Salai Cheettu Ammal

π-systems as lithium/hydrogen bond acceptors: Some theoretical observations

Ab initio calculations at the Hartree–Fock and correlated levels and density functional theory calculations have been performed with 6-31++G(d,p) and 6-311++G(d,p) basis sets on LiF and HF complexes of benzene, ethylene, and acetylene. Complex binding energies have been corrected for basis set superposition error, and zero point energy corrections have been done on Hartree–Fock binding energies. Computed results indicate that the complexes exist in different conformations and among them those with π-lithium and π-hydrogen bonds are the most stable. π-lithium bonds are stronger than π-hydrogen bonds. The computed binding energies and geometry of HF complexes correlate well with the available experimental results. LiF complexes with these π systems are found to be weaker than Li+ complexes but they are stronger than Li atom complexes. Natural bond orbital analysis traces the origin of the weak interactions that stabilize the complex. Li, as found in earlier cases, prefers the most symmetric site for interac...

Modeling the noble metal/TiO2 (110) interface with hybrid DFT functionals: A periodic electrostatic embedded cluster model study

The interaction of Au(n) and Pt(n) (n=2,3) clusters with the stoichiometric and partially reduced rutile TiO(2) (110) surfaces has been investigated using periodic slab and periodic electrostatic embedded cluster models. Compared to Au clusters, Pt clusters interact strongly with both stoichiometric and reduced TiO(2) (110) surfaces and are able to enhance the reducibility of the TiO(2) (110) surface, i.e., reduce the oxygen vacancy formation energy. The focus of this study is the effect of Hartree-Fock exchange on the description of the strength of chemical bonds at the interface of Au/Pt clusters and the TiO(2) (110) surface. Hartree-Fock exchange helps describing the changes in the electronic structures due to metal cluster adsorption as well as their effect on the reducibility of the TiO(2) surface. Finally, the performance of periodic embedded cluster models has been assessed by calculating the Pt adsorption and oxygen vacancy formation energies. Cluster models, together with hybrid PBE0 functional, are able to efficiently compute reasonable electronic structures of the reduced TiO(2) surface and predict charge localization at surface oxygen vacancies, in agreement with the experimental data, that significantly affect computed adsorption and reaction energies.

Ab initio and DFT investigations of lithium/hydrogen bonded complexes of trimethylamine, dimethyl ether and dimethyl sulfide

Ab initio and DFT computations have been carried out on LiF and HF complexes of a set of n-donors viz. trimethylamine, dimethyl ether and dimethyl sulfide with a 6-31++G(d,p) basis set. The effect of correlation has been included with MP2, MP4 and DFT calculations. NBO analyses of the wavefunctions have been performed to examine the intermolecular interaction at the orbital level. Calculations reveal that these donors form strong n→σ* complexes and computed binding energies of the (CH3)2O···HF complex agree very well with the experimental binding energies from IR spectroscopy. LiF forms stronger complexes than HF, and the effect of correlation on the hydrogen bond energy is considerable compared to the lithium bond energy. Though charge transfer interaction contributes to the stability of both LiF and HF complexes, it plays a less dominant role in lithium bonded complexes. While amine and ether donate their nσ lone pair, sulfide donates an nπ lone pair and this results in perpendicular intermolecular bonds in sulfide complexes.

Reaction kinetics of the electrochemical oxidation of CO and syngas fuels on a Sr2Fe1.5Mo0.5O6−δ perovskite anode

The performance of a Sr2Fe1.5Mo0.5O6−δ (SFMO) perovskite anode has been investigated under solid oxide fuel cell conditions while operating on CO and syngas fuels using periodic density functional theory and microkinetic modeling. Three surface models with different Fe/Mo ratios and oxygen vacancies on the gas exposed surface layer are used to identify the active site and electro-oxidation mechanism for CO and a mixture of CO and H2. Calculated current densities suggest that SFMO anodes exhibit lower performance while operating on CO compared to H2 fuel and a surface with a higher Mo concentration in the top most layer exhibits a higher activity for both fuels. Furthermore, the model predicts that desorption of CO2 and formation of an oxygen vacancy, which is found to be the charge transfer step, is rate-controlling for CO electro-oxidation at operating voltage. The CO oxidation activity can thus be improved by increasing the Mo content or by adding small amounts of an active transition metal on the surface that facilitates the oxygen vacancy formation process by delocalizing the electrons at the vacant site. In the presence of a mixture of CO, H2 and H2O gas, the water–gas shift reaction (CO + H2O ⇌ CO2 + H2) is rapid at operating voltage and H2 electro-oxidation contributes mostly to the overall observed electrochemical activity, while CO is primarily chemically oxidized to CO2. A higher Mo content in the SFMO surface promotes again a higher activity for syngas fuels with high CO content.