Simulation of spectroscopic properties of atoms and molecules

Chemical phenomena are largely determined by the behaviour of electrons in atoms or molecules. Knowledge on electronic properties is of key importance to understand chemical reactions. In this study, electronic structures of molecules are studied from the perspectives of both coordinate space and momentum space. The latter enables us to further simulate positron annihilation spectra of noble gas atoms and small molecules under appropriate conditions. The responses of perfluoro effects in benzene and structural modifications in cytidine nucleoside analogues are revealed through spectroscopic information, in addition to properties such as geometries, intra-molecular interactions, vibrational spectra and atomic site dependent properties such as, charges and Fukui functions. Simulated spectra and orbital momentum distributions (MDs) are validated with the available experimental results. It is observed that density functional theory (DFT) models, in combination with adequate basis sets, are able to produce optimal results for various properties. The orbital based signatures which are associated with the uniqueness of the chemical bonding of the molecules are revealed. From electron momentum distributions, we have calculated gamma-ray spectra for positron annihilation of noble gas atoms and small molecules based on an innovative “low energy plane wave positron” (LEPWP) approximation. We found that the annihilation line shapes, ε, depend significantly on their principal quantum number n and orbital angular momentum quantum number l of the noble gases, whereas in small molecules, the innermost valence orbital was found to produce reasonable agreement with the experiment.