Quantum chemical studies on radical cation clusters and X-ray emission spectroscopy

This thesis presents computational studies of two main projects, the first concerning the structure and bonding of mixed component radical cation clusters and the second relating to quantum chemical calculations of X-ray emission spectroscopy. Basin hopping in conjunction with second order Møller-Plesset perturbation theory is used to characterise the lowest energy isomers of mixed component radical cation clusters of the form [H2O-X]•+, [(H2O)2-X]•+ and [H2O-X2]•+, where X=PH3, H2S and HCl, with the relative energies refined using coupled cluster theory calculations. For the dimers where X=H2S or HCl, a proton transfer based structure comprising H3O+ and SH• or Cl• radicals has the lowest energy structure whereas for X=PH3 a hemibonded structure is most stable. For the trimers, a much wider range of possible isomers based upon both proton transfer and hemibonded structures are observed. The calculation of X-ray emission spectroscopy with equation of motion coupled cluster theory (EOM-CCSD), time dependent density functional theory (TDDFT) and resolution of the identity single excitation configuration interaction with second order perturbation theory (RI-CIS(D)) is studied. TDDFT with standard exchange- correlation functionals predicts transition energies that are much larger than exper- iment. Optimisation of a hybrid and short-range corrected functional to predict the X-ray emission transitions results in much closer agreement with EOM-CCSD. The most accurate exchange-correlation functional identified is a modified B3LYP hybrid functional with 66% Hartree-Fock exchange, denoted B66LYP, which predicts X-ray emission spectra for a range of molecules including fluorobenzene, nitrobenzene, acetone, dimethyl sulfoxide and CF3Cl in good agreement with experiment

Quantum chemical calculations of X-ray emission spectroscopy

The calculation of X-ray emission spectroscopy with equation of motion coupled cluster theory (EOM-CCSD), time dependent density functional theory (TDDFT) and resolution of the identity single excitation configuration interaction with second order perturbation theory (RI-CIS(D)) is studied. These methods can be applied to calculate X-ray emission transitions by using a reference determinant with a core-hole, and they provide a convenient approach to compute the X-ray emission spectroscopy of large systems since all of the required states can be obtained within a single calculation removing the need to perform a separate calculation for each state. For all of the methods, basis sets with the inclusion of additional basis functions to describe core orbitals are necessary, particularly when studying transitions involving the 1s or- bitals of heavier nuclei. EOM-CCSD predicts accurate transition energies when compared with experiment, however, its application to larger systems is restricted by its computational cost and difficulty in converging the CCSD equations for a core-hole reference determinant, which become increasing problematic as the size of the system studied increases. While RI-CIS(D) gives accurate transition energies for small molecules containing first row nuclei, its application to larger systems is limited by the CIS states providing a poor zeroth order reference for perturbation theory which leads to very large errors in the computed transition energies for some states. TDDFT with standard exchange-correlation functionals predicts transition energies that are much larger than experiment. Optimization of a hybrid and short-range cor- rected functional to predict the X-ray emission transitions results in much closer agreement with EOM-CCSD. The most accurate exchange-correlation functional identified is a modified B3LYP hybrid functional with 66% Hartree-Fock exchange, denoted B66LYP, which predicts X-ray emission spectra for a range of molecules including fluorobenzene, nitrobenzene, ace- tone, dimethyl sulfoxide and CF3Cl in good agreement with experiment

Quantum chemical studies on radical cation clusters and X-ray emission spectroscopy

This thesis presents computational studies of two main projects, the first concerning the structure and bonding of mixed component radical cation clusters and the second relating to quantum chemical calculations of X-ray emission spectroscopy. Basin hopping in conjunction with second order Møller-Plesset perturbation theory is used to characterise the lowest energy isomers of mixed component radical cation clusters of the form [H2O-X]•+, [(H2O)2-X]•+ and [H2O-X2]•+, where X=PH3, H2S and HCl, with the relative energies refined using coupled cluster theory calculations. For the dimers where X=H2S or HCl, a proton transfer based structure comprising H3O+ and SH• or Cl• radicals has the lowest energy structure whereas for X=PH3 a hemibonded structure is most stable. For the trimers, a much wider range of possible isomers based upon both proton transfer and hemibonded structures are observed. The calculation of X-ray emission spectroscopy with equation of motion coupled cluster theory (EOM-CCSD), time dependent density functional theory (TDDFT) and resolution of the identity single excitation configuration interaction with second order perturbation theory (RI-CIS(D)) is studied. TDDFT with standard exchange- correlation functionals predicts transition energies that are much larger than exper- iment. Optimisation of a hybrid and short-range corrected functional to predict the X-ray emission transitions results in much closer agreement with EOM-CCSD. The most accurate exchange-correlation functional identified is a modified B3LYP hybrid functional with 66% Hartree-Fock exchange, denoted B66LYP, which predicts X-ray emission spectra for a range of molecules including fluorobenzene, nitrobenzene, acetone, dimethyl sulfoxide and CF3Cl in good agreement with experiment

Mechanisms of monovacancy diffusion in graphene

A comprehensive investigation of monovacancy diffusion in graphene has been carried out with the use of density functional theory and the climbing image nudged elastic band method. An out-of-plane spiro structure is found for the first-order saddle point, which defines the transition state in the vacancy diffusion pathway. The obtained activation energy for diffusion is significantly lower than the reported values for the in-plane saddle point structures. The time between consecutive vacancy jumps in graphene is estimated to be in the range of 100-200 s at room temperature in a good agreement with experimental observations

Mechanisms of monovacancy diffusion in graphene

A comprehensive investigation of monovacancy diffusion in graphene has been carried out with the use of density functional theory and the climbing image nudged elastic band method. An out-of-plane spiro structure is found for the first-order saddle point, which defines the transition state in the vacancy diffusion pathway. The obtained activation energy for diffusion is significantly lower than the reported values for the in-plane saddle point structures. The time between consecutive vacancy jumps in graphene is estimated to be in the range of 100-200 s at room temperature in a good agreement with experimental observations