of linear triatomic molecules could be calculated. A method by which vibration-averaged structures are calculated was also developed and implemented. Analytical potential energy functions (PEFs) and dipole moment functions (DMFs) of (1A1)LiH+2, (1A1)NaH+2, (1A1)BeH2+2,(1A1)MgH2+2, (1Σ+g )BeHe2+2, (2Σ+)HBeHe2+, (1Σ+g )MgHe2+2 and (2Σ+)HMgHe2+ were developed using leastsquare regression techniques in conjunction with discrete ab initio grids. Vibrational structures and spectra of these species were subsequently calculated. In addition, the rovibrational spectra of (1A1)LiH+2, (1A1)NaH+2, (1A1)BeH2+2 and (1A1)MgH2+2 were calculated. For (1A1)LiH+2 and (1A1)LiD+2 , calculated rovibrational transition frequencies for J ≤ 10 and 0 ≤ K ≤ 3 were within ca. 0.1-0.2% of experimental values.

Research Doctorate - Doctor of Philosophy (PhD) / (**Note: this abstract is a plain text version of the author's abstract, the original of which contains characters and symbols which cannot be accurately represented in this format. The properly formatted abstract can be viewed in the Abstract and Thesis files above.**) The electronic structure and rovibrational spectroscopy of MH2, MHn+2, HMHen+ and MHen+2 (M = Li, Be, Na, Mg, K, Ca; n = 1, 2) have been investigated using correlated ab initio ansatz. In order to determine the efficacy of various electronic structure methods with respect to Group-I and II hydrides and helides, atomic properties of Li, Be,Na, Mg, K and Ca were calculated. Relativistically-corrected UCCSD(T) and ICMRCI(+Q) were deemed to be the most suitable ansatz with respect to both efficiency and accuracy. The lowest 2A1 and 2Σ- states of MH2 were found to be purely repulsive, in agreement with previous predictions. The main factor determining the structure and stability of the excited states of MH2 was the relative orientations and occupations of the valence p atomic orbital of M and the H2 1Ou orbital. The ground states of MHn+2 were found to be the result of the charge-quadrupole interaction between Mn+ and the H2 molecular subunit. The structures of the ground states of HMHe+ were extremely uxional with respect to the central bond angle co-ordinate. The ground state PESs of MHe+2 were also extremely sensitive to the ab initio ansatz by which they are modelled. The respective bonding of the H and He in both HMHe+ and HMHe2+ appeared to be charge-dependent in the case of Be, Mg and Ca. Despite the weak bonding observed for the Group-II hydrohelide and helide monocations, the corresponding dications each exhibit thermodynamically stable equilibria. The solution algorithm of von Nagy-Felsobuki and co-workers was employed in the calculation of vibrational and rovibrational spectra. This algorithm employed an Eckart-Watson Hamiltonian in conjunction with rectilinear normal co-ordinates. Vibrational and rovibrational Hamiltonian matrices were diagonalised using variational methods. This algorithm was extended so that the vibration transition moment integrals, and hence vibrational radiative properties, of linear triatomic molecules could be calculated. A method by which vibration-averaged structures are calculated was also developed and implemented. Analytical potential energy functions (PEFs) and dipole moment functions (DMFs) of (1A1)LiH+2, (1A1)NaH+2, (1A1)BeH2+2,(1A1)MgH2+2, (1Σ+g )BeHe2+2, (2Σ+)HBeHe2+, (1Σ+g )MgHe2+2 and (2Σ+)HMgHe2+ were developed using leastsquare regression techniques in conjunction with discrete ab initio grids. Vibrational structures and spectra of these species were subsequently calculated. In addition, the rovibrational spectra of (1A1)LiH+2, (1A1)NaH+2, (1A1)BeH2+2 and (1A1)MgH2+2 were calculated. For (1A1)LiH+2 and (1A1)LiD+2 , calculated rovibrational transition frequencies for J ≤ 10 and 0 ≤ K ≤ 3 were within ca. 0.1-0.2% of experimental values.